Throwing Back the Biggest Fish Better for Ecosystem

Scientists in San Diego reported Wednesday that commercial and sport fishing are destabilizing fish populations by targeting the oldest and biggest fish and allowing the faster-growing younger fish to wildly proliferate. Such fishing effectively alters the ‘age pyramid’ of the fish, the researchers said.

The scientists advised fisheries to encourage the taking of younger, smaller fish rather than requiring that they be thrown back.

"That type of regulation, which we see in many sport fisheries, is exactly wrong," George Sugihara of the Scripps Institution of Oceanography at the University of California San Diego said in a statement.

"It's not the young ones that should be thrown back, but the larger, older fish that should be spared. Not only do the older fish provide stability ... to the population, they provide more and better quality offspring," he added.

In conducting the research, Sugihara and his team used a study established by the California sardine fishery after its collapse in the 1940s, and examined 50 years worth of records of fished and non-fished species.

In a report on the research published in the journal Nature, Sugihara wrote that current policies that manage according to biomass targets rather than individual fish size can also destabilize the fish population.

He said a single large fish will simply lose a small amount of weight when food becomes scarce, or grow a little when food is more available.  However, a population of many smaller, younger fish could explode in number or collapse according to the availability of food.

Sugihara remarked that this is particularly important when trying to rebuild fish stocks.

"A high harvest target may be set after an especially abundant period when the population may be poised to decline on its own," he said.

Nils Stenseth of the University of Oslo said commercial and sport fishing practices that emphasize taking only the oldest and biggest fish can actually impose rapid evolutionary changes in the fish populations.

"Many recent studies have provided evidence for this ... effect, and show that the ecological-evolutionary consequences of harvesting can occur at a much faster rate than previously thought," Stenseth wrote in a commentary about the research.