Reversal of evolutionary downsizing caused by selective harvest of large fish

Abstract

Evolutionary responses to the long-term exploitation of individuals from a population may include reduced growth rate, age at maturation, body size and productivity. Theoretical models suggest that these genetic changes may be slow or impossible to reverse but rigorous empirical evidence is lacking. Here, we provide the first empirical demonstration of a genetically based reversal of fishing-induced evolution. We subjected six populations of silverside fish (Menidia menidia) to three forms of size-selective fishing for five generations, thereby generating twofold differences among populations in mean weight and yield (biomass) at harvest. This was followed by an additional five generations during which size-selective harvest was halted. We found that evolutionary changes were reversible. Populations evolving smaller body size when subjected to size-selective fishing displayed a slow but significant increase in size when fishing ceased. Neither phenotypic variance in size nor juvenile survival was reduced by the initial period of selective fishing, suggesting that sufficient genetic variation remained to allow recovery. By linear extrapolation, we predict full recovery in about 12 generations, although the rate of recovery may taper off near convergence. The recovery rate in any given wild population will also depend on other agents of selection determined by the specifics of life history and environment. By contrast, populations that in the first five generations evolved larger size and yield showed little evidence of reversal. These results show that populations have an intrinsic capacity to recover genetically from harmful evolutionary changes caused by fishing, even without extrinsic factors that reverse the selection gradient. However, harvested species typically have generation times of 3-7 years, so recovery may take decades. Hence, the need to account for evolution in managing fisheries remains.

Human-induced evolution caused by unnatural selection through harvest of wild animals

“…That is, time scales of evolutionary recovery are likely to be much longer than those on which undesirable evolutionary changes occur. Conover et al (101) provided the first experimental test of this expectation with laboratory populations of Menidia menidia. They found that the selective effects of fishing were reversible, but recovery took more than twice as many generations as the original period of fishery selection.…”

Recreational fishing selectively captures individuals with the highest fitness potential

“…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.…”

A Systematic Overview of Harvesting-Induced Maturation Evolution in Predator–Prey Systems with Three Different Life-History Tradeoffs

“…It is therefore not surprising that harvesting may cause phenotypic changes in heritable life-history traits in both terrestrial (Allendorf et al, 2008;Coltman et al, 2003) and marine species (Jørgensen et al, 2007). Concerns are mounting about the time it may take to reverse such adaptive changes (Conover et al, 2009;Law and Grey, 1989), if such reversals are feasible at all (de Roos et al, 2006). At this time there is no conclusive genetic evidence of harvesting-induced evolution, however, a growing number of empirical and experimental studies suggest that anthropogenic size-selective mortality in exploited wild populations can cause adaptive changes in traits influencing growth and maturation (Coltman et al, 2003;Jørgensen et al, 2007).…”