Assessing evolutionary consequences of size-selective recreational fishing on multiple life-history traits, with an application to northern pike (Esox lucius)

Abstract

ing are still scarce. We study selection differentials on three life-history traits -reproductive 4 investment, size at maturation, and growth capacity -under size-selective exploitation of 5 northern pike (Esox lucius L.) with recreational-fishing gear. An age-structured population 6 model is presented that accounts for the eco-evolutionary feedback arising from density-7 dependent and frequency-dependent selection. By introducing minimum-length limits, maxi-8 mum-length limits, and combinations of such limits (resulting in harvestable-slot length lim-9 its) into the model, we examine the potential of simple management tools for mitigating selec-10 tion pressures induced by recreational fishing. With regard to annual reproductive investment, 11 we find that size-selective fishing mortality exerts relatively small positive selection differen-12 tials. By contrast, selection differentials on size at maturation are large and consistently nega-13 tive. Selection differentials on growth capacity are often large and positive, but become nega-14 tive when a certain range of minimum-length limits are applied. In general, the strength of se-15 lection is reduced by implementing more stringent management policies, but each life-history 16 trait responds differently to the introduction of specific harvest regulations. Based on a simple 17 genetic inheritance model, we examine mid-and long-term evolutionary changes of the three 18 life-history traits and their impacts on the size spectrum and yield of pike. In general, fishing-19 induced evolution reduces sizes and yields, but details depend on a variety of factors such as 20 the specific regulation in place. We find no regulation that is successful in reducing to zero all 21 selection pressures on life-history traits induced by recreational fishing. Accordingly, we must 22 expect that inducing some degree of evolution through recreational fishing is inevitable. 23 24 4

Evolution of boldness and life history in response to selective harvesting

“…The size-selective term is described by a sigmoidal curve (typical for trawling and long-lining, Myers & Hoenig 1997), switching from zero 308 to one around a size ‫ݓ‬ ி . Variation in mesh sizes or changes in allowable landing size 16 (e.g., minimum or maximum-length limit or the combination, a harvest slot) alters size-310 selectivity (Jørgensen et al 2009;Matsumura et al 2011;Mollet et al 2016) and is simulated by changing ‫ݓ‬ (Figure 2a). To simulate a harvest slot fishery (not to be 312 confused with a protected slot-fishery) where only a certain intermediate size-range is targeted (as is common in gill-nets) the selectivity may again go down to zero at a size 314 ‫ݓ‬ (here taken to be 10 times larger than the size where fishing starts) (Figure 2b).…”

“…caused by fisheries will be magnified if fishing mortality is not only elevated but selective for fitness-related traits, such as body size (e.g., Edeline et al 2007; 70 Matsumura et al 2011;Heino et al 2015).…”

“…Fast life-histories can be characterized by fast juvenile growth, young age and small size at maturation and high reproductive investment, which reduce post-80 maturation growth rate and increase natural mortality. From a management 5 perspective, it is important to understand the population dynamical and social and 82 economic consequences of fisheries-induced evolution, to, if possible, design management tools that minimize undesired evolutionary effects on outcomes valued by 84 humans, such as yield, recovery or the catch prospects of exceptionally large fishes (Matsumura et al 2011;Eikeset et al 2013;Zimmermann & Jørgensen 2017). 86…”

Recreational fishing selectively captures individuals with the highest fitness potential

“…Suitable models to study the potential for fisheries-induced evolution include ecological feedbacks resulting in density-and frequencydependent selection that shapes fitness landscapes and evolutionary responses to fishing (7, 10). Many of these simulation models assume that fitness or fitness surrogates scale positively with body size (e.g., an exponential increase in individual female fecundity with size or a decline in natural mortality with increasing body length) (5,7,12,13). Under such model assumptions, sizedependent fishing mortality often means that larger, more-fit individuals are harvested at a higher rate than smaller, less-fit ones.…”

“…Several empirical studies on wild fish populations, however, have questioned the notion that larger fish generally exhibit higher reproductive fitness (14,15). As a result, although for many fisheries size-selective exploitation is well established (3)(4)(5)7), there is little empirical evidence from wild populations demonstrating that fishing truly targets reproductively more-fit individuals. One can speculate that certain fishing gear might even target less-fit individuals, that is, those that are either competitively inferior or in poorer condition and therefore more prone to attack fishing lures, more likely to encounter passive fishing gear, or less able to evade actively fished gear such as trawls.…”