Washington - An international team of researchers has demonstrated that commercial fishing compromises a species’ ability to buffer itself from environmental variations such as El Niño, resulting in a “double jeopardy” situation in which populations are depleted by the fishing itself and also made more vulnerable to threats other than fishing.

The availability of a 50-year larval fish survey of waters off California, begun when local sardine populations collapsed in the 1940s, was instrumental in the study. The survey provided the data supporting the study, which determined that fishing reduces a species’ resilience in ways that extend beyond the simple removal of individual fish from the population. The findings suggest that fisheries must be managed to maintain total biomass and the age structure of a population.

The research, published in the October 19 issue of the journal Nature, was conducted by scientists at the Scripps Institution of Oceanography at the University of California at San Diego; the Southwest Fisheries Science Center of the U.S. National Marine Fisheries Service; Imperial College, London; and the University of Oxford.

The work was funded by the National Science Foundation (NSF), the National Oceanic and Atmospheric Administration’s National Marine Fisheries Service, the Deutsche Bank Complexity Studies Fund and the Sugihara Family Trust.

The researchers tried to address a long-standing debate about the extent to which commercial fishing affects targeted fish populations, and the relative contributions of fishing and environmental factors to declines in the populations of commercially important species in recent decades, according to the NSF.

The study demonstrated that fishery exploitation not only causes declines in fish stocks but also amplifies natural population variability and increases the risk of fishery collapse.

"We found that the variability of the targeted populations was much higher, meaning that fishing tends to amplify both the peaks and the valleys of population numbers," said George Sugihara, a co-author of the paper and an oceanographer at Scripps. "Fishing can potentially not only lead to declining stock levels, but cause populations to fluctuate more through time, which could put them at greater risk of collapse than previously thought."

The researchers differentiated between fishing and nonfishing effects by analyzing the populations of exploited and unexploited species living in the same environments. This comparison cannot be made with traditional fisheries data based on "landings" (or catch) records because there are no landings records for unfished species. The comparison can be made with data on larval abundances of both fished and nonfished species, using larval abundance as an indicator of adult abundance.

The scientists reported that fished populations tend to show more extreme variations because fishing selectively culls the larger, older individuals - the fish that are most able to buffer random environmental variations. Those individuals also tend to be the most reproductively active. In a fished population, the average size and age of individuals decline and the resultant stock of near-juveniles is less able to cope with environmental stresses.

"This suggests that fisheries need to be managed not only to maintain a harvest target or total biomass level, but also to maintain a certain age structure in the stock," said Sugihara, who added that because the fluctuations identified precede systematic declines of populations, they can be viewed as a kind of early warning sign prior to collapse.

Philip Taylor, director of NSF's Biological Oceanography Program, said the study demonstrates the “significant value in conducting long-term ecological research in the oceans.”

The full text of the press release is available on the NSF Web site. Additional information about the study is available on Nature’s Web site.