Physiology and behaviour of free-swimming Atlantic cod (Gadus morhua) facing fluctuating temperature conditions

Spawning fish maintains trophic synchrony across time and space beyond thermal drivers

Increasing ocean temperature will speed up physiological rates of ectotherms. In fish, this is suggested to cause earlier spawning due to faster oocyte growth rates. Over time, this could cause spawning time to become decoupled from the timing of offspring food resources, a phenomenon referred to as trophic asynchrony. We used biological data, including body length, age, and gonad developmental stages collected from >125,000 individual Northeast Arctic cod (Gadus morhua) sampled between 59 and 73° N in 1980-2019. Combined with experimental data on oocyte growth rates, our analyses show that cod spawned progressively earlier by about a week per decade, partly due to ocean warming. It also appears that spawning time varied by more than 40 days, depending on year and spawning location. The significant plasticity in spawning time seems to be fine-tuned to the local phytoplankton spring bloom phenology. This ability to partly overcome thermal drivers and thus modulate spawning time could allow individuals to maximize fitness by closely tracking local environmental conditions important for offspring survival. Our finding highlights a new dimension for trophic match-mismatch and should be an important consideration in models used to predict phenology dynamics in a warmer climate.

Does heat tolerance actually predict animals' geographic thermal limits?

The "climate extremes hypothesis" is a major assumption of geographic studies of heat tolerance and climatic vulnerability. However, this assumption remains vastly untested across taxa, and multiple factors may contribute to uncoupling heat tolerance estimates and geographic limits. Our dataset includes 1000 entries of heat tolerance data and maximum temperatures for each species' known geographic limits (hereafter, Tmax). We gathered this information across major animal taxa, including marine fish, terrestrial arthropods, amphibians, non-avian reptiles, birds, and mammals. We first tested if heat tolerance constrains the Tmax of sites where species could be observed. Secondly, we tested if the strength of such restrictions depends on how high Tmax is relative to heat tolerance. Thirdly, we correlated the different estimates of Tmax among them and across species. Restrictions are strong for amphibians, arthropods, and birds but often weak or inconsistent for reptiles and mammals. Marine fish describe a non-linear relationship that contrasts with terrestrial groups. Traditional heat tolerance measures in thermal vulnerability studies, like panting temperatures and the upper set point of preferred temperatures, do not predict Tmax or are inversely correlated to it, respectively. Heat tolerance restricts the geographic warm edges more strongly for species that reach sites with higher Tmax for their heat tolerance. These emerging patterns underline the importance of reliable species' heat tolerance indexes to identify their thermal vulnerability at their warm range edges. Besides, the tight correlations of Tmax estimates across on-land microhabitats support a view of multiple types of thermal challenges simultaneously shaping ranges' warm edges for on-land species. The heterogeneous correlation of Tmax estimates in the ocean supports the view that fish thermoregulation is generally limited, too. We propose new hypotheses to understand thermal restrictions on animal distribution.

Behavioural responses of fishes to anthropogenic disturbances: Adaptive value and ecological consequences

Aquatic ecosystems are changing at an accelerating rate because of human activities. The changes alter the abundance and distribution of fishes, with potential consequences for ecosystem structure and function. Behavioural responses often underlie these changes in population dynamics, such as altered habitat choice or foraging activity. Here, we present a framework for understanding how and why behaviour is affected by human activities and how the behavioural responses in turn influence higher ecological levels. We further review the literature to assess the present state of the field and identify gaps in our knowledge. We begin with discussing the factors that determine how an individual responds to a change in the environment and whether the response is adaptive or not. In particular, we explain the importance of the evolutionary history of the species. We then search the literature to assess our current knowledge of the impact of human disturbances on the behaviour of fishes and the consequences for ecosystems. The search reveals that much attention has been directed to the impact of human activities on the behaviour of fishes, but that worryingly little is known about the consequences of these responses for populations, communities and ecosystems. Yet, behavioural responses can have profound ecological consequences given that behaviour underly many, if not most, species interactions. Thus, more attention should be paid to the mechanisms and pathways through which behavioural responses influence higher ecological levels. Such information is needed if we are to determine the ultimate effects of human activities on biodiversity and the function and stability of aquatic ecosystems.

Dehydrated snakes reduce postprandial thermophily

Transient thermophily in ectothermic animals is a common response during substantial physiological events. For example, ectotherms often elevate body temperature after ingesting a meal. In particular, the increase in metabolism during the postprandial response of pythons - known as specific dynamic action - is supported by a concurrent increase in preferred temperature. The objective of this study was to determine whether hydration state influences digestion-related behavioral thermophily. Sixteen (8 male and 8 female) Children's pythons (Antaresia childreni) with surgically implanted temperature data loggers were housed individually and provided with a thermal gradient of 25-45°C. Body temperature was recorded hourly beginning 6 days prior to feeding and for 18 days post-feeding, thus covering pre-feeding, postprandial and post-absorptive stages. Each snake underwent this 24 day trial twice, once when hydrated and once when dehydrated. Our results revealed a significant interaction between temperature preference, digestive stage and hydration state. Under both hydrated and dehydrated conditions, snakes similarly increased their body temperature shortly after consuming a meal, but during the later days of the postprandial stage, snakes selected significantly lower (∼1.5°C) body temperature when they were dehydrated compared with when they were hydrated. Our results demonstrate a significant effect of hydration state on postprandial thermophily, but the impact of this dehydration-induced temperature reduction on digestive physiology (e.g. passage time, energy assimilation) is unknown and warrants further study.

Behavioural temperature regulation is a low priority in a coral reef fish (Plectropomus leopardus): insights from a novel behavioural thermoregulation system

Current understanding of behavioural thermoregulation in aquatic ectotherms largely stems from systems such as "shuttle boxes", which are generally limited in their capacity to test large-bodied species. Here, we introduce a controlled system that allows large aquatic ectotherms to roam freely in a tank at sub-optimal temperatures, using thermal refuges to increase body temperature to their thermal optimum as desired. Of the 10 coral grouper (Plectropomus leopardus; length ∼400 mm) implanted with thermal loggers, three fish maintained themselves at the ambient tank temperature of 17.5-20.5oC for the entire 2-4 d trial. Of the other seven fish, body temperature never exceeded ∼21.5oC, which was well below the temperature available in the thermal refuges (∼31oC) and below the species' optimal temperature of ∼27oC. This study adds to a growing literature documenting an unexpected lack of behavioural thermoregulation in aquatic ectotherms in controlled, heterothermal environments.

Fine scale behaviour and time-budget in the cryptic ectotherm European pond turtle Emys orbicularis

For ectotherms, behaviour and associated energetic costs are directly related to thermal conditions. In the present context of global change, estimating time-budget for these species is relevant to assess and predict their capacity to adapt to near future. We tested the hypothesis that in ectotherms where reproduction is highly energy consuming, energy expenditure should vary throughout the breeding season with a maximum around nesting events. To test this hypothesis, we assessed the fine-scale behaviour, time-budget and estimated energetic costs in eight adult female European pond turtles Emys orbicularis equipped with data-loggers recording ambient temperature, pressure, light and the animals’ 3-axis acceleration. Deployments occurred over four months throughout the nesting season 2017 in semi-natural captive conditions in Alsace, France. All study turtles showed a clear daily pattern over the 24h cycle, with four distinct phases (referred to as Night, Morning, Midday and Evening), associated with different behaviours and activity levels. Before oviposition, turtles were mostly active during Morning, and activity was positively driven by ambient temperature. Activity levels doubled during the nesting period, mostly due to the increased activity in the Evening, when nesting events occurred. Throughout the active season, basking occurrence at Midday was related to air temperature but cloud coverage was an even more important factor. Our results are a first step in predicting the seasonal time and energy budgets of the European pond turtle, and demonstrate the usefulness of animal-borne accelerometers to study free living freshwater turtles over extended periods of time.

