Fishing throws targeted species off balance, Scripps study shows

Researchers say fishing disrupts age structure, making regulation difficult

Man looking at large tuna. Click here for more information.

Fishing activities can provoke volatile fluctuations in the populations they target, but it’s not often clear why. A new study published in the journal Nature by scientists at Scripps Institution of Oceanography at UC San Diego and colleagues has identified the general underlying mechanism.

Research led at Scripps with a distinguished team of government and international experts (including two chief scientific advisors to the United Kingdom) demonstrates that fishing can throw targeted fish populations off kilter. Fishing can alter the “age pyramid” by lopping off the few large, older fish that make up the top of the pyramid, leaving a broad base of faster-growing small younglings. The team found that this rapidly growing and transitory base is dynamically unstable—a finding having profound implications for the ecosystem and the fishing industries built upon it.

This schematic outlines variability on exploited and unexploited. Click here for more information.

“The data show that fished species appear to be significantly more nonlinear and less stable than unfished species,” said Professor George Sugihara of Scripps. “We think the mechanism involves systematic alteration of the demographic parameters—and especially increases in growth rates that magnify destabilization in many ways—which can happen as fishing truncates the age structure.”

Imagine a container of water with a 500-pound fish. With food, it grows a little bigger. Without food it gets a bit smaller. Imagine the same container with 500 one-pound fish. They eat, reproduce and the resulting thousands of fish boom, quickly outstripping the resources and the population crashes. These many smaller fish—with the same initial “biomass” as the larger fish—can’t average out the environmental fluctuations, and in fact amplify them through higher turnover rates that promote boom and bust cycles.