Energetics, cost reduction and functional consequences of fish morphology

The role of predation risk in structuring life-history traits of crucian carp (Carassius carassius) in a series of small boreal lakes

Predation is a major evolutionary force determining life-history traits in prey by direct and indirect mechanisms. This study focuses on life-history trait variation in crucian carp (Carassius carassius), a species well known for developing a deep body as an inducible morphological defence against predation risk. Here, the authors tested variation in growth and reproductive traits in 15 crucian carp populations in lakes along a predation risk gradient represented by increasingly efficient predator communities. Lakes were located in south-eastern Norway and were sampled in summer 2018 and 2019. The authors expected crucian carp to attain higher growth rate, larger size, and later age at maturity with increasing predation risk. In the absence of predators, they expected high adult mortality, early maturity and increased reproductive effort caused by strong intraspecific competition. They found that the life-history traits of crucian carp were clearly related to the presence of piscivores: with increasing predation risk, fish grew in body length and depth and attained larger asymptotic length and size at maturity. This growth was evident at young age, especially in productive lakes with pike, and it suggests that fish quickly outgrew the predation window by reaching a size refuge. Contrary to the authors' predictions, populations had similar age at maturity. High-predation lakes also presented low density of crucian carp. This suggests that fish from predator lakes may experience high levels of resource availability due to reduced intraspecific competition. Predation regulated life-history traits in crucian carp populations, where larger size, higher longevity and size at maturity were observed in lakes with large gaped predators.

Variations in cost of transport and their ecological consequences: a review

Movement is essential in the ecology of most animals, and it typically consumes a large proportion of individual energy budgets. Environmental conditions modulate the energetic cost of movement (cost of transport, COT), and there are pronounced differences in COT between individuals within species and across species. Differences in morphology affect COT, but the physiological mechanisms underlying variation in COT remain unresolved. Candidates include mitochondrial efficiency and the efficiency of muscle contraction-relaxation dynamics. Animals can offset increased COT behaviourally by adjusting movement rate and habitat selection. Here, we review the theory underlying COT and the impact of environmental changes on COT. Increasing temperatures, in particular, increase COT and its variability between individuals. Thermal acclimation and exercise can affect COT, but this is not consistent across taxa. Anthropogenic pollutants can increase COT, although few chemical pollutants have been investigated. Ecologically, COT may modify the allocation of energy to different fitness-related functions, and thereby influence fitness of individuals, and the dynamics of animal groups and communities. Future research should consider the effects of multiple stressors on COT, including a broader range of pollutants, the underlying mechanisms of COT and experimental quantifications of potential COT-induced allocation trade-offs.

Lotic Environment Affects Morphological Characteristics and Energy Metabolism of Juvenile Grass Carp Ctenopharyngodon idella

This study investigated the effect of a lotic environment on morphological characteristics and energy metabolism in juvenile grass carp Ctenopharyngodon idella. The fish were stocked in the lotic environment and forced to swim for 12 h per day for 4 weeks at three water current velocities of 0.5, 2, and 4 body length s−1 (Bl s−1). The control fish were stocked in the lentic environment with water current velocities of 0 Bl s−1. The results showed that lotic environment significantly increased body weight, body length, and condition factor of grass carp. The first principal component (PC1) characterized by measured overall body size suggested that fish in a lotic environment had body stoutness and wider tail stalk. Standard metabolic rate (SMR), maximum metabolic rate (MMR), and aerobic swimming performance (Ucrit) were elevated with the increased water flow and positively correlated with PC1. The 4 Bl s−1 group showed significantly decreased contents of serum glucose and muscular glycogen, and a significantly increased level of serum lactic acid. The mRNA expression levels of AMP-activated protein kinase-phosphorylate PPAR γ coactivator 1 α-nuclear respiratory factor 1 (AMPK-PGC1α-NRF1) pathway-related genes were significantly upregulated in red muscle of grass carp in the lotic environment. Water flow environment at 4 Bl s−1 significantly increased ratios of metabolic enzymes (lactate dehydrogenase/citrate synthase) and cytochrome c oxidase/citrate synthase) in the muscle. The relationship between morphological characteristics and metabolic capacity suggested that the body size of grass carp in a lotic environment was shaped to promote energy metabolism. The study identified the evidence of the mechanism and relationship of the trade-off between energy and morphology in grass carp.

Drone-Based Tracking of the Fine-Scale Movement of a Coastal Stingray (Bathytoshia brevicaudata)

Coastal ecosystems are under threat from a range of anthropogenic impacts that disrupt habitat connectivity and the ability for animals to move within them. Understanding fine-scale animal movement provides insight into how animals are responding to these pressures, and underpins effective ecological management and conservation strategies. This study used drones to investigate the drivers of the fine-scale movement of rays in coastal estuaries using the short-tail stingray (Bathytoshia brevicaudata) as a model species. Smaller rays swam with more regular bursts of speed and greater sinuosity than larger individuals, indicating that rays of different sizes alter their fine-scale movement behavior to maintain energetic efficiency. Rays were less likely to spend time resting and swam faster on the high tide compared to the outgoing tide. They were also more likely to exhibit bursts of speed at noon (11 am to 1 pm) than at other times of day. Body size, tide and time of day all influenced ray movement. Understanding the ecological variables that influence the fine-scale movement of rays and the potential for human activities to alter natural behaviors is integral to the implementation of effective management strategies for this group of animals and their ecosystems.