Cardiac and behavioural responses to hypoxia and warming in free-swimming gilthead seabream, Sparus aurata

Gilthead seabream were equipped with intraperitoneal biologging tags to investigate cardiac responses to hypoxia and warming, comparing when fish were either swimming freely in a tank with conspecifics or confined to individual respirometers. After tag implantation under anaesthesia, heart rate (fH) required 60 h to recover to a stable value in a holding tank. Subsequently, when undisturbed under control conditions (normoxia, 21°C), mean fH was always significantly lower in the tank than in the respirometers. In progressive hypoxia (100% to 15% oxygen saturation), mean fH in the tank was significantly lower than in the respirometers at oxygen levels down to 40%, with significant bradycardia in both holding conditions below this level. Simultaneous logging of tri-axial body acceleration revealed that spontaneous activity, inferred as the variance of external acceleration (VARm), was low and invariant in hypoxia. Warming (21 to 31°C) caused progressive tachycardia with no differences in fH between holding conditions. Mean VARm was, however, significantly higher in the tank during warming, with a positive relationship between VARm and fH across all temperatures. Therefore, spontaneous activity contributed to raising fH of fish in the tank during warming. Mean fH in respirometers had a highly significant linear relationship with mean rates of oxygen uptake, considering data from hypoxia and warming together. The high fH of confined seabream indicates that respirometry techniques may bias estimates of metabolic traits in some fishes, and that biologging on free-swimming fish will provide more reliable insight into cardiac and behavioural responses to environmental stressors by fish in their natural environment.

Seasonal influence on the bathymetric distribution of an endangered fish within a marine protected area

The spatio-temporal variability of fish distribution is important to better manage and protect the populations of endangered species. In this sense, the vertical movements of a vulnerable and protected species, Sciaena umbra, were assessed in a marine protected area (the Réserve Naturelle Marine de Cerbère-Banyuls, south of France) to study the variability of their bathymetric distribution at different time scales. Twenty adults were marked with acoustic transmitters and acoustically monitored over 2.5 years. This revealed that some individuals remained at shallow waters (< 8 m) all year round, while others presented vertical segregation at deeper waters during the cold months (mean depth of 22.5 ± 0.04 m) and all aggregated in shallow waters during the warm months. The brown meagre was more active during the night, except in June and July when peaks of activity were observed at dusk. These patterns are likely associated with foraging and reproductive behavior during the cold and warm periods, respectively, and likely regulated by water temperature and the depth of the thermocline. Here, we provide valuable information on when and where in the water column critical periods of S. umbra life cycle are expected to occur, which should be considered in management and protection plans.

Reductionist approaches to the study of ionoregulation in fishes

The mechanisms underlying ionoregulation in fishes have been studied for nearly a century, and reductionist methods have been applied at all levels of biological organization in this field of research. The complex nature of ionoregulatory systems in fishes makes them ideally suited to reductionist methods and our collective understanding has been dramatically shaped by their use. This review provides an overview of the broad suite of techniques used to elucidate ionoregulatory mechanisms in fishes, from the whole-animal level down to the gene, discussing some of the advantages and disadvantages of these methods. We provide a roadmap for understanding and appreciating the work that has formed the current models of organismal, endocrine, cellular, molecular, and genetic regulation of ion balance in fishes and highlight the contribution that reductionist techniques have made to some of the fundamental leaps forward in the field throughout its history.

Effects of polycyclic aromatic hydrocarbons and abiotic stressors on Fundulus grandis cardiac transcriptomics

Following the 2010 Deepwater Horizon oil spill, extensive research has been conducted on the toxicity of oil and polycyclic aromatic hydrocarbons (PAHs) in the aquatic environment. Many studies have identified the toxicological effects of PAHs in estuarine and marine fishes, however, only recently has work begun to identify the combinatorial effect of PAHs and abiotic environmental factors such as hypoxia, salinity, and temperature. This study aims to characterize the combined effects of abiotic stressors and PAH exposure on the cardiac transcriptomes of developing Fundulus grandis larvae. In this study, F. grandis larvae were exposed to varying environmental conditions (dissolved oxygen (DO) 2, 6 ppm; temperature 20, 30 °C; and salinity 3, 30 ppt) as well as to a single concentration of high energy water accommodated fraction (HEWAF) (∑PAHs 15 ppb). Whole larvae were sampled for RNA and transcriptional changes were quantified using RNA-Seq followed by qPCR for a set of target genes. Analysis revealed that exposure to oil and abiotic stressors impacts signaling pathways associated with cardiovascular function. Specifically, combined exposures appear to reduce development of the systemic vasculature as well as strongly impact the cardiac musculature through cardiomyocyte proliferation resulting in inhibited cardiac function and modulated blood pressure maintenance. Results of this study provide a holistic view of impacts of PAHs and common environmental stressors on the cardiac system in early life stage estuarine species. To our knowledge, this study is one of the first to simultaneously manipulate oil exposure with abiotic factors (DO, salinity, temperature) and the first to analyze cardiac transcriptional responses under these co-exposures.

Thermal biology and swimming performance of Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus)

Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) are two commercially important marine fishes impacted by both overfishing and climate change. Increasing ocean temperatures are affecting the physiology of these species and causing changes in distribution, growth, and maturity. While the physiology of cod has been well investigated, that of haddock has received very little attention. Here, we measured the metabolic response to increasing temperatures, as well as the critical thermal maximum (CT_max), of cod acclimated to 8 and 12 °C and haddock acclimated to 12 °C. We also compared the swimming performance (critical swimming speed, U_crit) of cod and haddock at 12 °C, as well as the U_crit of 12 °C-acclimated cod acutely exposed to a higher-than-optimal temperature (16 °C). The CT_max for cod was 21.4 and 23.0 °C for 8- and 12 °C-acclimated fish, respectively, whereas that for the 12 °C-acclimated haddock was 23.9 °C. These values were all significantly different and show that haddock are more tolerant of high temperatures. The aerobic maximum metabolic rate (MMR) of swimming cod remained high at 16 °C, suggesting that maximum oxygen transport capacity was not limited at a temperature above optimal in this species. However, signs of impaired swimming (struggling) were becoming evident at 16 °C. Haddock were found to reach a higher U_crit than cod at 12 °C (3.02 vs. 2.62 body lengths s⁻¹, respectively), and at a lower MMR. Taken together, these results suggest that haddock perform better than cod in warmer conditions, and that haddock are the superior swimmer amongst the two species.

