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.

Figure 1

Trends in mean length across five generations with size-selective fishing followed by five generations without selective fishing. (a) Mean length at day 90 (end of the larval stage). (b) Mean length at day 190 (harvest). Squares represent the small-size harvested populations, triangles are the randomly harvested controls and circles are the large-size harvested populations. Connecting lines represent the six separate populations with two replicates per treatment. Plotted are mean lengths as differences from the mean for the randomly harvested lines within each generation. Regression analysis (solid thick lines) showed that length at days 90 and 190 increased significantly after selective fishing ceased in the large-size harvested lines (day 90 slope=0.49, s.e.m.=0.18, two-tailed t-test, n=12, p=0.03; day 190 slope=0.86, s.e.m.=0.36, two-tailed t-test, n=12, p=0.04) but did not change significantly in the small-size harvested lines (day 90 slope=0.15, s.e.m.=0.11, two-tailed t-test, n=12, p=0.19; day 190 slope=-0.19, s.e.m.=0.30, two-tailed t-test, n=12, p=0.36).

Figure 2

Trends in the standard deviation (s.d.) of mean length for the six populations throughout the experiment. (a) Day 90 (end of the larval period). (b) Day 190 (harvest). Symbols are as defined in figure legend 1.

Figure 3

Mean daily mortality during days 90–190 in the six populations throughout the experiment. Symbols are as defined in figure legend 1. Vertical lines represent the range of the two replicates within each treatment.