Size-structured Fish Populations Research Articles

Temperature-Driven Regime Shifts in the Dynamics of Size-Structured Populations

Global warming impacts virtually all biota and ecosystems. Many of these impacts are mediated through direct effects of temperature on individual vital rates. Yet how this translates from the individual to the population level is still poorly understood, hampering the assessment of global warming impacts on population structure and dynamics. Here, we study the effects of temperature on intraspecific competition and cannibalism and the population dynamical consequences in a size-structured fish population. We use a physiologically structured consumer-resource model in which we explicitly model the temperature dependencies of the consumer vital rates and the resource population growth rate. Our model predicts that increased temperature decreases resource density despite higher resource growth rates, reflecting stronger intraspecific competition among consumers. At a critical temperature, the consumer population dynamics destabilize and shift from a stable equilibrium to competition-driven generation cycles that are dominated by recruits. As a consequence, maximum age decreases and the proportion of younger and smaller-sized fish increases. These model predictions support the hypothesis of decreasing mean body sizes due to increased temperatures. We conclude that in size-structured fish populations, global warming may increase competition, favor smaller size classes, and induce regime shifts that destabilize population and community dynamics.

Habitat selection by breeding waterbirds at ponds with size-structured fish populations

Fish may significantly affect habitat use by birds, either as their prey or as competitors. Fish communities are often distinctly size-structured, but the consequences for waterbird assemblages remain poorly understood. We examined the effects of size structure of common carp (Cyprinus carpio) cohorts together with other biotic and abiotic pond characteristics on the distribution of breeding waterbirds in a seminatural system of monocultured ponds, where three fish age classes were separately stocked. Fish age corresponded to a distinct fish size gradient. Fish age and total biomass, macroinvertebrate and amphibian abundance, and emergent vegetation best explained the differences in bird density between ponds. Abundance of animal prey other than fish (aquatic macroinvertebrates and larval amphibians) decreased with increasing carp age in the ponds. Densities of ducks and smaller grebes were strongly negatively associated with fish age/size gradient. The largest of the grebes, the piscivorous great crested grebe (Podiceps cristatus), was the only species that preferred ponds with medium-sized fish and was positively associated with total fish biomass. Habitat selection by bitterns and most rallids was instead strongly influenced by the relative amount of emergent vegetation cover in the ponds. Our results show that fish size structure may be an important cue for breeding habitat choice and a factor affording an opportunity for niche diversification in avian communities.

Size-structured effects of common carp on reproduction of pond-breeding amphibians

The role of fish in driving amphibian communities has been widely recognized. However, little is known about size-structured interactions between amphibian and fish populations. This study compared the taxonomic occurrence and densities of larval amphibians between unstocked ponds and ponds stocked with different age cohorts of common carp Cyprinus carpio differing in average body size. The average total densities of early and late breeding anurans known to be vulnerable to fish were by 1–2 orders of magnitude greater in the presence of young-of-the-year carp than that of older cohorts. The probabilities of occurrence of the most common taxa did not differ between ponds stocked with young-of-the-year fish and ponds free of carp, but were significantly larger in those ponds than in ponds stocked with large-size cohorts. No significant differences between pond categories were found for densities of unpalatable Bufo bufo larvae. In aquatic systems harbouring size-structured fish populations, a fish age/size gradient may explain differential habitat suitability for breeding amphibians better than the fish presence/absence dichotomy. When dominated by young cohorts incapable of predation or of adverse habitat alteration, fish-abundant waters are suitable for amphibian reproduction. Conversely, even a ‘non-predatory’ fish, after attaining large body size, may exert a detrimental impact on amphibian breeding success. These findings may be particularly important for amphibian conservation at pond fisheries characterized by spatial separation of age/size distributed stocks.