Thermal performance of fish is explained by an interplay between physiology, behaviour and ecology

Increasing temperatures under climate change are thought to affect individual physiology of fish and other ectotherms through increases in metabolic demands, leading to changes in species performance with concomitant effects on species ecology. Although intuitively appealing, the driving mechanism behind thermal performance is contested; thermal performance (e.g. growth) appears correlated with metabolic scope (i.e. oxygen availability for activity) for a number of species, but a substantial number of datasets do not support oxygen limitation of long-term performance. Whether or not oxygen limitations via the metabolic scope, or a lack thereof, have major ecological consequences remains a highly contested question. size and trait-based model of energy and oxygen budgets to determine the relative influence of metabolic rates, oxygen limitation and environmental conditions on ectotherm performance. We show that oxygen limitation is not necessary to explain performance variation with temperature. Oxygen can drastically limit performance and fitness, especially at temperature extremes, but changes in thermal performance are primarily driven by the interplay between changing metabolic rates and species ecology. Furthermore, our model reveals that fitness trends with temperature can oppose trends in growth, suggesting a potential explanation for the paradox that species often occur at lower temperatures than their growth optimum. Our model provides a mechanistic underpinning that can provide general and realistic predictions about temperature impacts on the performance of fish and other ectotherms and function as a null model for contrasting temperature impacts on species with different metabolic and ecological traits.

Acute and sub-lethal effects of an anionic polyacrylamide on sensitive early life stages of Atlantic cod (Gadus morhua)

Despite the possible increase in use of anionic polyacrylamide (APAM) in enhanced oil recovery operations, very little relevant information regarding ecotoxicity exists. The current study assessed acute and sub-lethal toxicity in sensitive early life stages (ELS) of Atlantic cod (Gadus morhua) exposed to 200 kDa APAM under controlled laboratory conditions. Two experiments (screening and long-term study) were conducted covering ecologically relevant endpoints (survival, hatching, growth, deformations, respiration and heart rate) in fish developing through embryogenesis, hatching, yolk-sac larvae stage and the first feeding period. The screening experiment was an 8-day exposure of embryos, whereas in the long-term experiments embryos and developing larvae were exposed continuously for 23 days. In the screening experiment, a significant reduction in embryonic heart rate was observed during exposure to 150 and 1500 mg APAM/L. However, we observed no effects on fitness-related endpoints (survival, hatching and growth) at concentrations up to 1500 mg L^-1 APAM. Also, for the long-term exposure from late embryo to first feeding larvae stage, we observed reduced heart rate at 125 mg L^-1. No consistent responses on survival, growth or respiration were observed except for the highest concentration tested (6000 mg L^-1). Dispersion modelling based on expected and relevant discharged polymer concentrations and durations showed that predicted environmental concentrations were orders of magnitude lower than the concentrations tested in our experiments, indicating that 200 kDa APAM will have a limited probability of causing fitness-related effects on Atlantic cod ELS.

GC bias lead to increased small amino acids and random coils of proteins in cold-water fishes

BACKGROUND

Temperature adaptation of biological molecules is fundamental in evolutionary studies but remains unsolved. Fishes living in cold water are adapted to low temperatures through adaptive modification of their biological molecules, which enables their functioning in extreme cold. To study nucleotide and amino acid preference in cold-water fishes, we investigated the substitution asymmetry of codons and amino acids in protein-coding DNA sequences between cold-water fishes and tropical fishes., The former includes two Antarctic fishes, Dissostichus mawsoni (Antarctic toothfish), Gymnodraco acuticeps (Antarctic dragonfish), and two temperate fishes, Gadus morhua (Atlantic cod) and Gasterosteus aculeatus (stickleback), and the latter includes three tropical fishes, including Danio rerio (zebrafish), Oreochromis niloticus (Nile tilapia) and Xiphophorus maculatus (Platyfish).

RESULTS

Cold-water fishes showed preference for Guanines and cytosines (GCs) in both synonymous and nonsynonymous codon substitution when compared with tropical fishes. Amino acids coded by GC-rich codons are favored in the temperate fishes, while those coded by AT-rich codons are disfavored. Similar trends were discovered in Antarctic fishes but were statistically weaker. The preference of GC rich codons in nonsynonymous substitution tends to increase ratio of small amino acid in proteins, which was demonstrated by biased small amino acid substitutions in the cold-water species when compared with the tropical species, especially in the temperate species. Prediction and comparison of secondary structure of the proteomes showed that frequency of random coils are significantly larger in the cold-water fish proteomes than those of the tropical fishes.

CONCLUSIONS

Our results suggested that natural selection in cold temperature might favor biased GC content in the coding DNA sequences, which lead to increased frequency of small amino acids and consequently increased random coils in the proteomes of cold-water fishes.

Drifting Away of a FET Endpoint: The Heart Rate in Danio rerio Embryos is Extremely Sensitive to Variation in Ambient Temperature

Beyond the mere detection of presence or absence of heart beat in zebrafish (Danio rerio) embryos in a fish embryo test conducted referring to the OECD TG 236 at 48 hpf (hours post fertilization) onwards, embryo heart rate may serve as an additional and very sensitive endpoint in ecotoxicological studies. But by including heart rate as a sublethal endpoint, care has to be taken of separating effects exerted by a tested compound from those exerted by temperature. Therefore, profound knowledge on the natural variation of zebrafish heart rates at defined temperatures as a basis for the assessment of gained results is mandatorily needed. As such continuous information in D. rerio is lacking from the literature, we designed a study covering a span of 12°C (from 18 to 30°C in steps of 2°C) to quantify the relationship between heart rate and temperature in D. rerio embryos 48 hpf. Conducting a multiple regression analysis, we found a considerably strong relationship between treatment temperature and the log10 of the heart rate, ranging from 82.8 beats per minute at 18°C to 218.0 beats per minute at 30°C. Our results therefore may serve as a reference for heart rates measured under normal conditions to be able to detect potential effects of contaminants in other studies when working under certain temperatures.

Remote bioenergetics measurements in wild fish: Opportunities and challenges

The generalized energy budget for fish (i.e., Energy Consumed=Metabolism+Waste+Growth) is as relevant today as when it was first proposed decades ago and serves as a foundational concept in fish biology. Yet, generating accurate measurements of components of the bioenergetics equation in wild fish is a major challenge. How often does a fish eat and what does it consume? How much energy is expended on locomotion? How do human-induced stressors influence energy acquisition and expenditure? Generating answers to these questions is important to fisheries management and to our understanding of adaptation and evolutionary processes. The advent of electronic tags (transmitters and data loggers) has provided biologists with improved opportunities to understand bioenergetics in wild fish. Here, we review the growing diversity of electronic tags with a focus on sensor-equipped devices that are commercially available (e.g., heart rate/electrocardiogram, electromyogram, acceleration, image capture). Next, we discuss each component of the bioenergetics model, recognizing that most research to date has focused on quantifying the activity component of metabolism, and identify ways in which the other, less studied components (e.g., consumption, specific dynamic action component of metabolism, somatic growth, reproductive investment, waste) could be estimated remotely. We conclude with a critical but forward-looking appraisal of the opportunities and challenges in using existing and emerging electronic sensor-tags for the study of fish energetics in the wild. Electronic tagging has become a central and widespread tool in fish ecology and fisheries management; the growing and increasingly affordable toolbox of sensor tags will ensure this trend continues, which will lead to major advances in our understanding of fish biology over the coming decades.

