Individual behaviour and resource use of thermally stressed brook trout Salvelinus fontinalis portend the conservation potential of thermal refugia

Individual aggression and thermal refuge use were monitored in brook trout Salvelinus fontinalis in a controlled laboratory to determine how fish size and personality influence time spent in forage and thermal habitat patches during periods of thermal stress. On average, larger and more exploratory fish initiated more aggressive interactions and across all fish there was decreased aggression at warmer temperatures. Individual personality did not explain changes in aggression or habitat use with increased temperature; however, larger individuals initiated comparatively fewer aggressive interactions at warmer temperatures. Occupancy of forage patches generally declined as ambient stream temperatures approached critical maximum and fish increased thermal refuge use, with a steeper decline in forage patch occupancy observed in larger fish. These findings suggest that larger individuals may be more vulnerable to stream temperature rise. Importantly, even at thermally stressful temperatures, all fish periodically left the thermal refuge to forage. This indicates that the success of refugia at increasing population survival during periods of stream temperature rise may depend on the location of thermal refugia relative to forage locations within the larger habitat mosaic. These results provide insights into the potential for thermal refugia to improve population survival and can be used to inform predictions of population vulnerability to climate change.     

Local habitat conditions explain the variation in the strength of self‐thinning in a stream salmonid

Self-thinning patterns are frequently used to describe density dependence in populations on timescales shorter than the organism''s life span and have been used to infer carrying capacity of the environment. Among mobile animals, this concept has been used to document density dependence in stream salmonids, which compete over access to food and space. The carrying capacity, growth conditions, and initial cohort sizes often vary between streams and stream sections, which would influence the onset and strength of the density dependence. Despite much effort in describing habitat relationships in stream fishes, few studies have explicitly tested how the physical environment affects the slope of the thinning curves. Here, we investigate the prevalence and strength of self-thinning in juvenile stages of a steelhead (Oncorhynchus mykiss) population in Idaho, USA. Further, we investigate the roles of local physical habitat and metabolic constraints in explaining the variation in thinning curves among study sites in the watershed. Only yearling steelhead exhibited an overall significant thinning trend, but the slope of the mass–density relationship (−0.53) was shallower than predicted by theory and reported from empirical studies. There was no detectable relationship in subyearling steelhead. Certain abiotic factors explained a relatively large portion of the variation in the strength of the self-thinning among the study reaches. For subyearling steelhead, the slopes were negatively associated with the average water depth and flow velocity in the study sites, whereas slopes in yearlings were steeper in sites that incurred a higher metabolic cost. Our results show that the prevalence and strength of density dependence in natural fish populations can vary across heterogeneous watersheds and can be more pronounced during certain stages of a species'' life history, and that environmental factors can mediate the extent to which density dependence is manifested in predictable ways.     

Resource-mediated demographic variation during the midsummer succession of a cladoceran community

SUMMARY. 1. The midsummer succession of cladoceran zooplankton in Lake Texoma was examined by demographic characterization of lake populations and a parallel set of life table experiments in which the dominant cladoceran species were raised on naturally occurring lake phytoplankton at lake temperatures. Diaphanosoma leuchtenbergianum occurred throughout the study period (mid June-mid August 1981) but Daphnia parvula, D. galeata mendotae and Ceriodaphnia lacustris were rarely detected after 15 July.
2. Analyses of size structure and size-specific fecundity of the cladoceran populations showed that (1) size structure shifted to smaller individuals in mid July, (2) size-specific fecundity was relatively constant in the smaller size classes and was constant or declined in larger females of each species, and (3) size at maturation did not vary significantly during the study of any population.
3. Individual growth rates of the two Daphnia species and Ceriodaphnia in the life table experiments were significantly lower in mid July when populations were declining and when size structure of each population was more dominated by smaller Individuals. The reduction in individual growth rates was short-lived in the life table experiments; growth rates returned to pre-decline levels by the end of July.
4. Two effects of temporary reductions in individual growth rates when size-specific fecundity and size at maturation are constant are (1) that size structure is temporarily shifted to smaller individuals, and (2) the potential rate of population growth is reduced because of delays in reproduction. Because data on size structure and size-specific fecundity are commonly used to evaluate the relative effects of predators and resource changes on populations it is noteworthy that erroneous conclusions about the mechanism of population decline may be reached when individual growth rates vary.     

