WASHINGTON, D.C. - The number of adult, female rockfish that successfully breed to help replenish the stocks of marine rockfish comprise less than 1 percent of the total fish population, new genetic studies have shown.

These findings were presented over the weekend at the annual meeting of the American Association for the Advancement of Science, and suggest that even though many females may spawn, the fish that survive to adulthood apparently are descended from just a tiny number of individuals.

"We already know that older fish are generally more prolific spawners, but even within this group, this research has found that the number of fish that actually breed successfully is much smaller than we thought," said Daniel Gomez-Uchida, a fisheries researcher at Oregon State University.

In studies that examined the genetic heritage of darkblotched rockfish off the Oregon coast, the entire fish populations could be linked to as few as two fish per 1,000 adult female breeders, or as little as 0.2 percent of the total population of fish, the OSU scientists have concluded.

Similar numbers have also been found in commercially important groundfish stocks ranging from red drum in the Gulf of Mexico to the New Zealand snapper and the Atlantic cod in British waters.

"The fact that in every generation a very small minority of the spawners is responsible to replace the majority of the population has profound consequences for the management of harvested populations," Gomez-Uchida said. "We could potentially identify the portion of the population that successfully breed, and target these fish for management protection."

There are many factors that can cause mortality among young fish, including predation, ocean conditions such as temperature and oxygen level, circulation patterns, starvation and other issues. But fishery managers in the past had assumed that all spawning fish had more or less an equal chance of producing young fish that would survive to adulthood. That apparently is not the case.

This research was done with molecular techniques such as polymerase chain reaction, along with population genetics and demography theory.

Many groundfish stocks on the West Coast of North America, and also in other places in the world, have been in serious decline in recent years, forcing closures or severe fishing restrictions in some cases.

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Daniel Gomez-Uchida, 541-758-8379