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...
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