Prey size,prey abundance,and temperature as correlates of growth in stream populations of cutthroat trout

Growth and maximum size of stream fishes can be highly variable across populations. For salmonid fishes in streams, individuals from populations confined to headwater streams often exhibit small size at maturity in comparison to populations with access to main-stem rivers. Differences in prey size, prey availability, and metabolic constraints based on temperature may explain patterns of maximum size and growth. In this study, cutthroat trout from headwater stream populations that were isolated above a waterfall were compared to individuals from populations in similar sized streams without a movement barrier and from large main-stem rivers. Cutthroat trout from smaller streams with or without a movement barrier were significantly smaller at a given age than fish from main-stem rivers, where individuals were able to achieve a much larger maximum size. Comparisons of invertebrate drift abundance and size in the three types of streams revealed that drift size did not differ between stream categories, but was highest per volume of water in large main-stem rivers. Across all stream types, prey abundance declined from summer to fall. Temperature declined over the course of the season in a similar manner across all stream types, but remained relatively high later in the season in main-stem river habitats. Prey availability and temperature conditions in main-stem rivers may provide more optimal growing conditions for fish as individuals increase in size and become constrained by prey availability and temperature conditions in small streams. Maintaining connectivity between small spawning and rearing tributary streams and main-stem river habitats may be critical in maintaining large-bodied populations of stream salmonids.     

Context‐specific influence of water temperature on brook trout growth rates in the field

1. Modelling the effects of climate change on freshwater fishes requires robust field‐based estimates accounting for interactions among multiple factors. 2. We used data from an 8‐year individual‐based study of a wild brook trout (Salvelinus fontinalis) population to test the influence of water temperature on season‐specific growth in the context of variation in other environmental (i.e. season, stream flow) or biotic factors (local brook trout biomass density and fish age and size) in West Brook, a third‐order stream in western Massachusetts, U.S.A. 3. Changes in ambient temperature influenced individual growth rates. In general, higher temperatures were associated with higher growth rates in winter and spring and lower growth rates in summer and autumn. However, the effect of temperature on growth was strongly context‐dependent, differing in both magnitude and direction as a function of season, stream flow and fish biomass density. 4. We found that stream flow and temperature had strong and complex interactive effects on trout growth. At the coldest temperatures (in winter), high stream flows were associated with reduced trout growth rates. During spring and autumn and in typical summers (when water temperatures were close to growth optima), higher flows were associated with increased growth rates. In addition, the effect of flow at a given temperature (the flow‐temperature interaction) differed among seasons. 5. Trout density negatively affected growth rate and had strong interactions with temperature in two of four seasons (i.e. spring and summer) with greater negative effects at high temperatures. 6. Our study provided robust, integrative field‐based estimates of the effects of temperature on growth rates for a species which serves as a model organism for cold‐water adapted ectotherms facing the consequences of environmental change. Results of the study strongly suggest that failure to derive season‐specific estimates, or to explicitly consider interactions with flow regime and fish density, will seriously compromise our ability to predict the effects of climate change on stream fish growth rates. Further, the concordance we found between empirical observations and likely energetic mechanisms suggests that our general results should be relevant at broader spatial and temporal scales.     

Density-dependent mortality and the evolution of social interactions

Existing models of kin selection implicitly assume that the intensity of density-dependent mortality depends on the size or density of the entire deme. If individuals compete for locally limited resources, then mortality may also depend on local population density. In this paper, I derive conditions for evolutionary equilibrium when the fitness of an individual depends on (1) the phenotypes of a local group of conspecifics with which it interacts and (2) the size of the local group. In general, altruistic behaviours which reduce the individual fitness of the actor but increase that of the recipient are favoured towards individuals whose coefficient of relationship exceeds a threshold value. ‘Harming’ behaviours which reduce the individual fitness of both the actor and the recipient are favoured towards individuals below this threshold value. The magnitude of this threshold depends on the nature of the processes that cause density-dependent mortality and the average coefficient of relationship in the group.     