Size-related effects and the influence of metabolic traits and morphology on swimming performance in fish

Energy metabolism fuels swimming and other biological processes. We compared the swimming performance and energy metabolism within and across eight freshwater fish species. Using swim tunnel respirometers, we measured the standard metabolic rate (SMR) and maximum metabolic rate (MMR) and calculated the critical swimming speed (U_crit). We accounted for body size, metabolic traits, and some morphometric ratios in an effort to understand the extent and underlying causes of variation. Body mass was largely the best predictor of swimming capacity and metabolic traits within species. Moreover, we found that predictive models using total length or SMR, in addition to body mass, significantly increased the explained variation of U_crit and MMR in certain fish species. These predictive models also underlined that, once body mass has been accounted for, U_crit can be independently affected by total length or MMR. This study exemplifies the utility of multiple regression models to assess within-species variability. At interspecific level, our results showed that variation in U_crit can partly be explained by the variation in the interrelated traits of MMR, fineness, and muscle ratios. Among the species studied, bleak Alburnus alburnus performed best in terms of swimming performance and efficiency. By contrast, pumpkinseed Lepomis gibbosus showed very poor swimming performance, but attained lower mass-specific cost of transport (MCOT) than some rheophilic species, possibly reflecting a cost reduction strategy to compensate for hydrodynamic disadvantages. In conclusion, this study provides insight into the key factors influencing the swimming performance of fish at both intra- and interspecific levels.

Behavioral and physiological adaptations to high-flow velocities in chubs (Gila spp.) native to Southwestern USA

Morphological streamlining is often associated with physiological advantages for steady swimming in fishes. Though most commonly studied in pelagic fishes, streamlining also occurs in fishes that occupy high-flow environments. Before the installation of dams and water diversions, bonytail (Cyprinidae, Gila elegans), a fish endemic to the Colorado River (USA), regularly experienced massive, seasonal flooding events. Individuals of G. elegans display morphological characteristics that may facilitate swimming in high-flow conditions, including a narrow caudal peduncle and a high aspect ratio caudal fin. We tested the hypothesis that these features improve sustained swimming performance in bonytail by comparing locomotor performance in G. elegans with that of the closely related roundtail chub (Gila robusta) and two non-native species, rainbow trout (Oncorhynchus mykiss) and smallmouth bass (Micropterus dolomieu), using a Brett-style respirometer and locomotor step-tests. Gila elegans had the lowest estimated drag coefficient and the highest sustained swimming speeds relative to the other three species. There were no detectible differences in locomotor energetics during steady swimming among the four species. When challenged by high-velocity water flows, the second native species examined in this study, G. robusta, exploited the boundary effects in the flow tank by pitching forward and bracing the pelvic and pectoral fins against the acrylic tank bottom to 'hold station'. Because G. robusta can station hold to prevent being swept downstream during high flows and G. elegans can maintain swimming speeds greater than those of smallmouth bass and rainbow trout with comparable metabolic costs, we suggest that management agencies could use artificial flooding events to wash non-native competitors downstream and out of the Colorado River habitat.

Why do placentas evolve? Evidence for a morphological advantage during pregnancy in live-bearing fish

A live-bearing reproductive strategy can induce large morphological changes in the mother during pregnancy. The evolution of the placenta in swimming animals involves a shift in the timing of maternal provisioning from pre-fertilization (females supply their eggs with sufficient yolk reserves prior to fertilization) to post-fertilization (females provide all nutrients via a placenta during the pregnancy). It has been hypothesised that this shift, associated with the evolution of the placenta, should confer a morphological advantage to the females leading to a more slender body shape during the early stages of pregnancy. We tested this hypothesis by quantifying three-dimensional shape and volume changes during pregnancy and in full-grown virgin controls of two species within the live-bearing fish family Poeciliidae: Poeciliopsis gracilis (non-placental) and Poeciliopsis turneri (placental). We show that P. turneri is more slender than P. gracilis at the beginning of the interbrood interval and in virgins, and that these differences diminish towards the end of pregnancy. This study provides the first evidence for an adaptive morphological advantage of the placenta in live-bearing fish. A similar morphological benefit could drive the evolution of placentas in other live-bearing (swimming) animal lineages.

