Evolutionary effects of fishing gear on foraging behavior and life-history traits

Abstract

Fishing gears are designed to exploit the natural behaviors of fish, and the concern that fishing may cause evolution of behavioral traits has been receiving increasing attention. The first intuitive expectation is that fishing causes evolution toward reduced boldness because it selectively removes actively foraging individuals due to their higher encounter rate and vulnerability to typical gear. However, life-history theory predicts that fishing, through shortened life span, favors accelerated life histories, potentially leading to increased foraging and its frequent correlate, boldness. Additionally, individuals with accelerated life histories mature younger and at a smaller size and therefore spend more of their life at a smaller size where mortality is higher. This life-history evolution may prohibit increases in risk-taking behavior and boldness, thus selecting for reduced risk-taking and boldness. Here, we aim to clarify which of these three selective patterns ends up being dominant. We study how behavior-selective fishing affects the optimal behavioral and life-history traits using a state-dependent dynamic programming model. Different gear types were modeled as being selective for foraging or hiding/resting individuals along a continuous axis, including unselective fishing. Compared with unselective harvesting, gears targeting hiding/resting individuals led toward evolution of increased foraging rates and elevated natural mortality rate, while targeting foraging individuals led to evolution of decreased foraging rates and lower natural mortality rate. Interestingly, changes were predicted for traits difficult to observe in the wild (natural mortality and behavior) whereas the more regularly observed traits (length-at-age, age at maturity, and reproductive investment) showed only little sensitivity to the behavioral selectivity.

Keywords: behavior; boldness; fishing‐induced evolution; foraging rate; life‐history traits; mortality; timidity.

Figure 1

Effect of vulnerability to gear γ on optimal life‐history and foraging strategies for fishing mortality of 0.1 year⁻¹. In the second row, dots indicate age at first maturation: black ones for populations adapted to natural mortality only, and colored ones after adaptation to fishing mortality (the thin arrow highlights the shift). The solid red line corresponds to the case where fishing is independent of behavior (γ = 0). The dashed green and dash‐dot blue lines correspond to the case where fishing targets passive (γ = −0.3) and active (γ = 0.3) individuals, respectively. Shaded areas correspond to the standard deviation of the trait within the population, and grey arrows show the direction of change due to adaptation to fishing

Figure 2

Effect of vulnerability to gear γ on predicted optimal values of length‐at‐age (a), the foraging strategy at age 14 (b), and the emergent natural mortality at age 14 (c). The shaded area for the foraging strategy corresponds to the standard deviation of the trait in the population. We use age 14 as the reference for adult life‐history traits because at this age all individuals are mature, even in the absence of fishing. This age is also far enough from the end of the modeled life span to be unaffected by terminal effects when using dynamic programming for optimization (Clark & Mangel, 2000)

Figure 3

Effects of fishing mortality on average individual‐level traits for different types of vulnerability to gear γ. The traits shown are the mean value of the foraging strategy at age 14 (a), total natural mortality at age 14 (note that total mortality includes also fishing mortality which is not shown here) (b), length‐at‐age 14 (c), reproductive investment (GSI) at age 14 (d), and mean age at maturity within a population, depending on γ. The red solid lines correspond to the case where fishing is not related to behavior (γ = 0). The green dashed (γ = −0.3) and blue dot‐dash (γ = 0.3) lines correspond to the case where fishing targets sheltering (purse seines) and foraging individuals (gill nets), respectively