A short piece in the April Scientific American confirms a hypothesis that Dorothy and I have developed. Namely that the common fisheries management “minimum size” rule is likely to seriously harm populations by selecting for small fish [the survivors]. An excerpt:
Ongoing experiments on captive fish reveal that harvesting only the largest individuals can actually force a species to evolve undesirable characteristics that diminish an overfished stock’s ability to recover, says David O. Conover, director of the Marine Sciences Research Center at Stony Brook University. The results may explain why many of the world’s most depleted stocks do not rebound as quickly as expected.
The genetic effects appeared in Atlantic silversides, a small, usually fastgrowing fish. Conover brought a batch of wild silversides to his laboratory in 1998. He and his students then reared six generations, each time removing the largest 90 percent from one group, the smallest 90 percent from another group, and a random 90 percent from a third.
By 2002 it was plain to see that killing off the largest fish had a dramatic effect. Individuals in that group were only about 70 percent the average weight of their randomly harvested counterparts; they were only 55 percent the weight of survivors in the group where only the largest individuals were spared. Because the compared fish were the same age, the scientists could attribute the shrinkage to selection of genes for slower growth.
Even more alarming, the slower growth came with a suite of deficiencies. Detailed examinations of the fifth- and sixth-generation fish, led by Matthew R. Walsh, now a doctoral student at the University of California, Riverside, revealed that members of the large-harvested group were less willing to forage for food and less able to outwit predators. They also produced smaller and fewer eggs, and a lesser portion of those eggs grew into healthy offspring.
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