Fish Species Face "Double Jeopardy" Due to Overharvesting, Study Says

Scott Norris
for National Geographic News
October 18, 2006
Commercial fishing policies should stop picking on larger, older fish, a
new study suggests.

Letting more of the big ones get away may help prevent heavily fished populations from swinging dangerously through cycles of boom and bust.

Such swings are a characteristic—but previously neglected—impact of commercial fishing, according to researchers who conducted the first systematic comparison of population changes in harvested and nonharvested fish species.

Fisheries managers have long encouraged the industry to target more mature fish, on the assumption that the younger generations left behind would thrive and ensure a stable population.

But big fish, the study authors say, may help buffer populations from the effects of random environmental variation, such as disease, pollution, or shifting water temperatures (related news: "Lice From Fish Farms Killing Wild Salmon" [October 2, 2006]).

Larger individuals are "better able to withstand lean times and are responsible for the lion's share of reproduction," said study co-author George Sugihara of the Scripps Institution of Oceanography in San Diego, California.

He says that stocks in which the big fish have all been caught face a kind of double jeopardy: The risk posed by long-term declines is magnified by erratic, shorter-term fluctuations.

The study, co-led by Sugihara and Scripps colleague Chih-hao Hsieh, implies that some overfished populations may be closer to collapse than had previously been thought.

But the finding also points to a new way of diagnosing problems in fisheries before it is too late.

"You might be able to use change in age structure and increase in variability as an early warning sign that a fishery is in trouble," Sugihara said.

Half a Century of Data

For years researchers have been struggling to understand exactly how commercial fishing affects fish populations.

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.

Restoring Order

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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