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

Abstract

The long‐term evolutionary effects of fishing on maturation schedules can depend on gear type, the shape of the gear type's size‐selectivity function, and the size and age structure of a population. Our goal was to better understand how environmentally induced differences in somatic growth influence the evolutionary effects of size‐selective fisheries, using lake whitefish (Coregonus clupeaformis) in Lake Huron as a case study. Using a state‐dependent optimization model of energy allocation parameterized for lake whitefish, we show that fishing with gill nets (bell‐shaped selectivity) and trap nets (sigmoid‐shaped selectivity) can be potent agents of selection on size thresholds for maturity. Compared to trap nets and large mesh (114 mm) gill nets, small mesh (89 mm) gill nets are better able to buffer populations from fishing‐induced evolution by safeguarding large, fecund fish, but only when overall fishing mortality is low and growth rates sufficiently fast such that fish can outgrow vulnerable size classes. Regardless of gear type, and all else being equal, high fishing mortality in combination with low growth rates is expected to intensify the long‐term evolutionary effects of fishing.

Ecological change alters the evolutionary response to harvest in a freshwater fish

“…The potential effects of providing a size refuge to larger fish under dome‐shaped selectivity, as predicted by Zimmermann and Jørgensen (), are experienced in our whitefish populations when harvest is low, mesh sizes are small enough, and ecosystem conditions are favorable. This was a similar result to that found by Morbey and Mema (), whereby small mesh gill nets buffered the evolution of size thresholds for maturation in lake whitefish fished at low rates under fast growing conditions. This is because more individuals are able to grow through and reach the protected size window.…”

“…). This particular aspect of size‐selectivity in the Great Lakes whitefish fishery could be one reason why maturation age has not undergone consistent declines over the past several decades (Appendix ; Morbey and Mema ) despite continued commercial harvest pressure, as has been observed in almost all fish stocks where fisheries‐induced evolution has been documented (Devine et al. ).…”

“…These changes included a reduction in growth potential and recruitment rate, consistent with a reduced carrying capacity (Gobin et al 2015(Gobin et al , 2016. Maturation ages and sizes of these stocks also shifted over time (Wang et al 2008, Morbey andMema 2018; Appendix S2: Fig. S1).…”

“…Maturation ages and sizes of these stocks also shifted over time (Wang et al. , Morbey and Mema ; Appendix ). We are aware of no previous studies examining the effects of dreissenid mussels on the eco‐evolutionary effects of fishing, even though dreissenid mussels have invaded aquatic systems across multiple countries.…”

“…A previous study (Gobin et al 2016) modeled the influence of the growth and recruitment changes on lake whitefish harvest sustainability, but did not consider potential evolutionary responses. Morbey and Mema (2018) considered how two different growth trajectories in a lake whitefish population influenced harvestinduced evolution of maturation size, but did not allow for continuous, reciprocal feedbacks between ecological and evolutionary processes and did not consider changes in recruitment. Here, we build on these previous studies by developing an eco-genetic model (Dunlop et al 2009) that enables a broader examination of how the ecological changes caused by an invasive species alter a stock's adaptation and resilience to harvest.…”