Measurement and relevance of maximum metabolic rate in fishes

Maximum (aerobic) metabolic rate (MMR) is defined here as the maximum rate of oxygen consumption (M˙O2max ) that a fish can achieve at a given temperature under any ecologically relevant circumstance. Different techniques exist for eliciting MMR of fishes, of which swim-flume respirometry (critical swimming speed tests and burst-swimming protocols) and exhaustive chases are the most common. Available data suggest that the most suitable method for eliciting MMR varies with species and ecotype, and depends on the propensity of the fish to sustain swimming for extended durations as well as its capacity to simultaneously exercise and digest food. MMR varies substantially (>10 fold) between species with different lifestyles (i.e. interspecific variation), and to a lesser extent (<three-fold) between individuals of the same species (i.e. intraspecific variation). MMR often changes allometrically with body size and is modulated by several environmental factors, including temperature and oxygen availability. Due to the significance of MMR in determining aerobic scope, interest in measuring this trait has spread across disciplines in attempts to predict effects of climate change on fish populations. Here, various techniques used to elicit and measure MMR in different fish species with contrasting lifestyles are outlined and the relevance of MMR to the ecology, fitness and climate change resilience of fishes is discussed.

The relationship between body temperature, heart rate, breathing rate, and rate of oxygen consumption, in the tegu lizard (Tupinambis merianae) at various levels of activity

The present study determined whether EEG and/or EMG recordings could be used to reliably define activity states in the Brazilian black and white tegu lizard (Tupinambis merianae) and then examined the interactive effects of temperature and activity states on strategies for matching O2 supply and demand. In a first series of experiments, the rate of oxygen consumption (VO2), breathing frequency (fR), heart rate (fH), and EEG and EMG (neck muscle) activity were measured in different sleep/wake states (sleeping, awake but quiet, alert, or moving). In general, metabolic and cardio-respiratory changes were better indictors of the transition from sleep to wake than were changes in the EEG and EMG. In a second series of experiments, the interactive effects of temperature (17, 27 and 37 °C) and activity states on fR, tidal volume (VT), the fraction of oxygen extracted from the lung per breath (FIO2-FEO2), fH, and the cardiac O2 pulse were quantified to determine the relative roles of each of these variables in accommodating changes in VO2. The increases in oxygen supply to meet temperature- and activity-induced increases in oxygen demand were produced almost exclusively by increases in fH and fR. Regression analysis showed that the effects of temperature and activity state on the relationships between fH, fR and VO2 was to extend a common relationship along a single curve, rather than separate relationships for each metabolic state. For these lizards, the predictive powers of fR and fH were maximized when the effects of changes in temperature, digestive state and activity were pooled. However, the best r(2) values obtained were 0.63 and 0.74 using fR and fH as predictors of metabolic rate, respectively.

Metabolic scope and interspecific competition in sculpins of Greenland are influenced by increased temperatures due to climate change

Ongoing climate change has led to an increase in sea surface temperatures of 2–4°C on the west coast of Greenland. Since fish are ectothermic, metabolic rate increases with ambient temperature. This makes these animals particularly sensitive to changes in temperature; subsequently any change may influence their metabolic scope, i.e. the physiological capacity to undertake aerobically challenging activities. Any temperature increase may thus disrupt species-specific temperature adaptations, at both the molecular level as well as in behavior, and concomitant species differences in the temperature sensitivity may shift the competitive balance among coexisting species. We investigated the influence of temperature on metabolic scope and competitive ability in three species of marine sculpin that coexist in Greenland coastal waters. Since these species have different distribution ranges, we hypothesized that there should be a difference in their physiological response to temperature; hence we compared their metabolic scope at three temperatures (4, 9 and 14°C). Their competitive ability at the ambient temperature of 9°C was also tested in an attempt to link physiological capacity with behaviour. The Arctic staghorn sculpin, the species with the northernmost distribution range, had a lower metabolic scope in the higher temperature range compared to the other two species, which had similar metabolic scope at the three temperatures. The Arctic staghorn sculpin also had reduced competitive ability at 9°C and may thus already be negatively affected by the current ocean warming. Our results suggest that climate change can have effects on fish physiology and interspecific competition, which may alter the species composition of the Arctic fish fauna.

Semidiurnal temperature changes caused by tidal front movements in the warm season in seabed habitats on the georges bank northern margin and their ecological implications

Georges Bank is a large, shallow feature separating the Gulf of Maine from the Atlantic Ocean. Previous studies demonstrated a strong tidal-mixing front during the warm season on the northern bank margin between thermally stratified water in the Gulf of Maine and mixed water on the bank. Tides transport warm water off the bank during flood tide and cool gulf water onto the bank during ebb tide. During 10 days in August 2009, we mapped frontal temperatures in five study areas along ∼100 km of the bank margin. The seabed "frontal zone", where temperature changed with frontal movment, experienced semidiurnal temperature maxima and minima. The tidal excursion of the frontal boundary between stratified and mixed water ranged 6 to 10 km. This "frontal boundary zone" was narrower than the frontal zone. Along transects perpendicular to the bank margin, seabed temperature change at individual sites ranged from 7.0°C in the frontal zone to 0.0°C in mixed bank water. At time series in frontal zone stations, changes during tidal cycles ranged from 1.2 to 6.1°C. The greatest rate of change (-2.48°C hr(-1)) occurred at mid-ebb. Geographic plots of seabed temperature change allowed the mapping of up to 8 subareas in each study area. The magnitude of temperature change in a subarea depended on its location in the frontal zone. Frontal movement had the greatest effect on seabed temperature in the 40 to 80 m depth interval. Subareas experiencing maximum temperature change in the frontal zone were not in the frontal boundary zone, but rather several km gulfward (off-bank) of the frontal boundary zone. These results provide a new ecological framework for examining the effect of tidally-driven temperature variability on the distribution, food resources, and reproductive success of benthic invertebrate and demersal fish species living in tidal front habitats.

Unsteady flow affects swimming energetics in a labriform fish (Cymatogaster aggregata)

Unsteady water flows are common in nature, yet the swimming performance of fishes is typically evaluated at constant, steady speeds in the laboratory. We examined how cyclic changes in water flow velocity affect the swimming performance and energetics of a labriform swimmer, the shiner surfperch, Cymatogaster aggregata. Using intermittent-flow respirometry, we measured critical swimming speed (Ucrit), oxygen consumption rate (ṀO2) and pectoral fin use in steady flow versus unsteady flows with either low (0.5 body lengths per second; BLs-1) or high amplitude (1.0 BLs-1) velocity fluctuations, with a 5 s period. Individuals in low amplitude unsteady flow performed as well as fish in steady flow. However, swimming costs in high amplitude unsteady flow were on average 25.3 % higher than in steady flow and 14.2% higher than estimated values obtained from simulations based on the non-linear relationship between swimming speed and oxygen consumption rate in steady flow. Time-averaged pectoral fin use (fin beat frequency measured over 300 s) was similar among treatments. However, measures of instantaneous fin use (fin beat period) and body movement in high amplitude unsteady flow indicate that individuals with greater variation in the duration of their fin beats were better at holding station and consumed less oxygen than fish with low variation in fin beat period. These results suggest that the costs of swimming in unsteady flows are context dependent in labriform swimmers, and may be influenced by individual differences in the ability of fishes to adjust their fin beats to the flow environment.

