Fish Evolution Plays Tricks on Fishermen

As fishermen catch the biggest fish, it becomes increasingly "unhealthy" for a fish to grow big. Thus small fishes get to multiply more often than bigger fishes and thus they pass on to the next generation their genes of smallness. It's a classic case of evolution - the fishermen are acting a selective force that gradually changes the size of the fish. In their drive for the best fish they diminish the quality of the fish stock.

According to the study conducted by Matthew R. Walsh of University of California, removing the large fish over several generations has caused the remaining fish in the population to become progressively smaller, have fewer and smaller eggs with lower survival and growth, and have lower feeding rates.

"We have shown for the first time that many traits correlated with fish body-size may be evolving in response to intense fishing pressure," he explained. Researchers conducted harvesting experiments along the east coast of North America over five generations of fish, selecting out the largest individuals from each generation and evaluating multiple traits, such as body size and the number of eggs. They observed declines in many traits spanning the life history, physiology and behavior of the fish.

"We know that commercially exploited populations of fish often are slow to recover when fishing pressure is reduced. Our research indicates that the over-harvested fish stocks are slow to rebound because fishing selects for evolutionary changes in the life history of the fish. Because the changes in the fish are genetic, they don't immediately go away when fishing ceases," he concluded.

This type of problem is known for centuries by the farmers - if for instance one would pick up the wheat with the largest seeds to use for consumption and plant the rest, one would get poorer and poorer quality wheat in just a few generations.

Picture credit: M. Walsh