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Genetic Diversity Threatened by Small Populations

When does a fish stock become endangered by overfishing? Recent research suggests it could be when the population is still much larger than was previously thought necessary to maintain a sustainable population. There is evidence to suggest that ocean populations of 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.

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 found is that the number of capable breeders is several magnitudes smaller than expected: In a population of New Zealand snapper that was fished down to about three 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.

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.