John R. Post

BOTTOM-UP AND TOP-DOWN IMPACTS ON FRESHWATER PELAGIC COMMUNITY STRUCTURE'

For freshwater pelagic ecosystems, the biodiversity and cascading trophic interaction theories both predict that decreased piscivore populations will result in direct, short—term (a few years) increases in planktivore biomass, reductions in crustacean herbivore biomass, and increases in chlorophyll a concentration and phytoplankton biomass. An Alternate view is offered by the bottom—up:top—down theory, which predicts that in eutrophic lakes changes in piscivore biomasses will have strong impacts on planktivore numbers, weaker but observable impacts on zooplankton biomass, and little or no long—term effects on phytoplankton biomass. A partial winterkill at Lake St. George, Ontario, Canada allowed us to test these predictions. The data set comprised measures of: (1) piscivore and planktivore numbers, (2) zooplankton species composition, size structure, and biomass, (3) chlorophyll a concentration and Secchi depth, and (4) water chemistry from 1980 through 1986. Prior to the winterkill of 1981—1982, the piscivore population was high (1000—2000 piscivores/ha), the planktivore population was intermediate (8000—10 000 planktivores/ha), zooplankton biomass was intermediate (2400 µg/L), and chlorophyll a concentration was high (5—12 µg/L). In the year following the winterkill (1982), piscivore and planktivore numbers were low, and zooplankton biomass and chlorophyll a concentration were high. During the next 2 yr (1983—1984) the planktivore population increased rapidly to densities >20 000 individuals/ha, zooplankton biomass density decreased to <1600 µg/L and chlorophyll a concentration decreased. During the final 2 yr of the study, piscivores recruited to near prewinterkill levels, planktivores were reduced to <8000 individuals/ha, zooplankton biomass increased, and chlorophyll a concentration decreased. Over the 7 yr data set, we found a strong negative correlation between numbers of piscivores and planktivores, a weaker correlation between numbers of planktivores and zooplankton biomass, and no between—year correlation between zooplankton biomass and chlorophyll a concentration. There was, however, a positive correlation between total epilimnetic phosphorus and chlorophyll a concentration. These data are consistent with predictions made by the bottom—up:top—down model, and the implication is that at Lake St. George, the trophic cascade uncouples at the zooplankton → phytoplankton link. We speculate that this may be due to the combined effects of lake trophy and Daphnia species composition and size.

Canada's Recreational Fisheries: The Invisible Collapse?

Abstract Fishing for recreation is a popular activity in many parts of the world and this activity has led to the development of a sector of substantial social and economic value worldwide. The maintenance of this sector depends on the ability of aquatic ecosystems to provide fishery harvest. We are currently witnessing the collapse of many commercial marine fisheries due to over-exploitation. Recreational fisheries are typically viewed as different from commercial fisheries in that they are self-sustaining and not controlled by the social and economic forces of the open market that have driven many commercial fisheries to collapse. Here we reject the view that recreational and commercial fisheries are inherently different and demonstrate several mechanisms that can lead to the collapse of recreational fisheries. Data from four high profile Canadian recreational fisheries show dramatic declines over the last several decades yet these declines have gone largely unnoticed by fishery scientists, managers, an...

Rapid depletion of genotypes with fast growth and bold personality traits from harvested fish populations

The possibility for fishery-induced evolution of life history traits is an important but unresolved issue for exploited fish populations. Because fisheries tend to select and remove the largest individuals, there is the evolutionary potential for lasting effects on fish production and productivity. Size selection represents an indirect mechanism of selection against rapid growth rate, because individual fish may be large because of rapid growth or because of slow growth but old age. The possibility for direct selection on growth rate, whereby fast-growing genotypes are more vulnerable to fishing irrespective of their size, is unexplored. In this scenario, faster-growing genotypes may be more vulnerable to fishing because of greater appetite and correspondingly greater feeding-related activity rates and boldness that could increase encounter with fishing gear and vulnerability to it. In a realistic whole-lake experiment, we show that fast-growing fish genotypes are harvested at three times the rate of the slow-growing genotypes within two replicate lake populations. Overall, 50% of fast-growing individuals were harvested compared with 30% of slow-growing individuals, independent of body size. Greater harvest of fast-growing genotypes was attributable to their greater behavioral vulnerability, being more active and bold. Given that growth is heritable in fishes, we speculate that evolution of slower growth rates attributable to behavioral vulnerability may be widespread in harvested fish populations. Our results indicate that commonly used minimum size-limits will not prevent overexploitation of fast-growing genotypes and individuals because of size-independent growth-rate selection by fishing.