An experimental analysis of self-thinning in juvenile steelhead trout

Mobile animal populations have been proposed to decline in density according to a slope based on the allometry of metabolic requirements or space requirements. In salmonid fishes, metabolic rate and food consumption scale to body mass by the exponent 0.87 and 0.73, respectively; whereas the territory size of steelhead trout scales to body mass by the exponent 0.86. Experimental cohorts of juvenile steelhead trout ( Oncorhynchus mykiss ) were used to test the hypothesis that mobile animal populations composed of individuals with indeterminate growth decline in density as a result of self-thinning. After controlling for experimentally manipulated levels of food abundance and stocking density, cohorts of steelhead trout declined in density with increasing body size according to a slope closest to the allometry of food consumption. Densities of steelhead trout were inversely related to average mass by the exponent −0.74. Despite the similarity to the food consumption slope, a relatively wide confidence interval also precluded distinguishing the slope either the metabolic rate or territory size slopes. Data from the literature were also examined to determine if there was general support for the idea of self-thinning in natural populations of stream-dwelling salmonid fish. Although not all data suggest that populations of salmonids in streams decline as a result of density-dependent intraspecific competition, at least some appear to fit the idea of self-thinning; especially when density is above a minimum level of habitat saturation.     

Larger eggs at lower water temperature as a measure to assure effective hatchling size in the landlocked form of Ayu, <Emphasis Type="Italic">Plecoglossus altivelis</Emphasis>

Egg size is a critical life-history trait in which maternal investment is optimized to maximize maternal fitness. The adaptive significance of variable egg size among spawning groups of Ayu (Plecoglossus altivelis) landlocked in the Lake Biwa system was examined through field investigations and rearing experiments. Observed egg size variations were explained by the water temperature around spawning grounds established near the mouths of inlet streams. Two typical streams with different incubation temperatures showed similar maternal body sizes and hatchling sizes, but eggs attached to the stream bed were larger in the colder stream. An experiment that used eggs from a single clutch showed that a smaller hatchling size was obtained with a lower incubation temperature, indicating that the effect of differences in egg size on hatchling size can be canceled out by variations in incubation temperature. In general, larvae that are less than a certain threshold of effective body size are not expected to be assured of early success among conspecifics competing for foods. It is proposed that environments in which the incubation temperature varies favor variability in egg size to ensure that sufficient food is accessible to larvae.     

Responses to competition effects of two anuran tadpoles according to life-history traits

Experimental manipulations of the densities of two larval anurans, Pelodytes punctatus and Bufo bufo , showed that these species compete asymmetrically in semi-natural conditions. Growth, mass at metamorphosis, date of metamorphosis, and survival were used as measures of response to interspecific competition. A mechanistic approach was used to collect information on the behaviour of the two species in different conditions. The competitive superiority of Pelodytes at individual level was correlated with a larger body, faster growth rate, increased per capita competitive impact on conspecifics, and greater reduction in the availability of trophic and spatial resources. In the presence of Pelodytes, Bufo showed slower growth, smaller size at metamorphosis and reduced survival. In the interspecific treatments Bufo individuals modified their behaviour by increasing activity and use of the water column while Pelodytes did not change their foraging activity or space use in the aquaria. However, the presence of Bufo resulted in a reduced larval period and smaller size at metamorphosis. We hypothesise that the presence of Bufo act as a signal of environmental degradation and shorten the larval period of Pelodytes, a typical temporal pond breeder . The smaller Bufo tadpoles are potentially stronger competitors at population level because they use relatively large amounts of energy (greater densities and higher metabolic rates). Consequently, they use larger proportions of the shared resources than their larger competitor. A possible evolutionary response for larger tadpoles is the development of interference mechanisms or "escaping" from ephemeral ponds where mortality by drying represent a high risk.     

