Let the big ones get away?
By JOHN RICHARDSON, Staff Writer March 4, 2009
Commercial fishing might be causing genetic changes in fish that swim in the ocean, making them smaller and less fertile.
The latest evidence comes from a laboratory study by New York-based researchers being published today in Proceedings of the Royal Society B, a British scientific journal. The study concludes that taking too many large fish out of a population leads to the birth of smaller fish over time.
However, the study also found that fish can grow larger again if the big ones are allowed to get away.
"There is building evidence that it's taking place in the wild," said David Conover, dean of Stony Brook University's School of Marine and Atmospheric Sciences and lead author of the study. Fishery conservation rules should be rewritten to protect older, larger fish instead of only the smaller ones, he said.
This study and others like it could have implications for fishing communities and regulators in Maine and other New England states.
New England's 400-year-old fishery for cod, haddock, flounder and other so-called groundfish has been struggling to rebuild depleted populations for decades. Efforts to bring back the fish still include rules – such as minimum sizes and large-mesh nets – that encourage fishermen to catch and kill the largest fish and spare the smaller ones.
That is sending the wrong message to the fish, genetically speaking, according to Conover, whose study was supported by the university's Institute for Ocean Conservation Science.
Conover worked with Atlantic silversides, a small ocean fish that produces a new generation every year. Other fish, such as cod, have a three- to five-year generational span, still allowing relatively rapid genetic changes.
Conover removed all the large silversides from one population – similar to the effect of intensive commercial fishing – and removed the smaller ones from a second group. After five years, fish in the first group were smaller than normal and half the size of the same-age fish in the second group, he said.
Shrinking fish sizes also mean a population reproduces at a slower rate, something that makes it more vulnerable to natural pressures such as predation and less able to recover from overfishing, Conover said. Larger fish are generally much more fertile than smaller ones.
"You've got to have old fish out there every year reproducing," he said.
Once the fish in the lab were allowed to survive to larger sizes, they experienced a slow evolutionary rebound in size, Conover said. The finding indicates changes can be reversed in the wild, too, but slowly.
"It took us five generations to create the differences (in the lab), but it's going to take 12 generations to recover," he said.
The study builds on a growing body of research that humans are driving rapid evolutionary changes in all kinds of creatures. Bighorn sheep, for example, now have smaller horns and are less likely to get shot by trophy hunters, according to one such study.
Conover said researchers are increasingly convinced that the same shrinkage in fish size is happening in the ocean with commercially valuable fish species.
"If you take a species like cod that has been heavily fished for centuries, the average size of the fish in the ocean is now, at best, probably half the size of the average fish in the historical cod population," he said.
New England cod can grow to more than 100 pounds, at least historically. One 180-pound cod was caught in 1838, and a 6-foot-long, 211-pound cod was landed in 1895, according to a University of New Hampshire report and other research.
A 10-pound cod is considered large in today's market, and one weighing 40 pounds or more is considered unusually large, fishermen say. The recreational record for a cod hauled in by rod-and-reel is a 77-pounder caught in 1989.
Conover said the mounting evidence should change the way fish
are managed. His study suggests that both the largest fish and the smallest fish should be kept out of fishing nets or thrown back alive.
"You harvest the fish that are in the middle of the range," he said. "From the fish's point of view, if people are going to harvest medium-sized fish, then the best strategy for that fish is to grow rapidly and get through that slot."
Conover's idea is similar to the way Maine's lobster fishery works: Both small and large lobsters have to be thrown back. Scientists believe one of the reasons that lobster populations have remained relatively healthy is the protected brood stock of large lobsters, which, like fish, produce many more babies as they get older.
The idea of setting a maximum size is clearly trickier for fish caught in nets, although fishermen could, in theory, use an excluder grate to let larger fish escape.
Maine fishermen say it's true there haven't been many 100-pound cod around for a while. But they are dubious that evolution is the reason.
"If you don't let the fish grow up (by catching it), it never gets big. If you let the fish grow up and they don't get big, something's changed. We're still in the first stage," said Ted Ames, a fisherman in Stonington who has studied historical fishing grounds along the Maine coast.
"What you have is a screening system that catches everything larger than the minimum-size fish," he said. "I'm not convinced that if, for example, you allow codfish to grow that they would not grow to be incredibly good-sized."
Glen Libby, a 30-year fisherman from Port Clyde, said there are a lot of factors at work in the ocean, such as predators, food abundance and habitat changes.
"The 100-pound cod? I haven't seen one," Libby said. But "I'd hesitate to jump on the bandwagon and say we killed off all the big fish genes. It could be happening, I guess."
Federal scientists also are not yet convinced.
"It's kind of a clouded issue in nature, for a lot of reasons. There are so many more variables," said Teri Frady, spokeswoman for the National Marine Fisheries Service. "You need to see what happens in the absence of high fishing pressure, because you can't tease out what's causing what."
Conover, meanwhile, said he's hopeful that fisheries management will evolve, too.
"It is still a new idea," he said. But "if you wait till you drive the stock all the way down you're going to wait a long time for a big fish to reappear again because you very likely have caused evolutionary changes."
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