ENERGY ALLOCATION STRATEGY IN YOUNG FISH: ALLOMETRY AND SURVIVAL

We observed substantial variation in seasonal growth rates, autumn body size, and growing-season mortality among eight experimental cohorts of age-O rainbow trout, Oncorhynchus mykiss. Wet mass, water, lipids (storage), and lipid-free dry mass (structure) had biphasic allometries with inflexions at - 10 cm in length. Dry:wet mass and storage:structure ratios were positively related to fish length, indicating that the relative quantities of these constituents change with body size. Lipid concentration varied according to a sigmoid relationship with wet mass which also had a growth-rate dependence. Inde- pendent assessments of the allometry of growing-season survival and winter metabolism facilitated assessment of the costs and benefits of two alternate energy allocation strategies of young fish. For cohorts with low growth rates and small autumn body size, a somatic growth rate maximization strategy is optimum, producing a 5% net survival advantage over an energy storage maximization strategy. For cohorts with intermediate growth rates and autumn mass, somatic growth and energy storage strategies lead to similar first-year sur- vival. The fastest growing cohorts are estimated to have a net survival advantage of 7%, by adopting an energy storage maximization strategy over a growth rate maximization strategy.

Predators select against high growth rates and risk-taking behaviour in domestic trout populations

Domesticated (farm) salmonid fishes display an increased willingness to accept risk while foraging, and achieve high growth rates not observed in nature. Theory predicts that elevated growth rates in domestic salmonids will result in greater risk–taking to access abundant food, but low survival in the presence of predators. In replicated whole–lake experiments, we observed that domestic trout (selected for high growth rates) took greater risks while foraging and grew faster than a wild strain. However, survival consequences for greater growth rates depended upon the predation environment. Domestic trout experienced greater survival when risk was low, but lower survival when risk was high. This suggests that animals with high intrinsic growth rates are selected against in populations with abundant predators, explaining the absence of such phenotypes in nature. This is, to our knowledge, the first large–scale field experiment to directly test this theory and simultaneously quantify the initial invasibility of domestic salmonid strains that escape into the wild from aquaculture operations, and the ecological conditions affecting their survival.

Instream flow needs in streams and rivers: the importance of understanding ecological dynamics

Resource managers have traditionally had to rely on simple hydrological and habitat-association methods to predict how changes in river flow regimes will affect the viability of instream populations and communities. Yet these systems are characterized by dynamic feedbacks among system components, a high degree of spatial and temporal variability, and connectivity between habitats, none of which can be adequately captured in the commonly employed management methods. We argue that process-oriented ecological models, which consider dynamics across scales and levels of biological organization, are better suited to guide flow regime management. We review how ecological dynamics in streams and rivers are shaped by a combination of the flow regime and internal feedbacks, and proceed to describe ecological modeling tools that have the potential to characterize such dynamics. We conclude with a suggested research agenda to facilitate the inclusion of ecological dynamics into instream flow needs assessments.

FROM INDIVIDUALS TO POPULATIONS: PREY FISH RISK-TAKING MEDIATES MORTALITY IN WHOLE-SYSTEM EXPERIMENTS

Recent research suggests that the behavior of individuals under risk of pre- dation could be a key link between individual behavior and population and community dynamics. Yet existing theory remains largely untested at large spatial and temporal scales. We manipulated food available to age-0 rainbow trout while at risk of cannibalism, in a replicated factorial whole-lake experiment, to test whether the trade-off between growth and mortality rates is mediated by foraging activity by young fish under predation risk. We found that this trade-off exists for young fish at the whole-system scale, and that food- dependent behavioral variation has large mortality consequences. In high-food lakes, age- 0 trout spent less time moving, fewer individuals swam continuously, and those swimming continuously swam at slower speeds relative to those in low-food lakes. Age-0 trout also used deep, risky habitats less when food was abundant. This lower activity, combined with avoidance of risky habitats, coincided with 68% higher survival in high-food lakes. If general, this trade-off may be a key mechanism linking individual behavior to population- level processes in size-structured populations.

