4 November 2002

Overfishing Threatens Genetic Diversity

by Kate Melville

Populations of marine fish may lose genetic diversity even if fishing stops while there are still several million individuals - a number previously assumed to be enough to preserve a diverse gene pool. Losing the diversity of key genes can render a population less productive and unable to adapt when faced with challenges such as global warming, pollution or changes in predators or prey. Rare genetic variation of little importance today might be the key to adaptability in the future, according to Lorenz Hauser, University of Washington assistant professor with the School of Aquatic and Fishery Sciences and lead author of a report recently published as part of the Proceedings of the National Academy of Sciences.

It could be that genetic diversity in a population needs to be considered when determining sustainable harvests for marine fish - fish such as snappers, rockfish and cod that spend their entire lives in the ocean.

For some animals studied by conservation biologists, such as pandas and elephants, as few as 500 individuals appear to be enough to maintain rare variances.

For reasons not yet understood, however, it appears that in marine fishes only a small proportion of individuals produce large numbers of offspring that survive. Therefore as a fish population declines the number of such capable breeders may reach levels that cannot sustain genetic diversity.

What Hauser and his co-authors found is that the number of capable breeders is several magnitudes smaller than they expected: In their work with a population of New Zealand snapper that was fished down to about 3 million, only one in 10,000 fish was a capable breeder. That means the genetic diversity for a population of a few million fish could be depending on as little as a few hundred fish.

If such low ratios are commonplace in marine species, many other kinds of marine fish stocks may be in danger of losing genetic variability, the paper says. Fish scales collected and archived at the New Zealand Ministry of Fisheries provided DNA from two isolated populations of New Zealand snappers for the research. The smaller population from Tasman Bay lost variations in six of seven gene markers as its numbers declined to 3 million while exploited between the 1950s and 1998. A larger population from Hauraki Gulf, composed of 37 million fish, showed no loss in variation between the 1950s and 1998. However, it had been fished since the late 1800s and by 1950 already had less genetic variation compared to the Tasman Bay population.

From a practical standpoint, fisheries managers are not going to be able to monitor genetic diversity for every stock, Hauser says. Instead, if scientists can learn which fish are the very successful breeders and why, then managers may be better able to predict the number of offspring produced and thus may be able to increase fishing when conditions are favorable for successful fish and decrease fishing when they aren't.