Stabilization of Population Fluctuations due to Cannibalism Promotes Resource Polymorphism in Fish

Resource polymorphism is a well-known phenomenon in many taxa, assumed to be a consequence of strong competition for resources and to be facilitated by stable environments and the presence of several profitable resources on which to specialize. In fish, resource polymorphism, in the form of planktivore-benthivore pairs, is found in a number of species. We gathered literature data on life-history characteristics and population dynamics for 15 fish species and investigated factors related to the presence of such resource polymorphism. This investigation indicated that early cannibalism and low overall population variability are typically associated with the presence of resource polymorphism. These findings match previously reported patterns of population dynamics for size-structured fish populations, whereby early cannibalism has been shown to decrease temporal variation in population dynamics and to equalize the profitability of the zooplankton and macroinvertebrate resources. Our study suggests that competition alone is not a sufficient condition for the development of resource polymorphism because overly strong competition is typically associated with increased temporal variation (environmental instability). We conclude that although resource competition is an important factor regulating the development of resource polymorphism, cannibalism may also play a fundamental role by dampening population oscillations and possibly by equalizing the profitability of different resources.

Asymmetric impact of piscivorous birds on size-structured fish populations

Fish are frequently considered the top predator in freshwater food web models despite evidence that predatory birds can impact fish populations. In this study, we quantified bird predation rates on experimental populations of rainbow trout ( Oncorhynchus mykiss (Walbaum, 1792)) created by stocking nine small lakes in British Columbia, Canada. Combining estimates of fish mortality with estimated bird predation rates allowed us to partition fish mortality into that due to birds versus cannibalism. Our results indicated that bird predators had significant impacts on age-1 trout populations, but little impact on age-0 trout. Common loons ( Gavia immer Brunnich, 1764) were the principle predator among eight predatory bird species present, apparently consuming nearly 50% of all stocked age-1 trout and explaining almost 50% of variation in mortality rates. Age-1 trout mortality did not differ significantly from zero in lakes without loons. Birds consumed a small proportion of age-0 trout, and estimated consumption explained none of the variation in age-0 trout mortality among lakes. We conclude that birds affect fish populations by asymmetric predation on different age (size) classes and can be important top predators that should not be ignored when characterizing freshwater food webs in lakes.

An advection-diffusion-reaction size-structured fish population dynamics model combined with a statistical parameter estimation procedure: Application to the Indian Ocean skipjack tuna fishery

We develop an advection-diffusion size-structured fish population dynamics model and apply it to simulate the skipjack tuna population in the Indian Ocean. The model is fully spatialized, and movements are parameterized with oceanographical and biological data; thus it naturally reacts to environment changes. We first formulate an initial-boundary value problem and prove existence of a unique positive solution. We then discuss the numerical scheme chosen for the integration of the simulation model. In a second step we address the parameter estimation problem for such a model. With the help of automatic differentiation, we derive the adjoint code which is used to compute the exact gradient of a Bayesian cost function measuring the distance between the outputs of the model and catch and length frequency data. A sensitivity analysis shows that not all parameters can be estimated from the data. Finally twin experiments in which pertubated parameters are recovered from simulated data are successfully conducted.

A multi-region nonlinear age–size structured fish population model

The goal of this paper is to present a generic multi-region nonlinear age–size structured fish population model, and to assess its mathematical well-posedness. An initial–boundary-value problem is formulated. Existence and uniqueness of a positive weak solution is proved. Eventually, a comparison result is derived: the population of all regions decreases as the mortality rate increases in at least one region.

Physiological Ecology Meets the Ideal-free Distribution: Predicting the Distribution of Size-structured Fish Populations Across Temperature Gradients