Coupling of methylmercury uptake with respiration and water pumping in freshwater tilapia Oreochromis niloticus

The relationships among the uptake of toxic methylmercury (MeHg) and two important fish physiological processes-respiration and water pumping--in the Nile tilapia (Oreochromis niloticus) were explored in the present study. Coupled radiotracer and respirometric techniques were applied to measure simultaneously the uptake rates of MeHg, water, and oxygen under various environmental conditions (temperature, dissolved oxygen level, and water flow). A higher temperature enhanced MeHg influx and the oxygen consumption rate but had no effect on the water uptake, indicating the influence of metabolism on MeHg uptake. The fish showed a high tolerance to hypoxia, and the oxygen consumption rate was not affected until the dissolved oxygen concentration decreased to extremely low levels (below 1 mg/L). The MeHg and water uptake rates increased simultaneously as the dissolved oxygen level decreased, suggesting the coupling of water flux and MeHg uptake. The influence of fish swimming performance on MeHg uptake was also investigated for the first time. Rapidly swimming fish showed significantly higher uptake rates of MeHg, water, and oxygen, confirming the coupling relationships among respiration, water pumping, and metal uptake. Moreover, these results support that MeHg uptake is a rate-limiting process involving energy. Our study demonstrates the importance of physiological processes in understanding mercury bioaccumulation in fluctuating aquatic environments.

Simultaneous biologging of heart rate and acceleration, and their relationships with energy expenditure in free-swimming sockeye salmon (Oncorhynchus nerka)

Monitoring the physiological status and behaviour of free-swimming fishes remains a challenging task, although great promise stems from techniques such as biologging and biotelemetry. Here, implanted data loggers were used to simultaneously measure heart rate (f (H)), visceral temperature, and a derivation of acceleration in two groups of wild adult sockeye salmon (Oncorhynchus nerka) held at two different water speeds (slow and fast). Calibration experiments performed with individual fish in a swim tunnel respirometer generated strong relationships between acceleration, f (H), tail beat frequency and energy expenditure over a wide range of swimming velocities. The regression equations were then used to estimate the overall energy expenditure of the groups of fish held at different water speeds. As expected, fish held at faster water speeds exhibited greater f (H) and acceleration, and correspondingly a higher estimated energy expenditure than fish held at slower water speeds. These estimates were consistent with gross somatic energy density of fish at death, as determined using proximate analyses of a dorsal tissue sample. Heart rate alone and in combination with acceleration, rather than acceleration alone, provided the most accurate proxies for energy expenditure in these studies. Even so, acceleration provided useful information on the behaviour of fish and may itself prove to be a valuable proxy for energy expenditure under different environmental conditions, using a different derivation of the acceleration data, and/or with further calibration experiments. These results strengthen the possibility that biologging or biotelemetry of f (H) and acceleration may be usefully applied to migrating sockeye salmon to monitor physiology and behaviour, and to estimate energy use in the natural environment.

The effects of acute changes in temperature and oxygen availability on cardiac performance in winter flounder (Pseudopleuronectes americanus)

Studies on how flatfish cardiovascular function responds to environmental challenges are limited, and have largely relied upon indirect methodologies (i.e. Fick principle). Thus, we measured dorsal aortic blood pressure (P(DA)) and cardiac function in 8 and 15 degrees C-acclimated flounder exposed to graded hypoxia, and in 8 degrees C-acclimated fish exposed to an acute temperature increase to their critical thermal maximum (CTM). The extent of bradycardia in 8 degrees C-acclimated fish (decrease in heart rate of 41%) was consistent with that observed for other teleosts, as was this species' CTM (25.8+/-0.5 degrees C) and its cardiac response to increasing temperature. However, this study provides further examples of how cardiovascular function is controlled differently in the flounder as compared with other fishes. First, the onset of bradycardia in 8 degrees C-acclimated fish occurred earlier than expected for this inactive and hypoxia-tolerant species (60% water air saturation). Second, resting cardiac output was similar in flounder acclimated to 8 and 15 degrees C (approximately 15 mL min(-1) kg(-1)), and hypoxic bradycardia was surprisingly absent at 15 degrees C. Finally, systemic vascular resistance decreased when flounder were exposed to elevated temperature, and this resulted in a 26% fall in P(DA). These are novel findings, however, the extent to which the flounder's behaviour influenced some of the results is unclear.

Stable oxygen isotope reconstruction of ambient temperature during the collapse of a cod (Gadus morhua) fishery

Changing environmental conditions set against a backdrop of high exploitation can result in severe consequences for commercially harvested stocks. The collapse of the Eastern Scotian Shelf cod (Gadus morhua L.) off eastern Canada was primarily due to overexploitation but may have been exacerbated by a widespread temperature decline. Recent studies have called for accurate determination of ambient temperature (the actual temperature exposure history of the fish) before discarding environmental conditions as a factor in the collapse. We used the stable oxygen isotope composition of otoliths (delta18O(oto)) to reconstruct the ambient temperature history of Eastern Scotian Shelf cod from 1970 to 2000 in order to determine whether the stock experienced the temperature decline or shifted their distribution to avoid it. To correct delta18O(oto) for seawater isotope content (deltaO(w)), we generated a new meta-equation for the relationship between delta18O(w) (per mil) and salinity (S, in psu) on the Eastern Scotian Shelf: delta18O(w) = 0.539 x S - 18.790. The ambient temperature series revealed that the large-scale geographic distribution of mature cod remained constant through the cooling period, although their ambient temperature was cooler than expected in warmer periods and warmer than expected in cooler periods, indicating small-scale thermoregulatory movement. Although the mean hydrographic temperature was 4 degrees C, mature cod usually inhabited the coldest available waters (mean ambient temperature = 3 degrees C), while the juveniles usually inhabited warmer waters (mean ambient temperature = 5.5 degrees C). Length-at-age was significantly related to ambient temperature, especially in the early years of growth, and therefore declining ambient temperatures were at least partially responsible for declines in asymptotic length (up to age 8 yr). The most active thermoregulatory movement occurred during a moderate warming period; therefore extreme warming events (such as those predicted under climate change) may force large-scale northward latitudinal shifts in this historically sedentary stock. Retroactive stable isotope chronologies can be an important tool in sustainable management strategies under the shifting climate conditions predicted for years to come.

Aerobic scope for activity in age 0 year Atlantic cod Gadus morhua

Key components of swimming metabolism: standard metabolism (R(s)), active metabolism (R(a)) and absolute aerobic scope for activity (R(a)-R(s)) were determined for small age 0 year Atlantic cod Gadus morhua. Gadus morhua juveniles grew from 0.50 to 2.89 g wet body mass (M(WB)) over the experimental period of 100 days, and growth rates (G) ranged from 1.4 to 2.9% day(-1), which decreased with increasing size. Metabolic rates were recorded by measuring changes in oxygen consumption over time at different activity levels using modified Brett-type respirometers designed to accommodate the small size and short swimming endurance of small fishes. Power performance relationships were established between oxygen consumption and swimming speed measurements were repeated for individual fish as each fish grew. Mass-specific standard metabolic rates (RsMWB-1) were calculated from the power performance relationships by extrapolating to zero swimming speed and decreased from 7.00 to 5.77 micromol O(2) g(-1) h(-1), mass-specific active metabolic rates (RaMWB-1) were calculated from extrapolation to maximum swimming speed (U(max)) and decreased from 26.18 to 14.35 micromol O(2) g(-1) h(-1) and mass-specific absolute scope for activity was calculated as the difference between active and standard metabolism (RaMWB-1-RsMWB-1) and decreased from 26.18 to 14.35 micromol O(2) g(-1) h(-1) as M(WB) increased. Small fish with low R(s) had bigger aerobic scopes but, as expected, R(s) was higher in smaller fish than larger fish. The measurements and results from this study are unique as R(s), R(a) and absolute aerobic scopes have not been previously determined for small age 0 year G. morhua.