Physiological mechanisms underlying animal social behaviour

Many species of animal live in groups, and the group represents the organizational level within which ecological and evolutionary processes occur. Understanding these processes, therefore, relies on knowledge of the mechanisms that permit or constrain group formation. We suggest that physiological capacities and differences in physiology between individuals modify fission-fusion dynamics. Differences between individuals in locomotor capacity and metabolism may lead to fission of groups and sorting of individuals into groups with similar physiological phenotypes. Environmental impacts such as hypoxia can influence maximum group sizes and structure in fish schools by altering access to oxygenated water. The nutritional environment determines group cohesion, and the increase in information collected by the group means that individuals should rely more on social information and form more cohesive groups in uncertain environments. Changing environmental contexts require rapid responses by individuals to maintain group coordination, which are mediated by neuroendocrine signalling systems such as nonapeptides and steroid hormones. Brain processing capacity may constrain social complexity by limiting information processing. Failure to evaluate socially relevant information correctly limits social interactions, which is seen, for example, in autism. Hence, functioning of a group relies to a large extent on the perception and appropriate processing of signals from conspecifics. Many if not all physiological systems are mechanistically linked, and therefore have synergistic effects on social behaviour. A challenge for the future lies in understanding these interactive effects, which will improve understanding of group dynamics, particularly in changing environments.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

Genetic composition of a population of natural common bream Abramis brama × roach Rutilus rutilus hybrids and their morphological characteristics in comparison with parent species

Common bream Abramis brama, roach Rutilus rutilus and their hybrids were collected in the Dobczyce Reservoir in southern Poland in 2006-2013 to study whether it is better for a hybrid individual to resemble and compete with one of its parents, or to minimize competition by having a distinctive phenotype. All hybrids were F₁ crosses and originated predominantly (93·2%) from matings between female A. brama and male R. rutilus parents. In morphometric analyses, a newly defined coefficient, L₃  = 2·5 (body mass) (L_S  × body depth)^-1 , which enables forms with similar length-depth proportions but different length-mass relationships to be distinguished was used. Morphometric and meristic characteristics of the hybrids were intermediate in comparison with the parental species, with small but significant deviation towards R. rutilus in longitudinal body dimensions (trunk and tail length) and towards A. brama in body cross-sectional shape (body depth and L₃ coefficient). This may result in a more R. rutilus like propulsion in hybrids and a more A. brama like ability to manoeuvre.

Short-term feed and light deprivation reduces voluntary activity but improves swimming performance in rainbow trout Oncorhynchus mykiss

Rainbow trout Oncorhynchus mykiss (~ 180 g, 16 °C and < 5 kg m^-3) that were feed deprived and kept in total darkness showed a significant increase in critical swimming speed (U _crit) between 1 and 12 days of deprivation (from 3.35 to 4.46 body length (BL) s^-1) with no increase in maximum metabolic rate (MMR). They also showed a significant decrease in the estimated metabolic rate at 0 BL s^-1 over 12 days which leads to a higher factorial aerobic metabolic scope at day 12 (9.38) compared to day 1 (6.54). Routine metabolic rates were also measured in ~ 90 g rainbow trout that were swimming freely in large circular respirometers at 16 °C. These showed decreasing consumption oxygen rates and reductions in the amount of oxygen consumed above standard metabolic rate (a proxy for spontaneous activity) over 12 days, though this happened significantly faster when they were kept in total darkness when compared to a 12:12-h light-dark (LD) photoperiod. Weight loss during this period was also significantly reduced in total darkness (3.33% compared to 4.98% total body weight over 12 days). Immunological assays did not reveal any consistent up- or downregulation of antipathogenic and antioxidant enzymes in the serum or skin mucus of rainbow trout between 1 and 12 days of feed and light deprivation. Overall, short periods of deprivation do not appear to significantly affect the performance of rainbow trout which appear to employ a behavioural energy-sparing strategy, albeit more so in darkness than under a 12:12-h LD regime.

Energetic cost determines voluntary movement speed only in familiar environments

Locomotor performance is closely related to fitness. However, in many ecological contexts, animals do not move at their maximal locomotor capacity, but adopt a voluntary speed that is lower than maximal. It is important to understand the mechanisms that underlie voluntary speed, because these determine movement patterns of animals across natural environments. We show that voluntary speed is a stable trait in zebrafish (Danio rerio), but there were pronounced differences between individuals in maximal sustained speed, voluntary speed and metabolic cost of locomotion. We accept the hypothesis that voluntary speed scales positively with maximal sustained swimming performance (Ucrit), but only in unfamiliar environments (1st minute in an open-field arena versus 10th minute) at high temperature (30°C). There was no significant effect of metabolic scope on Ucrit Contrary to expectation, we rejected the hypothesis that voluntary speed decreases with increasing metabolic cost of movement, except in familiar spatial (after 10 min of exploration) and thermal (24°C but not 18 or 30°C) environments. The implications of these data are that the energetic costs of exploration and dispersal in novel environments are higher than those for movement within familiar home ranges.

Comparison of swimming capacity and energetics of migratory European eel (Anguilla anguilla) and New Zealand short-finned eel (A. australis)

The spawning migration of the European eel (Anguilla anguilla) can cover more than 6000 km, while that of the New Zealand short-finned eel (A. australis) is assumed to be approximately 3000 km. Since these species are expected to show adaptive traits to such an important lifetime event, we hypothesized differences in swimming capacity and energetics as a response to this adaptation. In an experimental swimming respirometer set-up, critical swimming speed (Ucrit), optimal swimming speed (Uopt), mass specific oxygen consumption rate (ṀO2), standard metabolic rate (SMR), active metabolic rate at Ucrit (AMRcrit) and at Uopt (AMRopt), the minimum cost of transport at Uopt (COTmin), and the scope for activity, were assessed and compared between the species. With a similar body length and mass, European eels showed ca. 25% higher values for both Ucrit and Uopt, and 23% lower values for COTmin, compared to New Zealand short-finned eels. However, SMR, AMRcrit, AMRopt, and scope for activity did not differ between the species, indicating very similar swimming physiology traits. This study discusses physiological aspects of long distance migration and provides recommendations for (a) swimming respirometry in anguilliform fish, and (b) telemetry research using externally attached pop-up tags.

