Effects of food availability on temporal activity patterns and growth of Atlantic salmon

Activity strategy and pattern of the Siberian jerboa (Orientallactaga sibirica) in the Alxa desert region, China

Rodents exhibit seasonal changes in their activity patterns as an essential survival strategy. We studied the activity patterns and strategies of the Siberian jerboa (Orientallactaga sibirica) in the Alxa desert region to better understand the habitats and behavioural ecology of xeric rodents. We conducted an experiment using three plots to monitor the duration, time, and frequency of the active period of the Siberian jerboa using infrared cameras in the Alxa field workstation, Inner Mongolia, China in 2017. The relationships between the activity time and frequency, biological factors (perceived predation risk, food resources, and species composition), and abiotic factors (temperature, air moisture, wind speed) were analysed using Redundancy Analysis (RDA). Our results showed that: (1) relative humidity mainly affected activities in the springtime; temperature, relative humidity and interspecific competition mainly affected activities in the summertime; relative humidity and perceived predation risk mainly influenced activities in the autumn. (2) The activity pattern of the Siberian jerboa altered depending on the season. The activity of the Siberian jerboa was found to be bimodal in spring and summer, and was trimodal in autumn. The activity time and frequency in autumn were significantly lower than the spring. (3) Animals possess the ability to integrate disparate sources of information about danger to optimize energy gain. The jerboa adapted different responses to predation risks and competition in different seasons according to the demand for food resources.

Contingent trade-off decisions with feedbacks in cyclical environments: testing alternative theories

Many animals make contingent decisions, such as when and where to feed, as trade-offs between growth and risk when these vary not only with activity and location but also 1) in cycles such as the daily light cycle and 2) with feedbacks due to competition. Theory can assume an individual decides whether and where to feed, at any point in the light cycle and under any new conditions, by predicting future conditions and maximizing an approximate measure of future fitness. We develop four such theories for stream trout and evaluate them by their ability to reproduce, in an individual-based model, seven patterns observed in real trout. The patterns concern how feeding in four circadian phases—dawn, day, dusk, and night—varies with predation risk, food availability, temperature, trout density, physical habitat, day length, and circadian cycles in food availability. We found that theory must consider the full circadian cycle: decisions at one phase must consider what happens in other phases. Three theories that do so could reproduce almost all the patterns, and their ability to let individuals adapt decisions over time produced higher average fitness than any fixed behavior cycle. Because individuals could adapt by selecting among habitat patches as well as activity, multiple behaviors produced similar fitness. Our most successful theories base selection of habitat and activity at each phase on memory of survival probabilities and growth rates experienced 1) in the three previous phases of the current day or 2) in each phase of several previous days.

State-dependent behavior and alternative behavioral strategies in brown trout (Salmo trutta L.) fry

ABSTRACT

Animals generally adjust their behavior in response to bodily state (e.g., size and energy reserves) to optimize energy intake in relation to mortality risk, weighing predation probability against the risk of starvation. Here, we investigated whether brown trout Salmo trutta adjust their behavior in relation to energetic status and body size during a major early-life selection bottleneck, when fast growth is important. Over two consecutive time periods (P1 and P2; 12 and 23 days, respectively), food availability was manipulated, using four different combinations of high (H) and low (L) rations (i.e., HH, HL, LH, and LL; first and second letter denoting ration during P1 and P2, respectively). Social effects were excluded through individual isolation. Following the treatment periods, fish in the HL treatment were on average 15-21 % more active than the other groups in a forced open-field test, but large within-treatment variation provided only weak statistical support for this effect. Furthermore, fish on L-ration during P2 tended to be more actively aggressive towards their mirror image than fish on H-ration. Body size was related to behavioral expression, with larger fish being more active and aggressive. Swimming activity and active aggression were positively correlated, forming a behavioral syndrome in the studied population. Based on these behavioral traits, we could also distinguish two behavioral clusters: one consisting of more active and aggressive individuals and the other consisting of less active and aggressive individuals. This indicates that brown trout fry adopt distinct behavioral strategies early in life.

SIGNIFICANCE STATEMENT

This paper provides information on the state-dependence of behavior in animals, in particular young brown trout. On the one hand, our data suggest a weak energetic state feedback where activity and aggression is increased as a response to short term food restriction. This suggests a limited scope for behavioral alterations in the face of starvation. On the other hand, body size is linked to higher activity and aggression, likely as a positive feedback between size and dominance. The experiment was carried out during the main population survival bottleneck, and the results indicate that growth is important during this stage, as 1) behavioral compensation to increase growth is limited, and 2) growth likely increases the competitive ability. However, our data also suggests that the population separates into two clusters, based on combined scores of activity and aggression (which are positively linked within individuals). Thus, apart from an active and aggressive strategy, there seems to be another more passive behavioral strategy.