Home range, home range overlap, and species energy use among herbivorous mammals

Harestad & Bunnell (1979) showed that, at least for North American species, home ranges of large herbivorous mammals are relatively larger than we would expect on metabolic grounds, and suggested that the productivity of the environment for mammal species decreases with increasing body size. This interpretation assumes that the number of conspecifics that share an individual's home range is independent of body size. Data presented here show that this is not true for the species in their sample; the home range is shared with an increasing number of conspecifics in larger herbivore species. The productivity of the environment for a species is independent of body size and the area available to an individual for its own use scales approximately as do individual metabolic requirements. These results agree with conclusions based upon the scaling of population density with body mass and illustrate the interrelationship between home range and dietary and social organization trends among mammalian herbivores. Individual home range area is a function of the way in which the local population of a species, not merely an individual, exploits the environment.     

Thermal ecology of juvenile steelhead in a warm-water environment

How salmonids adjust to the cost of chronic exposure to warm water in nature, particularly in habitats where areas of cold water are unavailable, is largely unknown. In the hot dry climate of southern California, oversummering juvenile steelhead, Oncorhynchus mykiss, can experience a continual warm-water environment. Studies were performed in three streams (34°N) during the summers of 2002–2004 to assess how steelhead (10–28 cm total length) exploit the thermal environment and to develop an understanding of how existing preference and heat tolerance information reported for the species relate to the actual temperatures the local population experiences. Analyses of achievable body temperatures, an index of observed body temperatures, and behavioral time budgets, indicate steelhead did not behaviorally thermoregulate. Findings from associated field surveys suggest areas of cold water were not available. Steelhead held focal points at temperatures (17.4–24.8°C) that were at times higher than the temperature preferences and heat tolerances reported for the species. Oversummering juvenile steelhead accept an elevated body temperature and remain active and forage throughout the day, apparently as a means for coping with warm water at the southern extent of their range. The relatively high body temperatures that steelhead accept appear to represent a compromise in exchange for maintaining an expanded geographic (latitudinal) range.     

Bill size variation in northern cardinals associated with anthropogenic drivers across North America

Allen's rule predicts that homeotherms inhabiting cooler climates will have smaller appendages, while those inhabiting warmer climates will have larger appendages relative to body size. Birds’ bills tend to be larger at lower latitudes, but few studies have tested whether modern climate change and urbanization affect bill size. Our study explored whether bill size in a wide‐ranging bird would be larger in warmer, drier regions and increase with rising temperatures. Furthermore, we predicted that bill size would be larger in densely populated areas, due to urban heat island effects and the higher concentration of supplementary foods. Using measurements from 605 museum specimens, we explored the effects of climate and housing density on northern cardinal bill size over an 85‐year period across the Linnaean subspecies’ range. We quantified the geographic relationships between bill surface area, housing density, and minimum temperature using linear mixed effect models and geographically weighted regression. We then tested whether bill surface area changed due to housing density and temperature in three subregions (Chicago, IL., Washington, D.C., and Ithaca, NY). Across North America, cardinals occupying drier regions had larger bills, a pattern strongest in males. This relationship was mediated by temperature such that birds in warm, dry areas had larger bills than those in cool, dry areas. Over time, female cardinals’ bill size increased with warming temperatures in Washington, D.C., and Ithaca. Bill size was smaller in developed areas of Chicago, but larger in Washington, D.C., while there was no pattern in Ithaca, NY. We found that climate and urbanization were strongly associated with bill size for a wide‐ranging bird. These biogeographic relationships were characterized by sex‐specific differences, varying relationships with housing density, and geographic variability. It is likely that anthropogenic pressures will continue to influence species, potentially promoting microevolutionary changes over space and time.     