Physiological Trade-Offs Along a Fast-Slow Lifestyle Continuum in Fishes: What Do They Tell Us about Resistance and Resilience to Hypoxia?

It has recently been suggested that general rules of change in ecological communities might be found through the development of functional relationships between species traits and performance. The physiological, behavioural and life-history traits of fishes are often organised along a fast-slow lifestyle continuum (FSLC). With respect to resistance (capacity for population to resist change) and resilience (capacity for population to recover from change) to environmental hypoxia, the literature suggests that traits enhancing resilience may come at the expense of traits promoting resistance to hypoxia; a trade-off may exist. Here I test whether three fishes occupying different positions along the FSLC trade-off resistance and resilience to environmental hypoxia. Static respirometry experiments were used to determine resistance, as measured by critical oxygen tension (Pcrit), and capacity for (RC) and magnitude of metabolic reduction (RM). Swimming respirometry experiments were used to determine aspects of resilience: critical (Ucrit) and optimal swimming speed (Uopt), and optimal cost of transport (COTopt). Results pertaining to metabolic reduction suggest a resistance gradient across species described by the inequality Melanotaenia fluviatilis (fast lifestyle) < Hypseleotris sp. (intermediate lifestyle) < Mogurnda adspersa (slow lifestyle). The Ucrit and COTopt data suggest a resilience gradient described by the reverse inequality, and so the experiments generally indicate that three fishes occupying different positions on the FSLC trade-off resistance and resilience to hypoxia. However, the scope of inferences that can be drawn from an individual study is narrow, and so steps towards general, trait-based rules of fish community change along environmental gradients are discussed.

Streamwise vortices destabilize swimming bluegill sunfish (Lepomis macrochirus)

In their natural environment, fish must swim stably through unsteady flows and vortices, including vertical vortices, typically shed by posts in a flow, horizontal cross-flow vortices, often produced by a step or a waterfall in a stream, and streamwise vortices, where the axis of rotation is aligned with the direction of the flow. Streamwise vortices are commonly shed by bluff bodies in streams and by ships' propellers and axial turbines, but we know little about their effects on fish. Here, we describe how bluegill sunfish use more energy and are destabilized more often in flow with strong streamwise vorticity. The vortices were created inside a sealed flow tank by an array of four turbines with similar diameter to the experimental fish. We measured oxygen consumption for seven sunfish swimming at 1.5 body lengths (BL) s−1 with the turbines rotating at 2 Hz and with the turbines off (control). Simultaneously, we filmed the fish ventrally and recorded the fraction of time spent maneuvering side-to-side and accelerating forward. Separately, we also recorded lateral and ventral video for a combination of swimming speeds (0.5, 1.5 and 2.5 BL s−1) and turbine speeds (0, 1, 2 and 3 Hz), immediately after turning the turbines on and 10 min later to test for accommodation. Bluegill sunfish are negatively affected by streamwise vorticity. Spills (loss of heading), maneuvers and accelerations were more frequent when the turbines were on than in the control treatment. These unsteady behaviors, particularly acceleration, correlated with an increase in oxygen consumption in the vortex flow. Bluegill sunfish are generally fast to recover from roll perturbations and do so by moving their pectoral fins. The frequency of spills decreased after the turbines had run for 10 min, but was still markedly higher than in the control, showing that fish partially adapt to streamwise vorticity, but not completely. Coping with streamwise vorticity may be an important energetic cost for stream fishes or migratory fishes.

Accommodating the cost of growth and swimming in fish-the applicability of exercise-induced growth to juvenile hapuku (Polyprion oxygeneios)

Induced-swimming can improve the growth and feed conversion efficiency of finfish aquaculture species, such as salmonids and Seriola sp., but some species, such as Atlantic cod, show no or a negative productivity response to exercise. As a possible explanation for these species-specific differences, a recent hypothesis proposed that the applicability of exercise training, as well as the exercise regime for optimal growth gain (ERopt growth), was dependent upon the size of available aerobic metabolic scope (AMS). This study aimed to test this hypothesis by measuring the growth and swimming metabolism of hapuku, Polyprion oxygeneios, to different exercise regimes and then reconciling the metabolic costs of swimming and specific dynamic action (SDA) against AMS. Two 8-week growth trials were conducted with ERs of 0.0, 0.25, 0.5, 0.75, 1, and 1.5 body lengths per second (BL s(-1)). Fish in the first trial showed a modest 4.8% increase in SGR over static controls in the region 0.5-0.75 BL s(-1) whereas the fish in trial 2 showed no significant effect of ER on growth performance. Reconciling the SDA of hapuku with the metabolic costs of swimming showed that hapuku AMS is sufficient to support growth and swimming at all ERs. The current study therefore suggests that exercise-induced growth is independent of AMS and is driven by other factors.