We describe a habitat selection model that predicts the distribution of size-structured groups of fish in a habitat where food availability and water temperature vary spatially. This model is formed by combining a physiological model of fish growth with the logic of ideal free distribution (IFD) theory. In this model we assume that individuals scramble compete for resources, that relative competitive abilities of fish vary with body size, and that individuals select patches that maximize their growth rate. This model overcomes limitations in currently existing physiological and IFD-based models of habitat selection. This is because existing physiological models do not take into account the fact that the amount of food consumed by a fish in a patch will depend on the number of competitors there (something that IFD theory addresses), while traditional IFD models do not take into account the fact that fish are likely to choose patches based on potential growth rate rather than gross food intake (something that physiological models address). Our model takes advantage of the complementary strengths of these two approaches to overcome these weaknesses. Reassuringly, our model reproduces the predictions of its two constituent models under the simple conditions where they apply. When there is no competition for resources it mimics the physiological model of habitat selection, and when there is competition but no temperature variation between patches it mimics either the simple IFD model or the IFD model for unequal competitors. However, when there are both competition and temperature differences between patches our model makes different predictions. It predicts that input-matching between the resource renewal rate and the number of fish (or competitive units) in a patch, the hallmark of IFD models, will be the exception rather than the rule. It also makes the novel prediction that temperature based size-segregation will be common, and that the strength and direction of this segregation will depend on per capita resource renewal rates and the manner in which competitive weight scales with body size. Size-segregation should become more pronounced as per capita resource abundance falls. A larger fish/cooler water pattern is predicted when competitive ability increases more slowly than maximum ration with body size, and a smaller fish/cooler water pattern is predicted when competitive ability increases more rapidly than maximum ration with body size.

How does floodplain width affect floodplain river ecology? A preliminary exploration using simulations

Hydraulic food chain models allow us to explore the linkages of river discharge regimes and river-floodplain morphology to the structure and dynamics of modeled food webs. Physical conditions (e.g. depth, width, velocity) that vary with river discharge affect the performance (birth, growth, feeding, movement, or death rates) of organisms or trophic groups. Their performances in turn affect their impacts on food webs and ecosystems in channel and floodplain habitats. Here we explore the impact of floodplain width (modeled as 1 ×, 10× and 40× the channel width) on a food web with two energy sources (detritus and vegetation), invertebrates that consume these, a size structured fish population which consumes invertebrates and in which larger fish cannibalize small fish, and birds which feed on large fish. Hydraulic linkages to trophic dynamics are assumed to be mediated in three ways: birds feed efficiently only in shallow water; plant carrying capacity varies non-linearly with water velocity, and mobile and drifting organisms are diluted and concentrated with spillover of river discharge to the floodplain, and its reconfinement to the channel. Aspects of this model are based on field observations of Junk and Bailey from the Amazon, of Sparks from the Mississippi, and on our observations of the Fly River in Papua New Guinea. The model produced several counter-intuitive results. Biomass of invertebrates and fish increased with floodplain width, but much more rapidly from 1 × to 10 × floodplains than from 10 × to 40 × floodplains. For birds, maximum biomass occurred on the 10× floodplain. Initially high bird biomass on the 40 × floodplain declined to extinction over time, because although favorable fishing conditions (shallow water) were most prolonged on the widest floodplain, this advantage was more than offset by the greater dilution of prey after spillover. Bird predation on large fish sometimes increased their biomass, by reducing cannibalism and thereby increasing the abundance of small fish available to grow into the larger size class. Sensitivity analyses indicated that model results were relatively robust to variation in parameter values that we chose, but much more exploration and calibration with field data are needed before we know how specific our results are to the structure and other assumptions of this model. We share with others the opinion that progress towards understanding complex dynamic systems like floodplain river ecosystems requires frequent feedback between modeling and field observations and experimentation. This understanding is crucial for river management and restoration. Organisms in real rivers have adapted to track and quickly exploit favorable conditions, and to avoid or endure adverse conditions. It is when we engineer away this environmental variability that we threaten the long term persistence of river-adapted biota.

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...

A General Fishery Model for a Size-Structured Fish Population

This paper describes a fishery model based on size, rather than age, as the fundamental population variable. The model's key ingredient is a density function related to number of fish within any given size range. A key assumption is that selectivity in the fishery is determined by size, not age, and that fish can reach recruitment size at various ages. The theory easily yields moment equations comparable with those obtained recently by Fournier and Doonan; for example, the general moment equation here is derived in a four-line proof. The paper gives detailed recommendations for tailoring the new model to a specific fishery, where model complexity is determined by the available data. The new theory places earlier models of Deriso and Schnute in the context of generalized cohort analysis, where size-dependent mortality is allowed. In particular, the paper identifies a potential problem with these earlier models and proposes a solution. All results are interpreted biologically to provide both conceptual and analytical tools for stock assessment.