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Is man to blame for shrinking fish? scientists ask

It's not a fish tale, it's a fact: Fishermen did catch bigger fish years ago.

For years, the explanation seemed simple: Fishermen had caught all the big ones so only the small fish were left. But now, scientific evidence is stacking up to show that overfishing may be having a more far-reaching evolutionary effect.

With the biggest fish removed from the waters with increasingly sophisticated fishing gear, the smaller fish that are able to slip through the nets take on more dominance in seeding the next generation. Some of those smaller fish are sexually maturing earlier, in turn producing some offspring that are both small and programmed to be young mothers, a potentially dangerous evolutionary trend in some cases.

Boston haddock, which once served as a backbone of New England's fishing industry, is a case in point. In the 1960s, most haddock spawned at age 3 or later. Now, even 1-year-olds are spawning. Cod are also having offspring younger. And on the West Coast, the average size of pink salmon coming back to spawn (which they always do at the same age) decreased 30 percent in 40 years.

"It's like (eliminating) all the people who are 7 feet tall: They will become rarer and rarer and you'll have only small people left. It's fascinating to think humans are having this effect on fish," said Steven Murawski, chief of the population dynamic branch of National Marine Fisheries in Woods Hole, Mass. "And if you look at cod populations and other fisheries, they are maturing earlier. The only common factor is overfishing."

Scientists aren't exactly sure what is happening inside the fish. It's difficult to sort out what is true genetic or evolutionary change and what is a short-term physiological adaptation that will end when fishing pressure does. In some cases, it seems like both may be happening at the same time: Some fish are sexually maturing earlier but also growing faster than their ancestors.

Researchers are racing to find out the answer. As collapsed fishing stocks begin to rebound _ cod, haddock, and summer flounder are making startling recoveries _ how fishing pressures affect stocks will dictate the future of fisheries management.

Some fish may be maturing earlier because competition is less and they are finding more food. If this is true, the trend is not genetic and once populations reach high numbers again, scientists expect fish to eventually mature at traditional levels.

But here's the catch if a genetic change is occurring: Fish that have offspring earlier tend to produce ones that are less viable, a sort of genetic penalty if they spawn too early. In this scenario, just having more fish in the sea may not be the answer. Those fish could continue to produce fewer and fewer fertile offspring until the fishery could be in trouble of collapsing.

"It can be a downward spiral," said Joseph G. Kunkel, a professor at the University of Massachusetts at Amherst studying marine life. "If it's physiological, we'll have the same old codfish one day. But if it isn't it has tremendous consequences for codfish and haddock."

It's not that scientists believe a genetic change is all bad. Some studies show that early spawners can lay eggs repeatedly, and over time the eggs become healthier and produce more and more fertile offspring. Regardless, if it is a genetic change, fishery managers will have to be careful to ensure that there are fish of different ages to ensure the species' survival. That's a move that could keep harvest rates low and fishermen irate.

Of course, not everyone agrees why some fish seem to be maturing earlier or becoming smaller or lighter. Some research has indicated that striped bass, one of the few success stories of fishery management, may be lighter than their ancestors at the same age. But many fishermen scoff at the suggestion it's because of fishing.

"It's only because they don't have a lot of food now," said Anton Stetzko, of Orleans, Mass., who briefly held the world's record for catching the biggest striped bass in 1981. That fish was 73 pounds and Stetzo would regularly catch 40 and 50 pounders back then. Now, he figures the average catch is between 8 and 18 pounds.

The striped bass fishery, which collapsed in the late 1970s and early 1980s, is just making a comeback, he says, so the fish aren't old enough yet to weigh a lot. While he has noticed some small fish, he says it's probably because there are so many striped bass these days that they are having a tough time finding enough food.

"I think the big fish will be back," Stetzo said. "But we have to allow them to grow big."

Scientists say he could be right and add that scores of other reasons are possible for change in fish size or sexual maturity. There may be more pollution in water _ or less _ which could affect fish. Part of the trouble is that no one really knows what the "right" natural environment of fisheries looks like; studies haven't been going on that long and the fish are always changing naturally anyway.

Still, scientists say they are making headway. As stocks rebound, they are carefully studying fish to see if they mature later. The answer may be obvious within a decade, because fish maturity declined noticeably in even shorter time frames: The median age of maturity for Atlantic cod decreased 18 percent between 1986 and 1993, and researchers want to find out if they'll increase maturity rates at as fast a pace.