Always keeping the largest fish you catch has the unintended consequence of causing the remaining fish population to evolve smaller adult sizes.
Research published in the latest issue of the Proceedings of the Royal Society B: Biological Sciences suggests that this form of human-induced evolution is reversible.
David Conover, Stephan Munch and Stephen Arnott subjected six populations of Atlantic silverside (Menidia menidia) to three forms of size-selective fishing for five generations (each generation lasts about a year): two populations had the largest fish selectively removed (large-harvested), two had the smallest fish selectively removed (small-harvested) and the remaining two had fish of all sizes randomly removed (this was the control experiment).
The authors then allowed the populations to recover for five generations in which no size-selective harvesting was carried out (the fish were still harvested, but those of all sizes were randomly taken).
The authors found that the mean body size of the fish increased and decreased for the small- and large-harvested populations respectively.
After the recovery phase of the experiment, the authors found a slow but significant recovery in the large-harvested population, with body sizes rebounding towards the average; the small-harvested population showed no such recovery.
The authors predict that full recovery of fish size would require approximately 12 generations after harvest ceases, although it is likely that the rate of recovery will slow as size converges near its original state.
The authors continue, This is good news for fisheries managers because it means that evolutionary reversals are possible and not dependent solely on extrinsic selective factors.
The authors temper their optimism with the observation that generation time in many harvested species is generally approximately 3"7 years, meaning that, if our estimates are correct, recovery would take roughly three to eight decades
For more information, see the paper: Conover, DO, SB Munch and SA Arnott (2009) Reversal of evolutionary downsizing caused by selective harvest of large fish. Proceedings of the Royal Society B: Biological Sciences 276, pp. 2015"2020.