Many previous studies looked at fisheries' impacts over time using records of the amount of fish harvested. But such studies have a major limitation.
"You can't make any comparison between harvested and nonharvested fish," Sugihara said.
Without such a benchmark, he says, it is impossible to know if changes seen in harvested populations are due to fishing or to unrelated changes in the environment.
To get around this problem, Sugihara's team used data from larval fish surveys conducted annually since the 1950s in the waters off California.
Their work, which appears in tomorrow's edition of the journal Nature, provides the first hard evidence that fish species targeted by commercial fishers experience much higher variability in abundance than nonharvested species.
The data come from the California Cooperative Oceanic Fisheries Investigations (CalCOFI) program, which was launched in 1949 to better understand the collapse of California's sardine populations.
(Related news: "Fisheries Ebb and Flow in 50-Year Cycle, Study Says" [January 9, 2003].)
CalCOFI gave the present-day researchers exactly what they needed: a long-term series of data for 13 commercially fished and 16 unharvested fish species.
"It's like having a 50-year-long controlled ecological experiment," Sugihara said. "Both the unexploited and the exploited species were experiencing the same environment."
All of the fish species showed responses to periodic changes in ocean currents and temperatures, such as those triggered by the El Niño phenomenon—a periodic warming in the equatorial Pacific Ocean.
(Related feature: "El Niño/La Niña" in National Geographic magazine.)
But the highs and lows of harvested populations appeared to be amplified well beyond the range of variability of nonharvested species.
Statistical analysis ruled out the effects of different physical and behavioral characteristics between the two groups.
That meant that fishing pressure was the only explanation for the population swings in commercially fished species such as Pacific chub mackerel, Pacific sardine, and kelp bass.
The study may have several important implications for fisheries management.
Instituting maximum size limits, for example, could help ensure that the age structure of a harvested population is maintained.
Roger Hewitt directs fisheries resources at the National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center in La Jolla, California.
Hewitt praised the new study and the CalCOFI long-term survey program. But, he says, it is not yet clear how the findings may ultimately affect management regulations.
"I can imagine harvest strategies that would try to maintain the age structure, but they would be difficult to put in place," Hewitt said.
Sugihara agrees, and says that a more practical solution might be a system of marine reserves, in which populations of all species would be free from fishing pressure.
Such areas are already a top priority for conservationists concerned about decreasing fish populations worldwide.
Sugihara adds that his team's findings may strengthen the argument for ocean reserves—especially given the likelihood of increased rates of environmental change due to global warming.
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