Published on Mon, Feb. 21, 2005

Big, fat, older fish -- the very animals targeted under current fishing practices -- give birth to far more and healthier offspring than younger fish.

This suggests a sharp reversal of traditional fisheries management practices that enforce minimum catch sizes to remove adults while leaving smaller, younger animals to repopulate the species.

"I was astounded by the results," said Steven Berkeley, a fisheries biologist at the University of California at Santa Cruz, who presented his findings at a weekend conference in Washington. His results were published in May.

It was well known that bigger fish produce more offspring, but not that those offspring would be better survivors.

Comparing the larvae from 5-year-old to that from 17-year-old black rockfish, Berkeley found that offspring from the oldest fish grew more than three times faster than those from the youngest fish. The older fishes' larvae were also hardier, able to survive twice as long without food.

He said he has since repeated the experiments with other species of rockfish and seen a similar trend. He suspects other marine species show similar behavior.

"Big mothers make better babies -- that's what Steve has been able to show," said Stephen Palumbi, a marine biologist at Hopkins Marine Station in Pacific Grove who attended the conference.

The findings have created widespread praise for what rockfish scientists now refer to affectionately as "fat, fertile females."

The future of big fish may be threatened. On Friday, David Conover of the State University of New York in Stony Brook presented evidence that fishing is causing evolutionary changes that favor small fish.

In heavily fished species, as many as three out of four animals will end their life in a trawl net. This makes fishing the greatest pressure selecting for particular traits. In this situation, a fish that grows slowly or matures early has a distinct evolutionary advantage because it can produce more offspring before being caught. Also, fish that are naturally petite can slip through nets.

"What we do in fishing is make large size a liability, rather than a benefit," Conover said, "the opposite from in nature."

He raised Atlantic silverfish, Menidia menidia, in the lab. By continuously removing the largest animals, he was able to breed fish that had slower growth rates, lower feeding efficiencies, and smaller egg size than the original population.

"A simple process like size selection caused changes in many different traits," Conover said. "And changes in those traits have a very important impact on fish populations."

The combined findings -- that older fish are the most valuable and that their genes are being selected out of the population -- could have important consequences.

"If fishing continues over generations, we can cause evolutionary changes that favor slow-growing, smaller fish," Berkeley said. In the long term, this could mean that fishing effects would not be reversible, and depleted fish stocks would be slow to rebound.

The North Atlantic cod fishery has been closed for 13 years and fish have still not returned. Jeffrey Hutchings of Dalhousie University in Halifax, Nova Scotia, presented evidence Friday showing decreased size among the remaining fish. He said it now seems "highly probable" that evolutionary changes in the cod population are among the reasons for the slow comeback.

A possible management strategy to reverse current trends and bring back large, productive fish would be to introduce minimum and maximum size limits, or what are called "slot" limits. With rockfish this might be problematic, however, since the animals' swim bladders expand as they are hauled to the surface. Even if large fish were released on capture, Berkeley said, they would likely not survive.

He and Palumbi both favor marine protected areas that would allow natural populations to recover. These areas were mandated by the Marine Life Protection Act in 1999, but are still under discussion.

Zeke Grader, executive director of the Pacific Coast Federation of Fishermen's Associations, said by phone that he has not heard about the rockfish findings, but that the council has adopted upper size limits for other species. His group is working to get more money for fish assessments to increase knowledge for sustainable management.

"A lot of these (historic) fishery collapses, it wasn't so much out of greed, but out of ignorance," Grader said.