BREWSTER, Mass. – For years, the reason today’s fishermen caught smaller fish than their predecessors seemed simple: All the big ones had been caught, so only the small fish were left.

Now scientific evidence shows that overfishing may be having a more far-reaching evolutionary effect.

The biggest fish are removed from the waters with sophisticated fishing gear. The smaller fish can slip through the nets and can take on more dominance in seeding the next generation.

Some of those smaller fish are sexually maturing earlier, in turn producing offspring that are both small and programmed to be young mothers. That can be a potentially dangerous evolutionary trend in some cases.

“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,” Steven Murawski, chief of the population dynamic branch of National Marine Fisheries in Woods Hole, told The Boston Globe. “And if you look at cod populations and other fisheries, they are maturing earlier. And the only common factor is overfishing.”

In the 1960s, most Boston haddock spawned at age 3 or later.

Now, even 1-year-olds are spawning. Cod are also having offspring younger.

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.

Scientists say it is 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.

Some collapsed fishing stocks, such as cod, flounder, and haddock, are making startling recoveries. Scientists say fishing pressures that 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.

If a genetic change is occurring, however, there is a potential problem: Fish that have offspring earlier tend to produce ones that are less viable.

Those fish could continue to produce fewer and fewer fertile offspring until the fishery industry could be in danger 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.”

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, who briefly held the world’s record for catching the biggest striped bass off Nauset 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 somewhere between 8 and 18 pounds.

He says it’s probably because there are so many striped bass these days that they are having a tough time finding enough food right away.

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