Fisheries managers frequently set minimum size limits for captured animals, mandating that the smallest--and thus youngest--be freed to allow for full maturation. But findings detailed in the current issue of the journal Science suggest that these regulations may actually be shrinking the average size of wild fish over time. Plucking the largest fish from the gene pool, the authors report, leaves only hereditary information from the smallest fish for the next generation.

To examine the potential evolutionary effects of selective extraction , David Conover and Stephan Munch of the State University of New York at Stony Brook studied a common marine fish called Menidia menidia. Allowing groups of the fish to grow in separate tanks, the scientists removed and weighed the largest 90 percent of individuals from some tanks, the smallest 90 percent from others, and a random 90 percent assortment from the rest. After the remaining fish matured and spawned, the team repeated the process. Initially, the large fish-harvested tanks produced the highest yields. After four generations of such "fishing," however, the total weight of all the fish extracted from the small fish-harvested tanks, as well as the average weight of each creature, amounted to twice that of the large fish-harvested tanks. Additionally, since the reproductive capability of large fish is much greater than that of small ones, small fish-harvesting resulted in more fertile animals. Juvenile survival rates were about the same for all groups, indicating that evolved changes in growth, not viability, caused these results. The findings suggest that in the real world, taking only the largest fish may in the long run result in a calamitous decrease in yield, and thus income, for the entire industry.

Fishing is big business for many coastal communities. "In New York State alone, the commercial fishing, recreational fishing, and the seafood industries make a $11.5 billion contribution to the states economy and employ over 100,000 people," remarks Jack Mattice of New York Sea Grant, one of the funders of this project. A successful industry is based on a healthy fishery, however. "Our study illustrates how well-intentioned management plans that appear to maximize yield on ecological time scales may have the opposite effect after accounting for evolutionary dynamics," Conover notes. The researchers thus propose both creating no-fish areas to prevent an irreversible loss of important genetic diversity, and setting a maximum size limit in addition to the minimum.