ORIGINAL ARTICLE: Size‐selective fishing gear and life history evolution in the Northeast Arctic cod

Abstract

Industrial fishing has been identified as a cause for life history changes in many harvested stocks, mainly because of the intense fishing mortality and its size-selectivity. Because these changes are potentially evolutionary, we investigate evolutionarily stable life-histories and yield in an energy-allocation state-dependent model for Northeast Arctic cod Gadus morhua. We focus on the evolutionary effects of size-selective fishing because regulation of gear selectivity may be an efficient management tool. Trawling, which harvests fish above a certain size, leads to early maturation except when fishing is low and confined to mature fish. Gillnets, where small and large fish escape, lead to late maturation for low to moderate harvest rates, but when harvest rates increase maturation age suddenly drops. This is because bell-shaped selectivity has two size-refuges, for fish that are below and above the harvestable size-classes. Depending on the harvest rate it either pays to grow through the harvestable slot and mature above it, or mature small below it. Sustainable yield on the evolutionary time-scale is highest when fishing is done by trawling, but only for a small parameter region. Fishing with gillnets is better able to withstand life-history evolution, and maintains yield over a wider range of fishing intensities.

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

“…Fish that mature at smaller sizes have lower reproductive output, but they also avoid growing into vulnerable size classes where their survival would be low. In contrast to trap nets, gill nets present two size refuges for fish, and the presence of an upper size refuge has the potential to buffer a population from the long‐term evolutionary effects of fishing (Jørgensen et al., 2009; Law, 2007). However, this was not an outcome for Lake Huron lake whitefish when fishing with 114‐mm gill nets, the mesh size currently dominating in the commercial fishery.…”

“…Variation in lifetime patterns of growth and maturation emerge as the outcome of state‐dependent energy allocation decisions. This model has been used to explore the phenomenon of skipped spawning (Jørgensen, Ernande, Fiksen, & Dieckmann, 2006) and the fitness consequences of size‐selective fisheries for a variety of life history traits (Jørgensen et al., 2009). This type of model was well‐suited to explore the interaction between size‐selective fisheries and environmental variation in growth potential.…”

“…Indeed, evidence has emerged that fishing mortality of stocks, both marine and freshwater, can be sufficiently high for evolutionary changes in life history traits to occur at trackable, ecological timescales (Edeline et al., 2007; Heino et al., 2015; Jørgensen et al., 2007; Nusslé, Bornand, & Wedekind, 2009; Olsen et al., 2004). Further, the direction and intensity of fisheries‐induced selection are expected to depend on which sizes are targeted, either through size or gear restrictions (Dunlop, Heino, & Dieckmann, 2009; Hutchings, 2009; Jørgensen, Ernande, & Fiksen, 2009; Wang & Höök, 2009). For example, Jørgensen et al.…”

“…For example, Jørgensen et al. (2009) demonstrated that trawl nets, which have sigmoid‐shaped selectivity curves, consistently favor early maturation in Atlantic cod ( Gadus morhua ). In contrast, gill nets, which have bell‐shaped size selectivity curves, can select for fish that grow through vulnerable size classes before maturing.…”

“…Growth has been formulated in different ways in these models, but typically is not manipulated to generate predictions. In Atlantic cod, several modeling studies have considered the evolutionary and demographic consequences of size‐selective fisheries and different gear types under a specified growth regime (Hutchings, 2009; Jørgensen et al., 2009; Zimmerman & Jørgensen, 2017). In eco‐genetic models of fisheries‐induced evolution of maturation traits in lake whitefish, somatic growth rate is assumed to be negatively density‐dependent according to a specified functional form.…”