Chapter 3. Effects of climate change and commercial fishing on Atlantic cod Gadus morhua

During the course of the last century, populations of Atlantic cod Gadus morhua L. have undergone dramatic declines in abundance across their biogeographic range, leading to debate about the relative roles of climatic warming and overfishing in driving these changes. In this chapter, we describe the geographic distributions of this important predator of North Atlantic ecosystems and document extensive evidence for limitations of spatial movement and local adaptation from population genetic markers and electronic tagging. Taken together, this evidence demonstrates that knowledge of spatial population ecology is critical for evaluating the effects of climate change and commercial harvesting. To explore the possible effects of climate change on cod, we first describe thermal influences on individual physiology, growth, activity and maturation. We then evaluate evidence that temperature has influenced population-level processes including direct effects on recruitment through enhanced growth and activity, and indirect effects through changes to larval food resources. Although thermal regimes clearly define the biogeographic range of the species, and strongly influence many aspects of cod biology, the evidence that population declines across the North Atlantic are strongly linked to fishing activity is now overwhelming. Although there is considerable concern about low spawning stock biomasses, high levels of fishing activity continues in many areas. Even with reduced fishing effort, the potential for recovery from low abundance may be compromised by unfavourable climate and Allee effects. Current stock assessment and management approaches are reviewed, alongside newly advocated methods for monitoring stock status and recovery. However, it remains uncertain whether the rebuilding of cod to historic population sizes and demographic structures will be possible in a warmer North Atlantic.

Successful foraging zones of southern elephant seals from the Kerguelen Islands in relation to oceanographic conditions

Southern elephant seals, Mirounga leonina, undertake large-scale oceanic movements to access favourable foraging areas. Successful foraging areas of elephant seals from the Kerguelen Islands are investigated here in relation to oceanographic parameters. Movements and diving activity of the seals as well as oceanographic data were collected through a new generation of satellite relayed devices measuring and transmitting locations, pressure, temperature and salinity. For the first time, we have associated foraging behaviour, determined by high increased sinuosity in tracks, and dive density (i.e. number of dives performed per kilometre covered), and changes in body condition, determined by variations in drift rate obtained from drift dives, to identify the oceanographic conditions of successful foraging zones for this species. Two main sectors, one close to the Antarctic continent and the other along the Polar Front (PF), where both foraging activity and body condition increase, seem to be of particular interest for the seals. Within these regions, some seals tended to focus their foraging activity on zones with particular temperature signatures. Along the Antarctic continent, some seals targeted colder waters on the sea bottom during benthic dives, while at the PF the favourable zones tended to be warmer. The possible negative effect of colder waters in Antarctic on the swimming performances of potential fish or squid prey could explain the behaviour of elephant seals in these zones, while warmer waters within the PF could correspond to the optimal conditions for potential myctophid prey of elephant seals.

Linking environmental variability and fish performance: integration through the concept of scope for activity

Investigating the biological mechanisms linking environmental variability to fish production systems requires the disentangling of the interactions between habitat, environmental adaptation and fitness. Since the number of environmental variables and regulatory processes is large, straightening out the environmental influences on fish performance is intractable unless the mechanistic analysis of the 'fish-milieu' system is preceded by an understanding of the properties of that system. While revisiting the key points in our currently poorly integrated understanding of fish ecophysiology, we have highlighted the explanatory potential contained within Fry's (Fry 1947 Univ. Toronto Stud. Biol. Ser. 55, 1-62) concept of metabolic scope and categorization of environmental factors. These two notions constitute a pair of powerful tools for conducting an external (at the emerging property level) analysis of the environmental influences on fish, as well as an internal (mechanistic) examination of the behavioural, morphological and physiological processes involved. Using examples from our own and others work, we have tried to demonstrate that Fry's framework represents a valuable conceptual basis leading to a broad range of testable ecophysiological hypotheses.

Nonadditive genetic effects in animal behavior

Heritabilities, commonly used to predict evolutionary potential, are notoriously low for behaviors. Apart from strong contributions of environmental variance in reducing heritabilities, the additive genetic components can be very low, especially when they are camouflaged by nonadditive genetic effects. We first report the heritabilities of courtship traits in founder-flush and control populations of the housefly (Musca domestica L.). We estimated the heritability of each male and female display through the regression of the courtships involving daughters and sons (with randomly selected mates) onto the "midparental" courtship values of their parents. Overall, the average heritability was significantly (P = .012) higher for the parent-daughter assays than for the parent-son assays. We attributed the low (even negative) heritabilities to genotype-by-environment interactions whereby the male's behavior is influenced by the "environment" of his mating partner's preferences for the display, generating epistasis through indirect genetic effects. Moreover, bottlenecked lines had up to 800% of the heritability of the controls, suggesting "conversion" of additive genetic variance from nonadditive components. Second, we used line-cross assays on separate populations that had been selected for divergence in mating behavior to identify dominance and epistasis through heterosis and outbreeding depression in courtship. Finally, our literature review confirms the prevalence of such low heritabilities (i.e., a conservative mean of 0.38) and nonadditive genetics in other behavioral repertoires (64% of the studies). We conclude that animal behavior is especially prone to the gamut of quantitative genetic complexities that can result in negative heritabilities, negative selection responses, inbreeding depression, conversion, heterosis, and outbreeding depression.

The responses of Atlantic cod (Gadus morhua L.) to ultrasound-emitting predators: stress, behavioural changes or debilitation?

A previous study has reported that Atlantic cod can be conditioned to detect ultrasonic sound pulses of high intensity. This capability has been proposed as a mean for detection and avoidance of echolocating toothed whales that emit intense ultrasonic clicks. In this study, we use acoustic playbacks to test the hypotheses that unconditioned cod can detect and respond to intense ultrasound from toothed whales and from echosounders. Intense ultrasound exposure of 210 dB re. 1μPa (pp) did not cause a short-term stress response in the form of bradycardia in unconditioned cod. Free-swimming cod exposed to ultrasonic clicks and echosounder pulses with received levels of more than 204 dB re. 1 μPa (pp) did not elicit flight responses as seen in ultrasound detecting Alosinae. Furthermore, we tested the debilitating effects of high intensity ultrasound on swimming cod with no detected changes in swimming ability when exposed to more than 213 dB re. 1 μPa (pp). It is concluded that intense ultrasound exposure induces neither an antipredator nor a stress response in Atlantic cod, and that echosounder pulses and biosonar clicks therefore most probably play no ecophysiological role in wild cod populations.