Density-dependent diel activity in stream-dwelling Arctic charr Salvelinus alpinus

Intraspecific competition plays a significant role in shaping how animals use and share habitats in space and time. However, the way individuals may modify their diel activity in response to increased competition has received limited attention. We used juvenile (age 1+) Arctic charr Salvelinus alpinus to test the prediction that individuals at high population density are more active and distribute their foraging activity over a greater portion of the 24-h cycle than individuals at low population density. Individually tagged fish were stocked in seminatural stream enclosures at low (2 fish/m(2)) and high (6 fish/m(2)) density. During each of two 2-week experimental rounds, activity of all fish within each enclosure was recorded every 3 h over seven 24-h cycles. At high density, fish were more active and distributed their activity over a greater portion of the 24-h cycle, with increased activity particularly at crepuscular times. Fluctuations in ecological conditions (e.g., water temperature and light intensity) also affected activity. Fish at high density grew as fast as fish at low density. This study demonstrates that individuals exhibit a degree of behavioral flexibility in their response to changes in ecological conditions and suggests that intraspecific competition can cause animals to modify temporal aspects of their activity to gain access to resources and maintain growth.

Importance of fish behaviour in modelling conservation problems: food limitation as an example

Simulation experiments using the inSTREAM individual-based brown trout Salmo trutta population model explored the role of individual adaptive behaviour in food limitation, as an example of how behaviour can affect managers' understanding of conservation problems. The model includes many natural complexities in habitat (spatial and temporal variation in characteristics such as depth and velocity, temperature, hiding and feeding cover, drift-food supply and predation risk), fish physiology (especially, how food intake and growth vary with hydrodynamics, cover, fish size and temperature) and behaviour. When drift-food concentration was increased over a wide range in 7 year simulations, the simulated population was always food limited. In fact, as food supply increased, the population increased at an increasing rate and consumed a higher percentage of the food supply, apparently because higher food concentrations make more stream area energetically profitable for drift feeders. The behaviour most responsible for this response was activity selection: when food was abundant, fish chose to feed less frequently and more nocturnally, thereby reducing predation mortality so more fish survived longer. These results indicate that the traditional concept of food limitation, that food availability stops limiting population size when it exceeds some threshold level, may not be useful and can be misleading. Results also strongly contradict the concept that a salmonid population is not food limited if the total food supply is greater than the population's consumption. Explicit consideration of adaptive behaviour produced a novel but believable understanding of food effects on salmonid populations. Published 2011. This article is a U.S. Government work and is in the public domain in the USA.

Seasonal shift in the effect of predators on juvenile Atlantic salmon (Salmo salar) energetics

Predator effects on prey populations are determined by the number of prey consumed and effects on the traits of surviving prey. Yet, effects of predators on prey traits are rarely evaluated in field studies. We measured the effects of predators on energetic traits (consumption and growth rates) of juvenile Atlantic salmon (Salmo salar) in a large-scale field study. Salmon fry were released at 18 sites that encompassed a wide range in abundance of predatory slimy sculpin (Cottus cognatus). We sampled salmon after 21 and 140 days to measure salmon growth and estimate consumption using a mass-balance model of methylmercury accumulation. Salmon population density was reduced fivefold at sites with abundant sculpin. Over the early season, salmon consumed less where sculpin were abundant, suggesting that reduced foraging under predation risk contributed to predator-caused mortality. In contrast, over the late season, salmon grew more where sculpin were abundant, suggesting that compensatory growth at reduced salmon population density moderated predator-caused mortality. Predator effects on prey energetics can drive variation in survival and growth, with important consequences for population dynamics.

Photic induction of locomotor activity is correlated with photic habitat in Anolis lizards

A variety of ecologically important behaviors, including circadian rhythms and seasonal reproduction, are influenced by non-visual responses to light, yet very little is known about the relationship between photic habitat and non-visual photoreception. Puerto Rican Anolis lizards have diverged into multiple photic niches, making them a good model for non-visual photosensory ecology. We investigated the photic induction of locomotor activity, a non-visual response to light, in four species of Anolis comprising two pairs of closely related, ecomorphologically similar species whose microhabitats differ in solar irradiance. We developed a device for continuous, automated detection and recording of anole locomotor activity, and used it to characterize activity under 12:12 h light-dark cycles. Next, we administered a series of 2-h light pulses during the dark period of the light-dark cycle and measured the increase in locomotor activity relative to baseline dark activity. Five different irradiances (ranging from very dim to daytime levels) were given to each individual lizard on separate nights. As expected, light caused an irradiance-dependent increase in locomotor activity in all four species. The responses at the highest irradiances were significantly greater in species occupying relatively more shaded habitats, suggesting that non-visual photoreception may be adapted to habitat light in Anolis lizards.

