The 10-year fish size study experimented with Atlantic silverside (Menidia menidia). (Photo: Stony Brook University)

Small, stunted fish can bounce back to normal size: study

UNITED STATES
Wednesday, March 04, 2009, 21:30 (GMT + 9)

A tendency for commercially sought fish to shrink in body size and number - caused by fishers targeting the biggest fish in the sea - can be reversed, concluded a new 10-year study published this week in the journal Proceedings of the Royal Society B. 

Previous studies have found that animals harvested from the wild are decreasing two and a half time the natural rate, mostly as a result of humans catching the biggest specimens with highly efficient gear and technology, The Scientific American reports.

Funded by the Institute for Ocean Conservation Sciences, the new study shows the tendency is reversible but that a return to normal size would probably take more than twice as long as the original downsizing.

"There's a good news story in that the evolutionary changes are not permanent — on a contemporary timescale," said David Conover, lead study author and a Stony Brook University marine professor. "But the bad news is that it's slow."

Marine researcher David Conover. (Photo: Stony Brook University)

University scientists tested 10 generations of Atlantic silverside (Menidia menidia) that had been captured off the coast of New York state and introduced to laboratory tanks. They found that the group of fish that remained following the removal of the largest specimens experienced a decrease in average body size during the first five generations, or 5 years.

Starting with the sixth generation, fish were removed at random. By the 10th and final generation, the previously small-sized fish groups had almost returned to average size.

Based on the results of the experiment, researchers estimate that it would take about 12 generations for the body size of the shrunken group of Atlantic silverside to completely return to normal. However, recovery time for longer-lived fish species like cod, which has five-year generations, could take 60 years or longer. 

The team of scientists also switched the experimental test on other groups and removed the smallest fish. In this case, the body size of the fish actually expanded and did not decrease in a long-term situation of random harvesting.

Theoretically then, Conover suspects, wild fish probably would return to their specifically evolved size as they faced food shortages and the presence of natural predators not replicated in the lab.

"We really didn't think the recovery would be fast enough to be measurable in five generations," Conover said.

Chris Darimont, an evolutionary ecologist at the University of California Santa Cruz, believes the next step would be to carry out similar experiments in the wild, as a recovery to full size might depend on other factors, including remaining genetic diversity and natural predators.

"I think [a similar 12-generation recovery rate] might be highly variable, among even fish species," he said, adding that there are still too many marine variables to test for.

Conover believes size changes "definitely" stem from genetic changes rather than plasticity (individual adaptation to the environment), which is still debatable among marine biologists.

The study's findings may eventually benefit natural marine predators. Larger fish such as Pacific salmon, bluefin tuna and even Mediterranean dolphins, are stressed and underweight due to the overfishing of their preferred prey, states a recent report issued by Oceana, a US environmental group.

Despite the positive findings, Conover believes "it is still better to avoid these evolutionary changes in the first place." He and Darimont agree that commercial rules should be changed to require fisheries to only harvest smaller and mid-sized fish.

By Denise Recalde
editorial@fis.com
www.fis.com