Predator-driven intra-species variation in locomotion, metabolism and water velocity preference in pale chub (Zacco platypus) along a river

Fish inhabit environments that vary greatly in terms of predation intensity, and these predation regimes are generally expected to be a major driver of divergent natural selection. To test whether there is predator-driven intra-species variation in the locomotion, metabolism and water velocity preference of pale chub (Zacco platypus) along a river, we measured unsteady and steady swimming and water velocity preference among fish collected from both high- and low-predation habitats in the Wujiang River. We also measured the routine metabolic rate (RMR), maximum metabolic rate (MMR) and cost of transport (COT) and calculated the optimal swimming speed (Uopt). The fish from the high-predation populations showed a shorter response latency, elevated routine metabolism, lower swimming efficiency at low swimming speed and lower water velocity preference compared with those from the low-predation populations. Neither of the kinematic parameters fast-start and critical swimming speed (Ucrit) showed a significant difference between the high- and low-predation populations. The fish from the high-predation populations may improve their predator avoidance capacity primarily through an elevated routine metabolism and shorter response latency to achieve advanced warning and escape, rather than an improved fast-start swimming speed or acceleration. Thus, the cost of this strategy is an elevated RMR, and no trade-off between unsteady and steady swimming performance was observed in the pale chub population under various predation stresses. It was interesting to find that the high-predation fish showed an unexpected lower velocity preference, which might represent a compromise between predation avoidance, foraging and energy saving.

The effect of drag and attachment site of external tags on swimming eels: experimental quantification and evaluation tool

Telemetry studies on aquatic animals often use external tags to monitor migration patterns and help to inform conservation effort. However, external tags are known to impair swimming energetics dramatically in a variety of species, including the endangered European eel. Due to their high swimming efficiency, anguilliform swimmers are very susceptibility for added drag. Using an integration of swimming physiology, behaviour and kinematics, we investigated the effect of additional drag and site of externally attached tags on swimming mode and costs. The results show a significant effect of a) attachment site and b) drag on multiple energetic parameters, such as Cost Of Transport (COT), critical swimming speed (U_crit) and optimal swimming speed (U_opt), possibly due to changes in swimming kinematics. Attachment at 0.125 bl from the tip of the snout is a better choice than at the Centre Of Mass (0.35 bl), as it is the case in current telemetry studies. Quantification of added drag effect on COT and U_crit show a (limited) correlation, suggesting that the U_crit test can be used for evaluating external tags for telemetry studies until a certain threshold value. U_opt is not affected by added drag, validating previous findings of telemetry studies. The integrative methodology and the evaluation tool presented here can be used for the design of new studies using external telemetry tags, and the (re-) evaluation of relevant studies on anguilliform swimmers.

Integrating water flow, locomotor performance and respiration of Chinese sturgeon during multiple fatigue-recovery cycles

The objective of this study is to provide information on metabolic changes occurring in Chinese sturgeon (an ecologically important endangered fish) subjected to repeated cycles of fatigue and recovery and the effect on swimming capability. Fatigue-recovery cycles likely occur when fish are moving through the fishways of large dams and the results of this investigation are important for fishway design and conservation of wild Chinese sturgeon populations. A series of four stepped velocity tests were carried out successively in a Steffensen-type swimming respirometer and the effects of repeated fatigue-recovery on swimming capability and metabolism were measured. Significant results include: (1) critical swimming speed decreased from 4.34 bl/s to 2.98 bl/s; (2) active oxygen consumption (i.e. the difference between total oxygen consumption and routine oxygen consumption) decreased from 1175 mgO₂/kg to 341 mgO₂/kg and was the primary reason for the decrease in U_crit; (3) excess post-exercise oxygen consumption decreased from 36 mgO₂/kg to 22 mgO₂/kg; (4) with repeated step tests, white muscle (anaerobic metabolism) began contributing to propulsion at lower swimming speeds. Therefore, Chinese sturgeon conserve energy by swimming efficiently and have high fatigue recovery capability. These results contribute to our understanding of the physiology of the Chinese sturgeon and support the conservation efforts of wild populations of this important species.

Body fineness ratio as a predictor of maximum prolonged-swimming speed in coral reef fishes

The ability to sustain high swimming speeds is believed to be an important factor affecting resource acquisition in fishes. While we have gained insights into how fin morphology and motion influences swimming performance in coral reef fishes, the role of other traits, such as body shape, remains poorly understood. We explore the ability of two mechanistic models of the causal relationship between body fineness ratio and endurance swimming-performance to predict maximum prolonged-swimming speed (Umax ) among 84 fish species from the Great Barrier Reef, Australia. A drag model, based on semi-empirical data on the drag of rigid, submerged bodies of revolution, was applied to species that employ pectoral-fin propulsion with a rigid body at U max. An alternative model, based on the results of computer simulations of optimal shape in self-propelled undulating bodies, was applied to the species that swim by body-caudal-fin propulsion at Umax . For pectoral-fin swimmers, Umax increased with fineness, and the rate of increase decreased with fineness, as predicted by the drag model. While the mechanistic and statistical models of the relationship between fineness and Umax were very similar, the mechanistic (and statistical) model explained only a small fraction of the variance in Umax . For body-caudal-fin swimmers, we found a non-linear relationship between fineness and Umax , which was largely negative over most of the range of fineness. This pattern fails to support either predictions from the computational models or standard functional interpretations of body shape variation in fishes. Our results suggest that the widespread hypothesis that a more optimal fineness increases endurance-swimming performance via reduced drag should be limited to fishes that swim with rigid bodies.

