Roles of density-dependent growth and life history evolution in accounting for fisheries-induced trait changes

Abstract

The relative roles of density dependence and life history evolution in contributing to rapid fisheries-induced trait changes remain debated. In the 1930s, northeast Arctic cod (Gadus morhua), currently the world's largest cod stock, experienced a shift from a traditional spawning-ground fishery to an industrial trawl fishery with elevated exploitation in the stock's feeding grounds. Since then, age and length at maturation have declined dramatically, a trend paralleled in other exploited stocks worldwide. These trends can be explained by demographic truncation of the population's age structure, phenotypic plasticity in maturation arising through density-dependent growth, fisheries-induced evolution favoring faster-growing or earlier-maturing fish, or a combination of these processes. Here, we use a multitrait eco-evolutionary model to assess the capacity of these processes to reproduce 74 y of historical data on age and length at maturation in northeast Arctic cod, while mimicking the stock's historical harvesting regime. Our results show that model predictions critically depend on the assumed density dependence of growth: when this is weak, life history evolution might be necessary to prevent stock collapse, whereas when a stronger density dependence estimated from recent data is used, the role of evolution in explaining fisheries-induced trait changes is diminished. Our integrative analysis of density-dependent growth, multitrait evolution, and stock-specific time series data underscores the importance of jointly considering evolutionary and ecological processes, enabling a more comprehensive perspective on empirically observed stock dynamics than previous studies could provide.phenotypic plasticity | eco-evolutionary dynamics | management | genetic adaptation | genetic variance

Reaction norm analysis reveals rapid shifts toward delayed maturation in harvested Lake Erie yellow perch (Perca flavescens)

“…A negatively sloped PMRN indicates that individuals who grow slowly have a greater tendency to mature at a smaller size than fast‐growing individuals (Heino & Dieckmann, ). Other modeling studies have predicted little evolution of PMRN slopes in response to harvest (Dunlop, Heino et al, ; Eikeset et al, ). Superficially, the shift to a negative sloped PMRN for yellow perch in MU1 where harvest declined somewhat but remained high relative to other MUs is consistent with the effects of size‐selective harvest (Figure ).…”

Size‐selective fishing and the potential for fisheries‐induced evolution in lake whitefish

“…In experimental studies of captive Menidia menidia , harvesting of large fish leads to the evolution of slower growth rates, whereas harvesting of small fish leads to the evolution of faster growth rates (Conover & Munch, 2002). Evolutionary models show that fishing mortality can favor higher or lower growth rate, depending on the size selectivity of fishing mortality and other factors (Dunlop, Heino et al., 2009; Eikeset et al., 2016). …”

“…Eco‐genetic modeling also has been used to explore how different intensities of density‐dependent growth affect population dynamics and stock productivity (Gobin, Lester, Fox, & Dunlop, 2016). Intrinsic growth rate also has been treated as a decision variable in models of size‐selective fisheries (Dunlop, Heino et al., 2009; Eikeset et al., 2016). …”

Evolution of boldness and life history in response to selective harvesting

“…Another limitation of our modelling approach is the absence of population regulation 624 through density-dependent processes, which is a key ingredient of alternative individual-based eco-genetic models of fisheries-induced evolution (Dunlop et al 626 2015b;Eikeset et al 2016). A previous modelling study that examined the impact of a range of density-dependent processes acting on fecundity and mortality on selection 628 31 differentials on reproductive investment in pike (Esox lucius) (Arlinghaus et al 2009) indeed found that the predictions were rather sensitive to density-dependent somatic 630 growth but insensitive to density-dependent mortality.…”