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

Abstract

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.

Figure 1

Hypothesized relationships between whole-body lipid content and body length for age-0 trout responding only to the risk of lipid-dependent overwinter starvation mortality (H2) or to the size-dependent risk of predation when small and vulnerable, and risk of overwinter starvation when larger (H1). Maximum and minimum lipid contents in relation to body size bound the range of possible outcomes for fishes' lipid content with body size (actual observed minimum and maximum lipid allometries are shown in figure 3).

Figure 2

(a) Relative use of the refuge habitat (areas less than 1.5 m depth) by age-0 trout and (b) population mean mass of age-0 trout, in relation to food treatment and sampling date. Each datum represents the response observed in a single lake (n=9 lakes). Mean (±s.e.m.) for each is shown. Refuge use by age-0 trout is significantly higher in high-food lakes than in low-food lakes in late July; all other differences in refuge use and mean mass between food treatments are not different (p>0.05; data redrawn from Biro et al. 2003a). Food-dependent differences in habitat use by age-0 trout are similarly accompanied by food-dependent differences in individual activity rates (see Biro et al. 2003a). Areas less than 1.5 m deep represent refuges because adult rainbow trout are typically only observed in deeper habitats (Biro et al. 2003a). Thin horizontal lines in (b) represent the predicted maximum mean mass of rainbow trout (see §2).

Figure 3

Allometric relationships between fork length of individual age-0 trout from low- and high-food lakes and (a, b) their lipid concentration (g lipid g wet mass ⁻¹) and (c, d) ratio of storage/structure (g lipid per g lipid-free dry mass). Solid symbols represent late July samples (18 days post stocking) and open symbols represent late August samples (48 days post stocking). Regression lines represent the significant interaction effect between sampling date and fork length, and the dashed line in the upper panels represents the minimum lipid concentration for survival in age-0 rainbow trout (Biro et al. 2004a,b). The heavy solid lines (a, b) represent the lipid allometry of rainbow trout raised in the hatchery (in the absence of predator cues) and fed to satiation for approximately two weeks following hatching (data from Biro et al. 2004a,b).