George Nardi, vice president and director of a commercial fish farm in Newington, N.H., says the problem makes breeding the fish a tricky proposition.

"We invest in our brood stocks, the parents of the young fish, much as a thoroughbred horse farm invests in mares and stallions," said Nardi. "It doesn't do us much good if we always have to go out and get new females."

Berlinsky and his colleagues so far have discovered the fish are more likely to become males if raised at constant temperatures or when kept in crowded tanks.

Berlinsky is also collaborating with Canadian researchers to determine the biochemical mechanisms causing the gender changes.