Harvest-induced maturation evolution under different life-history trade-offs and harvesting regimes

Abstract

The potential of harvesting to induce adaptive changes in exploited populations is now increasingly recognized. While early studies predicted that elevated mortalities among larger individuals select for reduced maturation size, recent theoretical studies have shown conditions under which other, more complex evolutionary responses to size-selective mortality are expected. These new predictions are based on the assumption that, owing to the trade-off between growth and reproduction, early maturation implies reduced growth. Here we extend these findings by analyzing a model of a harvested size-structured population in continuous time, and by systematically exploring maturation evolution under all three traditionally acknowledged costs of early maturation: reduced fecundity, reduced growth, and/or increased natural mortality. We further extend this analysis to the two main types of harvest selectivity, with an individual's chance of getting harvested depending on its size and/or maturity stage. Surprisingly, we find that harvesting mature individuals not only favors late maturation when the costs of early maturation are low, but promotes early maturation when the costs of early maturation are high. To our knowledge, this study therefore is the first to show that harvesting mature individuals can induce early maturation.

Fisheries-induced disruptive selection

“…We assume that early-maturing individuals face several life-history tradeoffs, since energy allocation to maturation 66 reduces the energy available for other life-history processes, including growth, survival, and reproduction (Poos et al, 2011;Bodin et al, 2012). We make the simplest possible assumptions for these three tradeoffs, by considering the 68 mortality of small early-maturing individuals to be increased relative to small late-maturing individuals according to m2(x) = m 2 (1 + β m x), the growth rate of small early-maturing individuals to be decreased relative to small late-70 maturing individuals according to r2(x) = r 2 (1 − β r x) + , and the fecundity of large early-maturing individuals to be decreased relative to large late-maturing individuals according to f3(x) = f 3 (1 − β f x) + .…”

“…To date, most studies considering the genetic and phenotypic responses of fish stock to fishing have focused on fisheries-induced directional 6 selection on life-history traits such as age and size at maturation (Barot et al, 2004;Ernande et al, 2004;de Roos et al, 2006;Gårdmark and Dieckmann, 2006;Dunlop et al, 2009;Poos et al, 2011).…”

“…We use a discretely size-structured life-history model, similar to that employed in Poos et al (2011) and Bodin et al (2012), to describe an adaptively harvested fish population divided into three size classes ( Figure 1). Individuals can 40 mature either in the second or in the third size class, and accordingly differ in their sizes at maturation.…”

“…We consider ten different fishing policies, with different selectivity according to size and maturity (Ajiad et al, 1999;Law, 2000;Poos et al, 2011;Bodin et al, 2012). These are detailed in Table 2.…”

“…Norwegian spring-spawning herring have been harvested at their spawning grounds throughout the 20th century, while mature individuals of Northeast Arctic cod have been harvested during their spawning migration until the mid-20th cen-366 tury (Poos et al, 2011). Therefore, adaptive harvesting under policies that allow the targeting of large individuals alters natural adaptive landscapes in a way that selects for increased reproductive investment early in life.…”