Fishermen generally assume that biggest is best. We use lures and bait and nets that target the larger individuals, and we make regulations requiring smaller fish to be released.
There are unquestionably positive results from some of these regulations, such as allowing individuals of a species to achieve spawning age before they can be taken. But many recent and wide-ranging studies are showing that this approach is also having unintended consequences that are negative.
Fishing pressure on the biggest fish often selects against the traits we favor: fast growth, active feeding behavior and large size. In essence, we are seeing that many fishing strategies are inducing evolutionary responses in fish populations, such as early maturation at small size and reduced reproduction.
In one study on North Atlantic cod, analysis revealed probable genetic changes in growth in this population in response to size-selective fishing. The main question being investigated was why cod stocks have not rebounded in areas where fishing pressure has been reduced. Results indicated that there had been genetic changes in growth rates in this population in response to size-selective fishing, accounting for the continued small size-at-age despite good conditions for growth and little fishing for over a decade.
In another revealing experiment, two small Canadian lakes were stocked with equal densities of two types of rainbow trout. One genotype was selected for its fast growth and aggressive feeding behavior (fast/bold) and the other for traits of slow growth and cautious behavior (slow/shy). The fish were the same size at stocking. Then both lakes were fished intensively, but evenly, with gill nets. Fifty percent of the fast/bold fish were captured, but only 30 percent of the slow/shy fish were taken.
“Given that growth is heritable in fishes, we speculate that evolution of slower growth rates attributable to behavioral vulnerability may be widespread in harvested fish populations,” the authors stated.
“Our results indicate that commonly used minimum size-limits will not prevent over-exploitation of fast-growing genotypes and individuals because of size independent growth-rate selection by fishing.”
The next step will be to apply this knowledge to real-world fisheries management. Some research has already begun on how to best avoid these problems. Each fishery will require a somewhat different approach, since response to selective harvesting depends on specific life histories, varying environments and different community structures. In the future, managers will need to consider these factors of fisheries-induced evolution. Better genetics data will be required, along with sound understanding of existing ecological processes and changing aquatic environments.
Glenn Thomas is an associate professor of fisheries for the Louisiana Sea Grant Program and the Louisiana State University AgCenter School of Renewable Natural Resources.