Lactate kinetics of rainbow trout during graded exercise: do catheters affect the cost of transport?

Changes in lactate kinetics as a function of exercise intensity have never been measured in an ectotherm. Continuous infusion of a tracer is necessary to quantify rates of lactate appearance (Ra) and disposal (Rd), but it requires double catheterization, which could interfere with swimming. Using rainbow trout, our goals were to: (1) determine the potential effects of catheters and blood sampling on metabolic rate (O2), total cost of transport (TCOT), net cost of transport (NCOT) and critical swimming speed (Ucrit), and (2) monitor changes in lactate fluxes during prolonged, steady-state swimming or graded swimming from rest to Ucrit. This athletic species maintains high baseline lactate fluxes of 24 μmol kg(-1) min(-1) that are only increased at intensities >2.4 body lengths (BL) s(-1) or 85% Ucrit. As the fish reaches Ucrit, Ra is more strongly stimulated (+67% to 40.4 μmol kg(-1) min(-1)) than Rd (+41% to 34.7 μmol kg(-1) min(-1)), causing a fourfold increase in blood lactate concentration. Without this stimulation of Rd during intense swimming, lactate accumulation would double. By contrast, steady-state exercise at 1.7 BL s(-1) increases lactate fluxes to ~30 μmol kg(-1) min(-1), with a trivial mismatch between Ra and Rd that only affects blood concentration minimally. Results also show that the catheterizations and blood sampling needed to measure metabolite kinetics in exercising fish have no significant impact on O2 or TCOT. However, these experimental procedures affect locomotion energetics by increasing NCOT at high speeds and by decreasing Ucrit.

Phenotypic and genetic divergence within a single whitefish form - detecting the potential for future divergence

Human-induced nutrient input can change the selection regime and lead to the loss of biodiversity. For example, eutrophication caused speciation reversal in polymorphic whitefish populations through a flattening of littoral–pelagic selection gradients. We investigated the current state of phenotypic and genetic diversity in whitefish (Coregonus macrophthalmus) in a newly restored lake whose nutrient load has returned to pre-eutrophication levels and found that whitefish spawning at different depths varied phenotypically and genetically: individuals spawning at shallower depth had fewer gill rakers, faster growth, and a morphology adapted to benthic feeding, and they showed higher degrees of diet specialization than deeper spawning individuals. Microsatellite analyses complemented the phenotype analyses by demonstrating reproductive isolation along different spawning depths. Our results indicate that whitefish still retain or currently regain phenotypic and genetic diversity, which was lost during eutrophication. Hence, the population documented here has a potential for future divergence because natural selection can target phenotypes specialized along re-established littoral–pelagic selection gradients. The biodiversity, however, will have better chances to return if managers acknowledge the evolutionary potential within the local whitefish and adapt fishing and stocking measures.

Indirect trophic interactions with an invasive species affect phenotypic divergence in a top consumer

While phenotypic responses to direct species interactions are well studied, we know little about the consequences of indirect interactions for phenotypic divergence. In this study we used lakes with and without the zebra mussel to investigate effects of indirect trophic interactions on phenotypic divergence between littoral and pelagic perch. We found a greater phenotypic divergence between littoral and pelagic individuals in lakes with zebra mussels and propose a mussel-mediated increase in pelagic and benthic resource availability as a major factor underlying this divergence. Lakes with zebra mussels contained higher densities of large plankton taxa and large invertebrates. We suggest that this augmented resource availability improved perch foraging opportunities in both the littoral and pelagic zones. Perch in both habitats could hence express a more specialized foraging morphology, leading to an increased divergence of perch forms in lakes with zebra mussels. As perch do not prey on mussels directly, we conclude that the increased divergence results from indirect interactions with the mussels. Our results hence suggest that species at lower food web levels can indirectly affect phenotypic divergence in species at the top of the food chain.