The spatial scale of competition from recruits on an older cohort in Atlantic salmon

Competitive effects of younger cohorts on older ones are frequently assumed to be negligible in species where older, larger individuals dominate in pairwise behavioural interactions. Here, we provide field estimates of such competition by recruits on an older age class in Atlantic salmon (Salmo salar), a species where observational studies have documented strong body size advantages which should favour older individuals in direct interactions. By creating realistic levels of spatial variation in the density of underyearling (YOY) recruits over a 1-km stretch of a stream, and obtaining accurate measurements of individual growth rates of overyearlings (parr) from capture–mark–recapture data on a fine spatial scale, we demonstrate that high YOY density can substantially decrease parr growth. Models integrating multiple spatial scales indicated that parr were influenced by YOY density within 16 m. The preferred model suggested parr daily mass increase to be reduced by 39% when increasing YOY density from 0.0 to 1.0 m⁻², which is well within the range of naturally occurring densities. Reduced juvenile growth rates will in general be expected to reduce juvenile survival (via increased length of exposure to freshwater mortality) and increase generation times (via increased age at seaward migrations). Thus, increased recruitment can significantly affect the performance of older cohorts, with important implications for population dynamics. Our results highlight that, even for the wide range of organisms that rely on defendable resources, the direction of competition among age classes cannot be assumed a priori or be inferred from behavioural observations alone.

Interactions between riparian shading and food supply: a seasonal comparison of effects on time budgets, space use and growth in Atlantic salmon Salmo salar

This study examines seasonal (winter v. summer) differences in space-time budgets, food intake and growth of Atlantic salmon Salmo salar parr in a controlled, large-scale stream environment, to examine the direction and magnitude of shifts in behaviour patterns as influenced by the availability of overhead cover and food supply. Salmo salar parr tested in the presence of overhead cover were significantly more nocturnal and occupied more peripheral positions than those tested in the absence of overhead cover. This increase in nocturnal activity was driven primarily by increased activity at night, accompanied by a reduction in daytime activity during winter. The presence of overhead cover had no effect on rates of food intake or growth for a given food supply in a given season. Growth rates were significantly higher for fish subjected to a high food supply than those subjected to a low food supply. Food supply did not affect the extent to which S. salar parr were nocturnal. These results were consistent between winter and summer. The use of riparian shading as a management technique to mitigate the effects of warming allows the adoption of more risk-averse foraging behaviour and may be particularly beneficial in circumstances where it serves also to increase the availability of food.

Bioaccumulation syndrome: identifying factors that make some stream food webs prone to elevated mercury bioaccumulation

Mercury is a ubiquitous contaminant in aquatic ecosystems, posing a significant health risk to humans and wildlife that eat fish. Mercury accumulates in aquatic food webs as methylmercury (MeHg), a particularly toxic and persistent organic mercury compound. While mercury in the environment originates largely from anthropogenic activities, MeHg accumulation in freshwater aquatic food webs is not a simple function of local or regional mercury pollution inputs. Studies show that even sites with similar mercury inputs can produce fish with mercury concentrations ranging over an order of magnitude. While much of the foundational work to identify the drivers of variation in mercury accumulation has focused on freshwater lakes, mercury contamination in stream ecosystems is emerging as an important research area. Here, we review recent research on mercury accumulation in stream-dwelling organisms. Taking a hierarchical approach, we identify a suite of characteristics of individual consumers, food webs, streams, watersheds, and regions that are consistently associated with elevated MeHg concentrations in stream fish. We delineate a conceptual, mechanistic basis for explaining the ecological processes that underlie this vulnerability to MeHg. Key factors, including suppressed individual growth of consumers, low rates of primary and secondary production, hydrologic connection to methylation sites (e.g., wetlands), heavily forested catchments, and acidification are frequently associated with increased MeHg concentrations in fish across both streams and lakes. Hence, we propose that these interacting factors define a syndrome of characteristics that drive high MeHg production and bioaccumulation rates across these freshwater aquatic ecosystems. Finally, based on an understanding of the ecological drivers of MeHg accumulation, we identify situations when anthropogenic effects and management practices could significantly exacerbate or ameliorate MeHg accumulation in stream fish.

Size-dependent survival of brook trout Salvelinus fontinalis in summer: effects of water temperature and stream flow

A 5 year individual-based data set was used to estimate size-specific survival rates in a wild brook trout Salvelinus fontinalis population in a stream network encompassing a mainstem and three tributaries (1.5-6 m wetted width), western Massachusetts, U.S.A. The relationships between survival in summer and temperature and flow metrics derived from continuous monitoring data were then tested. Increased summer temperatures significantly reduced summer survival rates for S. fontinalis in almost all size classes in all four sites throughout the network. In contrast, extreme low summer flows reduced survival of large fish, but only in small tributaries, and had no significant effects on fish in smaller size classes in any location. These results provide direct evidence of a link between season-specific survival and environmental factors likely to be affected by climate change and have important consequences for the management of both habitats and populations.