Morphological predictors of swimming speed: a case study of pre-settlement juvenile coral reef fishes

The swimming abilities of fishes are of vital importance to their ecology, and studies on fish swimming have been the focus of research for over a century. Here we explore the relationship between swimming ability and external body morphology, using data on U(crit) swimming speeds of 100 species of pre-settlement juvenile coral reef fishes (at the transition between the larval and adult habitats), comprising 26 different families from 5 orders. The taxonomic diversity of this methodologically consistent dataset provides a unique opportunity to examine the relationship between form and function in fish swimming across a broad taxonomic range. Overall, we found that a predictive model incorporating total length (TL), the square of caudal peduncle depth factor (CPDF(2)) and aspect ratio (AR) can be used to accurately predict swimming performance of a wide range of fish families, and was able to explain 69% of the variability in swimming performance of these pre-settlement juvenile fishes. The model was also able to successfully predict the swimming speed of an out-group salmonid species (Oncorhynchus mykiss). There was no evidence that the model fit differed among taxonomic groups, despite the inclusion of five different orders of fishes, suggesting that body morphology sufficiently explains the bulk of differences in swimming performance. Furthermore, the model appears to work equally well for fishes from the Great Barrier Reef and the Caribbean, and for families with different adult habitat associations and swimming modes. It remains to be determined how well the model predicts the swimming abilities of temperate species as well as adults of these same species. This model provides an invaluable means of predicting swimming abilities of pre-settlement juvenile fishes that are unable to be reared in the laboratory, do not perform well in swimming flumes or are unable to be captured live in the field.

Mechanisms responsible for the enhanced pumping capacity of the in situ winter flounder heart (Pseudopleuronectes americanus)

In situ Starling and power output curves and in vitro pressure-volume curves were determined for winter flounder hearts, as well as the hearts of two other teleosts (Atlantic salmon and cod). In situ maximum cardiac output was not different between the three species (approximately 62 ml.min(-1).kg(-1)). However, because of the small size of the flounder heart, maximum stroke volume per milliliter per gram ventricle was significantly greater (2.3) compared with cod (1.7) and salmon (1.4) and is the highest reported for teleosts. The maximum power output of the flounder heart (7.6 mW/g) was significantly lower than that measured in the salmon (9.7) and similar to the cod (7.8) but was achieved at a much lower output pressure (4.9 vs. 8.0 and 6.2 kPa, respectively). Although the flounder heart could not perform resting levels of cardiac function at subambient pressures, it was much more sensitive to filling pressure, a finding supported by pressure-volume curves, which showed that the flounder's heart chambers were more compliant. Finally, we report that the flounder's bulbus:ventricle mass ratio (0.59) was significantly higher than in the cod (0.37) and salmon (0.22). These data, which support previous studies suggesting that the flatfish cardiovascular system is a high-volume, low-pressure design, show that vis-à-fronte filling is not important in flatfish, and that some fish can achieve high levels of cardiac output by vis-à-tergo filling alone; and suggest that a large compliant bulbus assists the flounder heart in delivering extremely large stroke volumes at pressures that do not become limiting.

Thermal sensitivity of metabolic rates and swimming performance in two latitudinally separated populations of cod, Gadus morhua L

Atlantic cod populations live in a wide thermal range and can differ genetically and physiologically. Thermal sensitivity of metabolic capacity and swimming performance may vary along a latitudinal gradient, to facilitate performance in distinct thermal environments. To evaluate this hypothesis, we compared the thermal sensitivity of performance in two cod stocks from the Northwest Atlantic that differ in their thermal experience: Gulf of St Lawrence (GSL) and Bay of Fundy (BF). We first compared the metabolic, physiological and swimming performance after short-term thermal change to that at the acclimation temperature (7 degrees C) for one stock (GSL), before comparing the performance of the two stocks after short-term thermal change. For cod from GSL, standard metabolism (SMR) increased with temperature, while active metabolism (AMR, measured in the critical swimming tests), EMR (metabolic rate after an exhaustive chase protocol), aerobic scope (AS) and critical swimming speeds (U (crit) and U (b-c)) were lower at 3 degrees C than 7 or 11 degrees C. In contrast, anaerobic swimming (sprint and burst-coasts in U (crit) test) was lower at 11 than 7 or 3 degrees C. Factorial AS (AMR SMR(-1)) decreased as temperature rose. Time to exhaustion (chase protocol) was not influenced by temperature. The two stocks differed little in the thermal sensitivities of metabolism or swimming. GSL cod had a higher SMR than BF cod despite similar AMR and AS. This led factorial AS to be significantly higher for the southern stock. Despite these metabolic differences, cod from the two stocks did not differ in their U (crit) speeds. BF cod were better sprinters at both temperatures. Cod from GSL had a lower aerobic cost of swimming at intermediate speeds than those from BF, particularly at low temperature. Only the activity of cytochrome C oxidase (CCO) in white muscle differed between stocks. No enzymatic correlates were found for swimming capacities, but oxygen consumption was best correlated with CCO activity in the ventricle for both stocks. Overall, the stocks differed in their cost of maintenance, cost of transport and sprint capacity, while maintaining comparable thermal sensitivities.

Cardiovascular and haematological responses of Atlantic cod (Gadus morhua) to acute temperature increase

For fish to survive large acute temperature increases (i.e. >10.0 degrees C) that may bring them close to their critical thermal maximum (CTM), oxygen uptake at the gills and distribution by the cardiovascular system must increase to match tissue oxygen demand. To examine the effects of an acute temperature increase ( approximately 1.7 degrees C h(-1) to CTM) on the cardiorespiratory physiology of Atlantic cod, we (1) carried out respirometry on 10.0 degrees C acclimated fish, while simultaneously measuring in vivo cardiac parameters using Transonic probes, and (2) constructed in vitro oxygen binding curves on whole blood from 7.0 degrees C acclimated cod at a range of temperatures. Both cardiac output (Q) and heart rate (fh) increased until near the fish's CTM (22.2+/-0.2 degrees C), and then declined rapidly. Q(10) values for Q and fh were 2.48 and 2.12, respectively, and increases in both parameters were tightly correlated with O(2) consumption. The haemoglobin (Hb)-oxygen binding curve at 24.0 degrees C showed pronounced downward and rightward shifts compared to 20.0 degrees C and 7.0 degrees C, indicating that both binding capacity and affinity decreased. Further, Hb levels were lower at 24.0 degrees C than at 20.0 degrees C and 7.0 degrees C. This was likely to be due to cell swelling, as electrophoresis of Hb samples did not suggest protein denaturation, and at 24.0 degrees C Hb samples showed peak absorbance at the expected wavelength (540 nm). Our results show that cardiac function is unlikely to limit metabolic rate in Atlantic cod from Newfoundland until close to their CTM, and we suggest that decreased blood oxygen binding capacity may contribute to the plateau in oxygen consumption.

Activation of a sensorimotor pathway in response to a water temperature drop in a teleost fish

When common carp, Cyprinus carpio L., experience a rapid temperature drop, the cerebral blood volume is strongly reduced to dampen the temperature drop in the brain. Simultaneously, the preoptic area and pituitary gland are activated to launch whole-body adaptive responses. However, the preferred reaction of fish to a temperature change is an escape reaction,which implies activation of a sensorimotor pathway. Here, we used blood oxygenation level-dependent (BOLD)- and cerebral blood volume (CBV)-weighted functional magnetic resonance imaging (fMRI) to identify a sensorimotor pathway, during a 10°C temperature drop in common carp. Transient activation was observed in the region where the sensory root of the trigeminal nerve enters the brain, and in the valvula cerebelli. In both regions,metabolic activity increased (increased deoxyhemoglobin content demonstrated by a decreased BOLD signal) within 30 s after the onset of the temperature drop, peaked after 2-3 min, and then decreased, even though the temperature continued to drop for another 2 min. These brain structures appear to respond to temperature change, rather than to the absolute temperature. Thus, during a temperature drop, the sensorimotor pathway consisting of the trigeminal nerve,the primary sensory trigeminal nucleus, the valvula cerebelli and some motornuclei, is active, in line with perception of temperature change in the buccal cavity, leading to motor activity for escape. This pathway operates in parallel to an acclimation pathway, which involves the preoptic area to pituitary gland pathway.