Habitat-specific locomotor variation among Chinese hook snout carp (Opsariichthys bidens) along a river

The Wujiang River is a tributary of the upper Yangtze River that shows great variations in its flow regime and habitat condition. Dams have been built along the Wujiang River and have altered the habitats profoundly enough that they may give rise to reproductive isolation. To test whether the swimming performance and morphology of the Chinese hook snout carp (Opsariichthys bidens), varied among habitats and whether the possible differences had a genetic basis, we measured the steady and unsteady swimming performance, external body shape and genetic distance among fish collected from both the main and tributary streams of the upper, middle and lower reaches along the river. We also measured the routine energy expenditure (RMR), maximum metabolic rate (MMR), cost of transport (COT) and calculated the optimal swimming speed. The steady swimming capacity, RMR, MMR and optimal swimming speed were all higher and the COT was lower in the upper reach or tributary streams compared with the lower reach or main stream. However, unsteady swimming performance showed no variation among collecting sites. Flow regimes as suggested by river slope and water velocity were positively correlated with steady swimming performance but not with unsteady swimming performance. Predation stress were significantly related with body morphology and hence energy cost during swimming but not U(crit) value. The fish from only one population (Hao-Kou) showed relatively high genetic differentiation compared with the other populations. Fish from the upper reach or tributary streams exhibited improved steady swimming performance through improved respiratory capacity and lower energy expenditure during swimming at the cost of higher maintenance metabolism. There was no correlation between the steady and unsteady swimming performance at either the population or the individual levels. These results suggest that a trade-off between steady and unsteady swimming does not occur in O. bidens.

Comparative metabolic rates of common western North Atlantic Ocean sciaenid fishes

The resting metabolic rates (R(R)) of western North Atlantic Ocean sciaenids, such as Atlantic croaker Micropogonias undulatus, spot Leiostomus xanthurus and kingfishes Menticirrhus spp., as well as the active metabolic rates (R(A)) of M. undulatus and L. xanthurus were investigated to facilitate inter and intraspecific comparisons of their energetic ecology. The R(R) of M. undulatus and L. xanthurus were typical for fishes with similar lifestyles. The R(R) of Menticirrhus spp. were elevated relative to those of M. undulatus and L. xanthurus, but below those of high-energy-demand species such as tunas Thunnus spp. and dolphinfish Coryphaena hippurus. Repeated-measures non-linear mixed-effects models were applied to account for within-individual autocorrelation and corrected for non-constant variance typical of noisy R(A) data sets. Repeated-measures models incorporating autoregressive first-order [AR(1)] and autoregressive moving average (ARMA) covariances provided significantly superior fits, more precise parameter estimates (i.e. reduced s.e.) and y-intercept estimates that more closely approximated measured R(R) for M. undulatus and L. xanthurus than standard least-squares regression procedures.

Morphological differences in parr of Atlantic salmon Salmo salar from three regions in Norway

Morphological characters were compared in parr (total length 33-166 mm) of Atlantic salmon Salmo salar sampled from eight wild populations in three regions, three in northern, two in the middle and three in southern Norway, covering a distance of 1700 km (from 70° N to 58° N). On the basis of morphological characters 94·6% of the individuals were correctly classified into the three regions. Discrimination between populations within these three regions also had a high degree of correct classification (89·0-95·8%). Principle component analysis identified largest differences to be in head characters, notably eye diameter and jawbone, with the smallest diameter and head size among the northernmost populations. Fish from the southern rivers had a deeper body form whereas fish from the middle region had larger heads and pectoral fins. This illustrates that S. salar already in the early parr stage has morphological traits, which can be used in discrimination between regions and populations and that these differences are discernible in spite of the volume of escaped farmed fish spawning in Norwegian rivers during the past 30 years.

Hydrodynamics-based functional forms of activity metabolism: a case for the power-law polynomial function in animal swimming energetics

The first-degree power-law polynomial function is frequently used to describe activity metabolism for steady swimming animals. This function has been used in hydrodynamics-based metabolic studies to evaluate important parameters of energetic costs, such as the standard metabolic rate and the drag power indices. In theory, however, the power-law polynomial function of any degree greater than one can be used to describe activity metabolism for steady swimming animals. In fact, activity metabolism has been described by the conventional exponential function and the cubic polynomial function, although only the power-law polynomial function models drag power since it conforms to hydrodynamic laws. Consequently, the first-degree power-law polynomial function yields incorrect parameter values of energetic costs if activity metabolism is governed by the power-law polynomial function of any degree greater than one. This issue is important in bioenergetics because correct comparisons of energetic costs among different steady swimming animals cannot be made unless the degree of the power-law polynomial function derives from activity metabolism. In other words, a hydrodynamics-based functional form of activity metabolism is a power-law polynomial function of any degree greater than or equal to one. Therefore, the degree of the power-law polynomial function should be treated as a parameter, not as a constant. This new treatment not only conforms to hydrodynamic laws, but also ensures correct comparisons of energetic costs among different steady swimming animals. Furthermore, the exponential power-law function, which is a new hydrodynamics-based functional form of activity metabolism, is a special case of the power-law polynomial function. Hence, the link between the hydrodynamics of steady swimming and the exponential-based metabolic model is defined.