ANGLER NUMERICAL RESPONSE ACROSS LANDSCAPES AND THE COLLAPSE OF FRESHWATER FISHERIES

Recreational angling opportunities in lakes are distributed across landscapes and attract anglers based on the combination of angling quality, travel distance, and availability of facilities. The relationship between angler density and fishing quality, as measured by catch rate, represents a numerical response that is analogous to a predator numerical response to variability in prey abundance. We quantified this numerical response of anglers to rainbow trout, Oncorhynchus mykiss, populations distributed over a large lake district in south-central British Columbia, Canada. We developed a harvest dynamics model by linking this empirical description of the spatial numerical response of anglers to a logistic population growth rate model. The model was parameterized for rainbow trout and simulated spatial patterns of angler density and catch rates over a landscape. At locations distant from urban centers, angler density is low and catch rate high, suggesting near pristine conditions; at intermediate distances angler density is higher while catch rates are lower and approximate maximum sustainable levels; and at short distances angler density is sufficiently high to harvest to local extirpation. We extrapolated the model to other lake districts varying in human population size using an empirically derived angling participation rate relationship. Extrapolation to lake districts with one-tenth the human population maintained viable fisheries close to the urban area, and districts with 10 times the human populations could not maintain viable fisheries across much of their lake district. Landscape-scale spatial patterns differed quantitatively for species varying in rates of intrinsic population growth and carrying capacity, but the qualitative spatial patterns were consistent among species, demonstrating the pervasive impacts of the angler numerical response. To achieve a management goal of sustaining fisheries across landscapes, a change in management perspective is necessary, from that of individual lakes to one of dynamic harvest processes across landscapes. This new approach makes it clear that a one-size-fits-all management approach must be replaced with a mosaic of approaches cognizant of landscape-scale processes.

Density-dependent growth and competitive asymmetries in size-structured fish populations: a theoretical model and recommendations for field experiments

Abstract High stocking or recruitment rates of juvenile fish can result in growth rate depression, so fisheries managers may need to consider a trade-off between producing large numbers or large sizes of fish. Analysis of food production, feeding, and bioenergetics of growth leads to a model for predicting such density-dependent growth depression. This model can be expressed as a simple hyperbolic equation whose parameters can be estimated from field data on maximum growth rates and growth depressions under controlled field conditions, If feeding rates are assumed proportional to the square of body length, the equation leads to a von Bertalanffy growth curve with only the asymptotic size varying with density, When all sizes of fish consume the same foods, the model predicts that increasing density will lead to parallel Ford–Walford plots with intercepts that are inversely proportional to density, A key complication regards partitioning of the food resource by small and large fish, which may lead to differ...

Ontogeny of energy allocation reveals selective pressure promoting risk-taking behaviour in young fish cohorts

Given limited food, prey fishes in a temperate climate must take risks to acquire sufficient reserves for winter and/or to outgrow vulnerability to predation. However, how can we distinguish which selective pressure promotes risk-taking when larger body size is always beneficial? To address this question, we examined patterns of energy allocation in populations of age-0 trout to determine if greater risk-taking corresponds with energy allocation to lipids or to somatic growth. Trout achieved maximum growth rates in all lakes and allocated nearly all of their acquired energy to somatic growth when small in early summer. However, trout in low-food lakes took greater risks to achieve this maximal growth, and therefore incurred high mortality. By late summer, age-0 trout allocated considerable energy to lipids and used previously risky habitats in all lakes. These results indicate that: (i) the size-dependent risk of predation (which is independent of behaviour) promotes risk-taking behaviour of age-0 trout to increase growth and minimize time spent in vulnerable sizes; and (ii) the physiology of energy allocation and behaviour interact to mediate growth/mortality trade-offs for young animals at risk of predation and starvation.