An asymmetric producer-scrounger game: body size and the social foraging behavior of coho salmon

A tension between cooperation and conflict characterizes the behavioral dynamics of many social species. The foraging benefits of group living include increased efficiency and reduced need for vigilance, but social foraging can also encourage theft of captured prey from conspecifics. The payoffs of stealing prey from others (scrounging) versus capturing prey (producing) may depend not only on the frequency of each foraging strategy in the group but also on an individual’s ability to steal. By observing the foraging behavior of juvenile coho salmon (Oncorhynchus kisutch), we found that, within a group, relatively smaller coho acted primarily as producers and took longer to handle prey, and were therefore more likely to be targeted by scroungers than relatively larger coho. Further, our observations suggest that the frequency of scrounging may be higher when groups contained individuals of different sizes. Based on these observations, we developed a model of phenotype-limited producer-scrounger dynamics, in which rates of stealing were structured by the relative size of producers and scroungers within the foraging group. Model simulations show that when the success of stealing is positively related to body size, relatively large predators should tend to be scroungers while smaller predators should be producers. Contrary to previous models, we also found that, under certain conditions, producer and scrounger strategies could coexist for both large and small phenotypes. Large scroungers tended to receive the highest payoff, suggesting that producer-scrounger dynamics may result in an uneven distribution of benefits among group members that—under the right conditions—could entrench social positions of dominance.     

Asymmetric competition in plant populations

Recently there has been much interest in the hypothesis that competition between individual plants is asymmetric or onesided: larger individuals obtain a disproportionate share of the resources (for their relative size) and suppress the growth of smaller individuals. This has important implications for population structure, for the analysis of competition between plants at the individual, population and community levels, and for our understanding of competition as a selective force in the evolution of plant populations.     

The combined effect of body size and temperature on oxygen consumption rates and the size-dependency of preferred temperature in European perch Perca fluviatilis

The present study determined the effect of body mass and acclimation temperature (15–28°C) on oxygen consumption rate (ṀO₂) and the size dependency of preferred temperature in European perch Perca fluviatilis. Standard metabolic rate (SMR) scaled allometrically with body mass by an exponent of 0.86, and temperature influenced SMR with a Q₁₀ of 1.9 regardless of size. Maximum metabolic rate (MMR) and aerobic scope (MMR-SMR) scaled allometrically with body mass by exponents of 0.75–0.88. The mass scaling exponents of MMR and aerobic scope changed with temperature and were lowest at the highest temperature. Consequently, the optimal temperature for aerobic scope decreased with increasing body mass. Notably, fish <40 g did not show a decrease aerobic scope with increasing temperature. Factorial aerobic scope (MMR × SMR⁻¹) generally decreased with increasing temperatures, was unaffected by size at the lower temperatures, and scaled negatively with body mass at the highest temperature. Similar to the optimal temperature for aerobic scope, preferred temperature declined with increasing body mass, unaffectedly by acclimation temperature. The present study indicates a limitation in the capacity for oxygen uptake in larger fish at high temperatures. A constraint in oxygen uptake at high temperature may restrict the growth of larger fish with environmental warming, at least if food availability is not limited. Furthermore, behavioural thermoregulation may be contributing to regional changes in the size distribution of fish in the wild caused by global warming as larger individuals will prefer colder water at higher latitudes and at larger depths than smaller conspecifics with increasing environmental temperatures.     

Interacting effects of temperature and density on individual growth performance in a wild population of brown trout

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Cooperative growth regulation in coral-dwelling fishes

Dominant individuals often grow faster than subordinates because they gain a greater share of important resources. However, dominants should also strategically adjust their growth rates, relative to the size of subordinates, if this improves their reproductive success. Here, we show that individuals in breeding pairs of the coral-dwelling fish Gobiodon histrio regulate their growth to reduce the size difference between partners. In pairs where one individual was larger than the other, the smaller individual increased its growth rate and the larger individual decreased its growth rate, compared to individuals in size-matched pairs. The reproductive success of breeding pairs is limited by the size of the smallest individual in the pair. Therefore, it appears that the larger individual trades-off its own growth against that of the smaller individual, thereby improving the reproductive success of both individuals in the pair. This demonstrates a remarkable ability of individuals to strategically adjust their body size to suit the local social environment, and reveals a novel mechanism for size-assortative mating.     