Postexercise physiology and repeat performance behaviour of free-swimming smallmouth bass in an experimental raceway

We studied postexercise physiology and behaviour of smallmouth bass (Micropterus dolomieu) that voluntarily ascended experimental raceways of varying length (20-50 m) against water velocities ranging from 8 to 120 cm/s. Our first objective was to link mean swimming speed to metabolism using patterns in postexercise muscle glycogen, muscle lactate, and plasma lactate. Our second objective was to examine several behavioural indices (attempt rate, success rate, and recovery time between an ascent and a subsequent attempt) and determine whether patterns in these data reflected those from the physiological measurements. Postexercise muscle glycogen and plasma lactate data suggest that smallmouth bass powered swimming speeds up to 70-80 cm/s using energy from aerobic processes. However, lactate did not begin to accumulate in the white muscle until speeds in excess of 120-130 cm/s were reached. The behavioural parameters measured did not indicate the presence of a physiological threshold at 70-80 cm/s; however, patterns in all factors changed appreciably when fish maintained speeds in excess of 120-130 cm/s. Therefore, it is clear that behaviour and physiology are tightly linked in this species and that maximum aerobic swimming capacity may not limit performance (or re-performance) during short-duration swims.

Factorial Aerobic Scope Is Independent of Temperature and Primarily Modulated by Heart Rate in Exercising Murray Cod (Maccullochella peelii peelii)

Several previous reports, often from studies utilising heavily instrumented animals, have indicated that for teleosts, the increase in cardiac output (Vb) during exercise is mainly the result of an increase in cardiac stroke volume (V(S)) rather than in heart rate (fH). More recently, this contention has been questioned following studies on animals carrying less instrumentation, though the debate continues. In an attempt to shed more light on the situation, we examined the heart rates and oxygen consumption rates (Mo2; normalised to a mass of 1 kg, given as Mo2kg) of six Murray cod (Maccullochella peelii peelii; mean mass+/-SE = 1.81+/-0.14 kg) equipped with implanted fH and body temperature data loggers. Data were determined during exposure to varying temperatures and swimming speeds to encompass the majority of the biological scope of this species. An increase in body temperature (Tb) from 14 degrees C to 29 degrees C resulted in linear increases in Mo2kg (26.67-41.78 micromol min(-1) kg(-1)) and fH (22.3-60.8 beats min(-1)) during routine exercise but a decrease in the oxygen pulse (the amount of oxygen extracted per heartbeat; 1.28-0.74 micromol beat(-1) kg(-1)). During maximum exercise, the factorial increase in Mo2kg was calculated to be 3.7 at all temperatures and was the result of temperature-independent 2.2- and 1.7-fold increases in fH and oxygen pulse, respectively. The constant factorial increases in fH and oxygen pulse suggest that the cardiovascular variables of the Murray cod have temperature-independent maximum gains that contribute to maximal oxygen transport during exercise. At the expense of a larger factorial aerobic scope at an optimal temperature, as has been reported for species of salmon and trout, it is possible that the Murray cod has evolved a lower, but temperature-independent, factorial aerobic scope as an adaptation to the largely fluctuating and unpredictable thermal climate of southeastern Australia.

Brain Responses to Ambient Temperature Fluctuations in Fish: Reduction of Blood Volume and Initiation of a Whole-Body Stress Response

Spatial and temporal ambient temperature variations directly influence cellular biochemistry and thus the physiology of ectotherms. However, many aquatic ectothermic species maintain coordinated sensorimotor function during large acute body-temperature changes, which points to a compensatory mechanism within the neural system. Here we used high-resolution functional magnetic resonance imaging to study brain responses to a drop of 10°C of ambient water temperature in common carp. We observed a strong drainage of blood out of the brain as of 90 s after the onset of the temperature drop, which would be expected to reduce entry of cold blood arriving from the gills so that the change in brain temperature would be slower. Although oxygen content in the brain thus decreased, we still found specific activation in the preoptic area (involved in temperature detection and stress responses), the pituitary pars distalis (stress response), and inactivation of the anterior part of the midbrain tegmentum and the pituitary pars intermedia. We propose that the blood drainage from the brain slows down the cooling of the brain during an acute temperature drop. This could help to maintain proper brain functioning including sensorimotor activity, initiation of the stress response, and the subsequent behavioral responses.

Oxygen limitation of thermal tolerance in cod,Gadus morhuaL., studied by magnetic resonance imaging and on-line venous oxygen monitoring

The hypothesis of an oxygen-limited thermal tolerance due to restrictions in cardiovascular performance at extreme temperatures was tested in Atlantic cod, Gadus morhua (North Sea). Heart rate, changes in arterial and venous blood flow, and venous oxygen tensions were determined during an acute temperature change to define pejus (“getting worse”) temperatures that border the thermal optimum range. An exponential increase in heart rate occurred between 2 and 16°C (Q10= 2.38 ± 0.35). Thermal sensitivity was reduced beyond 16°C when cardiac arrhythmia became visible. Flow-weighted magnetic resonance imaging (MRI) measurements of temperature-dependent blood flow revealed no exponential but a hyperbolic increase of blood flow with a moderate linear increase at temperatures >4°C. Therefore, temperature-dependent heart rate increments are not mirrored by similar increments in blood flow. Venous Po2(PvO2), which reflects the quality of oxygen supply to the heart of cod (no coronary circulation present), followed an inverse U-shaped curve with highest PvO2levels at 5.0 ± 0.2°C. Thermal limitation of circulatory performance in cod set in below 2°C and beyond 7°C, respectively, characterized by decreased PvO2. Further warming led to a sharp drop in PvO2beyond 16.1 ± 1.2°C in accordance with the onset of cardiac arrhythmia and, likely, the critical temperature. In conclusion, progressive cooling or warming brings cod from a temperature range of optimum cardiac performance into a pejus range, when aerobic scope falls before critical temperatures are reached. These patterns might cause a shift in the geographical distribution of cod with global warming.

Environmental hypoxia does not constrain the diurnal depth distribution of free-swimming Nautilus pompilius

The behaviour of Nautilus pompilius swimming freely in a controlled mesocosm (tower tank, 4 m diameter x 10.5 m deep) was monitored using ultrasonic depth telemetry. Initially depths were monitored in water equilibrated with air. Then the bottom 3.5 m were rendered hypoxic (Po(2) <20 mmHg) and depths monitored again. A thermocline at 7-m depth (17 degrees C below, 20 degrees C above) prevented mixing with the top, normoxic water. Mean depth was significantly greater during the light phase (8.9 m) of the 12L : 12D photoperiod than the dark phase (5.6 m), but this was not affected by hypoxia. During the light phase animals preferred the bottom 2.5 m of the tank but showed no specific preference for any depth range during the dark phase. Hypoxia did not alter these patterns of depth preference, though one animal made regular excursions toward normoxic water during the light phase. Vertical swimming activity was almost twofold greater during the dark phase and was not affected by hypoxia. These data suggest that, at least over the short term, Nautilus are not constrained from entering areas with low dissolved oxygen. This hypoxia tolerance may be attributed to the large onboard oxygen stores and suppressed metabolism during hypoxia.