Optimal swimming speed in head currents and effects on distance movement of winter-migrating fish

Migration is a commonly described phenomenon in nature that is often caused by spatial and temporal differences in habitat quality. However, as migration requires energy, the timing of migration may depend not only on differences in habitat quality, but also on temporal variation in migration costs. Such variation can, for instance, arise from changes in wind or current velocity for migrating birds and fish, respectively. Whereas behavioural responses of birds to such changing environmental conditions have been relatively well described, this is not the case for fish, although fish migrations are both ecologically and economically important. We here use passive and active telemetry to study how winter migrating roach regulate swimming speed and distance travelled per day in response to variations in head current velocity. Furthermore, we provide theoretical predictions on optimal swimming speeds in head currents and relate these to our empirical results. We show that fish migrate farther on days with low current velocity, but travel at a greater ground speed on days with high current velocity. The latter result agrees with our predictions on optimal swimming speed in head currents, but disagrees with previously reported predictions suggesting that fish ground speed should not change with head current velocity. We suggest that this difference is due to different assumptions on fish swimming energetics. We conclude that fish are able to adjust both swimming speed and timing of swimming activity during migration to changes in head current velocity in order to minimize energy use.

Mechanical and energetic factors underlying gait transitions in bluegill sunfish (Lepomis macrochirus)

As their swimming speed increased, bluegill sunfish (Lepomis macrochirus) switched from pectoral-fin-powered labriform swimming to undulations of the body axis. This gait transition occurred at a mean swimming speed of 0.24±0.01 m s–1 and a pectoral fin beat frequency of 2.79±0.11 Hz (mean ± s.e.m., N=6). The power output available from the main upstroke (adductor profundus) and downstroke (abductor superficialis) muscles, measured using the work-loop technique was maximal at the gait transition point. The cost of transport,measured by respirometry, increased as the fish switched from labriform to undulatory swimming. Our data show that bluegill changed gait as swimming speed increased to recruit additional muscle mass, rather than to maximize economy, as is the case for many terrestrial animals.

Efficiency of labriform swimming in the bluegill sunfish (Lepomis macrochirus)

Bluegill sunfish (Lepomis macrochirus) swim in the labriform mode at low speeds, generating lift and thrust by beating their pectoral fins. The maximal power output available from the two largest pectoral fin adductor and abductor muscles, constituting half of the total pectoral girdle muscle mass, was measured in vitro and used to estimate the muscle mechanical power output during maximal labriform swimming (Pmech; 0.15-0.21 W kg(-1) body mass). Respirometry was used to estimate the total metabolic power input (Ptotal; 0.95 W kg(-1) body mass) and the metabolic power available to the active muscle mass (Pmuscle; Ptotal minus standard metabolic rate, 0.57 W kg(-1) body mass) at this swimming speed. Drag measurements made on towed, dead fish were used to estimate the mechanical power required to overcome body drag (Pdrag; 0.028 W kg(-1) body mass). Efficiency estimates based on these data fell into the following ranges: overall swimming efficiency (etagross=Pmech/Ptotal), 0.16-0.22; muscle efficiency (etamuscle=Pmech/Pmuscle), 0.26-0.37; and propeller efficiency (etaprop=Pdrag/Pmech), 0.15-0.20. Comparison with other studies suggests that labriform swimming may be more efficient than swimming powered by undulations of the body axis.

Effects of temperature, swimming speed and body mass on standard and active metabolic rate in vendace (Coregonus albula)

This study gives an integrated analysis of the effects of temperature, swimming speed and body mass on standard metabolism and aerobic swimming performance in vendace (Coregonus albula (L.)). The metabolic rate was investigated at 4, 8 and 15 degrees C using one flow-through respirometer and two intermittent-flow swim tunnels. We found that the standard metabolic rate (SMR), which increased significantly with temperature, accounted for up to 2/3 of the total swimming costs at optimum speed (U (opt)), although mean U (opt) was high, ranging from 2.0 to 2.8 body lengths per second. Net swimming costs increased with swimming speed, but showed no clear trend with temperature. The influence of body mass on the metabolic rate varied with temperature and activity level resulting in scaling exponents (b) of 0.71-0.94. A multivariate regression analysis was performed to integrate the effects of temperature, speed and mass (AMR = 0.82M (0.93) exp(0.07T) + 0.43M (0.93) U (2.03)). The regression analysis showed that temperature affects standard but not net active metabolic costs in this species. Further, we conclude that a low speed exponent, high optimum speeds and high ratios of standard to activity costs suggest a remarkably efficient swimming performance in vendace.

Swimming efficiency and the influence of morphology on swimming costs in fishes

Swimming performance is considered a main character determining survival in many aquatic animals. Body morphology highly influences the energetic costs and efficiency of swimming and sets general limits on a species capacity to use habitats and foods. For two cyprinid fishes with different morphological characteristics, carp (Cyprinus carpio L.) and roach (Rutilus rutilus (L.)), optimum swimming speeds (U(mc)) as well as total and net costs of transport (COT, NCOT) were determined to evaluate differences in their swimming efficiency. Costs of transport and optimum speeds proved to be allometric functions of fish mass. NCOT was higher but U(mc) was lower in carp, indicating a lower swimming efficiency compared to roach. The differences in swimming costs are attributed to the different ecological demands of the species and could partly be explained by their morphological characteristics. Body fineness ratios were used to quantify the influence of body shape on activity costs. This factor proved to be significantly different between the species, indicating a better streamlining in roach with values closer to the optimum body form for efficient swimming. Net swimming costs were directly related to fish morphology.