Ontogenetic and spatial variation in size‐selective mortality of a marine fish

Although body size can affect individual fitness, ontogenetic and spatial variation in the ecology of an organism may determine the relative advantages of size and growth. During an 8‐year field study in the Bahamas, we examined selective mortality on size and growth throughout the entire reef‐associated life phase of a common coral‐reef fish, Stegastes partitus (the bicolour damselfish). On average, faster‐growing juveniles experienced greater mortality, though as adults, larger individuals had higher survival. Comparing patterns of selection observed at four separate populations revealed that greater population density was associated with stronger selection for larger adult size. Large adults may be favoured because they are superior competitors and less susceptible to gape‐limited predators. Laboratory experiments suggested that selective mortality of fast‐growing juveniles was likely because of risk‐prone foraging behaviour. These patterns suggest that variation in ecological interactions may lead to complex patterns of lifetime selection on body size.     

The origin and development of individual size variation in early pelagic stages of fish

Size variation among individuals born at the same time in a common environment (within cohorts) is a common phenomenon in natural populations. Still, the mechanisms behind the development of such variation and its consequences for population processes are far from clear. We experimentally investigated the development of early within-cohort size variation in larval perch (Perca fluviatilis). Specifically we tested the influence of initial variation, resulting from variation in egg strand size, and intraspecific density for the development of size variation. Variation in egg strand size translated into variation in initial larval size and time of hatching, which, in turn, had effects on growth and development. Perch from the smallest egg strands performed on average equally well independent of density, whereas larvae originating from larger egg strands performed less well under high densities. We related this difference in density dependence to size asymmetries in competitive abilities leading to higher growth rates of groups consisting of initially small individuals under high resource limitation. In contrast, within a single group of larvae, smaller individuals grew substantially slower under high densities whereas large individuals performed equally well independent of density. As a result, size variation among individuals within groups (i.e. originating from the same clutch) increased under high densities. This result may be explained by social interactions or differential timing of diet shifts and a depressed resource base for the initially smaller individuals. It is concluded that to fully appreciate the effects of density-dependent processes on individual size variation and size-dependent growth, consumer feedbacks on resources need to be considered.     

INTRASPECIFIC VARIATION IN SEX ALLOCATION IN A SIMULTANEOUS HERMAPHRODITE: THE EFFECT OF INDIVIDUAL SIZE

Within a population of simultaneous hermaphrodites, individuals may vary in both their current reproductive investment (biomass invested in gonads) and in how they allocate that investment between male and female function. In the chalk bass, Serranus tortugarum, estimates of both reproductive allocation and reproductive success as a male and a female can be made for individuals of different sizes. As individuals increase in size, their investment in gamete production increases, and there is a shift in allocation to a stronger female bias. Spawning frequency as a female in pair spawnings and as a male in both pair spawning and streaking (an alternative mating tactic) does not vary with individual size. As a result, larger individuals should release more sperm or eggs per spawn. Size-assortative pair spawning in this species leads to larger individuals having higher potential returns in total male reproductive success than smaller individuals, which should lead to increases in absolute levels of sperm production in larger individuals when individuals compete for fertilizations through sperm competition. However, smaller individuals contribute a smaller proportion of the sperm released in spawns with multiple spawners and thus are under more intense sperm competition than larger individuals, which should select for increases in male allocation in smaller individuals, all else equal. A local-mate-competition (LMC) model predicts that these factors select for increasing absolute male and female investment with individual size but a relative shift to more female-biased allocation as individual size increases. These predictions are supported by gonadal data. The predictions of average male allocation from the quantitative LMC model were 21.6% and 25.7%, whereas the collections averaged 21.3%. This close agreement of both the mean male allocation and its relative shift with individual size between model and data support the hypothesis that size-specific shifts in sex allocation in this species represent an adaptive response to patterns of mating success and sperm competition.