Effects of discharge and local density on the growth of juvenile Atlantic salmon Salmo salar

The study explored the combined effects of density, physical habitat and different discharge levels on the growth of juvenile Atlantic salmon Salmo salar in artificial streams, by manipulating flow during both summer and winter conditions. Growth was high during all four summer trials and increased linearly with discharge and mean velocity. Differences in fish densities (fish m(-3)) due to differences in stream volume explained a similar proportion of the variation in mean growth among discharge treatments. Within streams, the fish aggregated in areas of larger sediment size, where shelters were probably abundant, while growth decreased with increasing densities. Fish appeared to favour the availability of shelter over maximization of growth. Mean growth was negative during all winter trials and did not vary among discharge treatments. These results suggest that increased fish densities are a major cause of reduced summer growth at low discharge, and that habitat-mediated density differences explain the majority of the growth variation across habitat conditions both during summer and winter.

Sensory complement model helps to predict diel alarm response patterns in juvenile Atlantic salmon (Salmo salar) under natural conditions

Fish rely on both chemical and visual cues to evaluate predation risk. Decisions with respect to activity partitioning in time (i.e., night vs. day) rely on accurate assessment of predation risk relative to energy intake; predation risk is generally thought to be lower at night at the expense of feeding opportunities. At night, the sensory complement model predicts greater reliance on chemical perception of risk. Under this condition, a lower ability to use vision should result in a more conservative response to chemical cues than during the day. We tested this hypothesis under natural conditions by comparing the alarm response of young-of-the-year Atlantic salmon ( Salmo salar L., 1758) under summer day and night conditions in salmon nursery streams. We found that salmon responded to the alarm cues to a significantly greater extent at night. This suggests that the sensory complement model may be correct and that nocturnal perception of risk may be generally higher than previously believed for juvenile salmon in the wild. In the absence of a more precise indicator of risk (e.g., vision), a greater reliance on chemosensory risk assessment at night may cause fish to shift to more risk-adverse behaviour.

Rapid, efficient growth reduces mercury concentrations in stream-dwelling Atlantic salmon

Mercury (Hg) is a potent toxin that biomagnifies in aquatic food webs. Large fish generally have higher Hg concentrations than small fish of the same species. However, models predict that fish that grow large faster should have lower Hg concentrations than small, slow-growing fish due to somatic growth dilution (SGD). We examined the relationship between Hg concentrations and growth rate in fish using a large-scale field experiment. Atlantic salmon (Salmo salar) fry hatched under uniform initial conditions were released at eighteen sites in natural streams, collected after one growing season, and Hg concentration and growth measured. As expected for Hg accumulation from food, mercury concentrations in salmon tracked Hg concentrations in their prey. Nonetheless, large, fast-growing salmon had lower Hg concentrations than small, slow-growing salmon, consistent with SGD. While prey Hg concentration accounted for 59% of the explained variation in salmon Hg concentration across sites, salmon growth rate accounted for 38% of the explained variation independent of prey Hg concentration. A mass-balance Hg accumulation model shows that such SGD occurs when fast growth is associated with high growth efficiency. Fish growth is tremendously variable and sensitive to anthropogenic impacts, so SGD of Hg has important implications for fisheries management.

Mechanisms for climate-induced mortality of fish populations in whole-lake experiments

The effects of climate change on plant and animal populations are widespread and documented for many species in many areas of the world. However, projections of climate impacts will require a better mechanistic understanding of ecological and behavioral responses to climate change and climate variation. For vertebrate animals, there is an absence of whole-system manipulative experiments that express natural variation in predator and prey behaviors. Here we investigate the effect of elevated water temperature on the physiology, behavior, growth, and survival of fish populations in a multiple whole-lake experiment, by using 17 lake-years of data collected over 2 years with differing average temperatures. We found that elevated temperatures in excess of the optimum reduced the scope for growth through reduced maximum consumption and increased metabolism in young rainbow trout, Oncorhynchus mykiss. Increased metabolism at high temperatures resulted in increased feeding activity (consumption) by individuals to compensate and maintain growth rates similar to that observed at cooler (optimum) temperatures. However, greater feeding activity rates resulted in greater vulnerability to predators that reduced survival to only half that of the cooler year. Our work therefore identifies temperature-dependent physiology and compensatory feeding behavior as proximate mechanisms for substantial climate-induced mortality in fish populations at the scale of entire populations and waterbodies.