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| Fewer fish: Fishermen unload tuna on North Carolina's Outer Banks. Experts give various reasons – notably overfishing in European
waters and shifting currents – for reduced catches in the US. tyrone turner/national geographic/getty images |
Can the world act fast enough to save the disappearing tuna?
Scientists say drastic measures need to be taken to restore the bluefin.
from the December 27, 2007 edition
Page 3 of 3
The few tuna that still swim the western Atlantic are showing up quite lean. "The quality of the fish that I've seen has definitely declined over the last few years," says Bob Campbell, manager of the Yankee Fisherman's Cooperative in Seabrook, N.H.
After analyzing 14 years' worth of Mr. Campbell's logbooks, Walter Golet, a PhD candidate in Lutcavage's lab, agreed. Bluefin "come here for one reason – they come here to feed," says Mr. Golet, who recently published his findings in the journal Fishery Bulletin. But "they don't seem to be obtaining the forage that they were getting 10 to 15 years ago." (This year, which was not included in the study, has seen a slight rebound in tuna quality, says Campbell.)
This thinning has some wondering about what bluefin eat, particularly herring, a keystone species in the northern Atlantic.
Herring biomass seems high, similar to the way it was before the advent of modern industrial fishing fleets in the 1960s. But both fishermen and conservationists consider this five-inch-long fish important enough that they've formed an alliance devoted to its protection. Known as CHOIR, the Coalition for the Atlantic Herring Fishery's Orderly, Informed, and Responsible Long-Term Development, has repeatedly called for large herring trawlers to leave New England waters. The coalition fears that if herring numbers greatly diminish, everything that feeds on them, from tuna to cod, will suffer.
Indeed, while herring biomass seems healthy, scientists have noted that an individual four-year-old herring is smaller than in the past. "The size at that age has decreased over the last decade or so," says Bill Overholtz, a senior scientist with NMFS in Woods Hole, Mass. The most obvious cause is the "density effect," he says: There are so many herring that each one eats less, making them smaller. And smaller herring could mean that feeding tuna have to work harder for each meal.
Climate change may shrink stocks
But some think that changing oceanic conditions driven, perhaps, by global warming, may be responsible for the tuna's woes. Warmer temperatures explain the abundance of young tuna in the Gulf of Maine. "These are fish normally found off the mid-Atlantic states," says Lutcavage. "Are these fish shifting north because of ocean warming?" (Mayhew says they've always been abundant, but they never return as adults.) Climate change could also explain why Canadian fishermen continue to catch bluefin while their US counterparts don't. "It may be that since herring and bluefin are associated with cooler water … those conditions are better farther north than they used to be," says Dr. Overholtz.
A warming Arctic could also be having a more direct effect on the food chain. In the late 1990s, the western North Atlantic saw an increased inflow of low-salinity water from, scientists think, melting ice and permafrost. (The influx seems to have tapered off more recently.) Andrew Pershing and Jeff Runge, two marine scientists who divide their time between the University of Maine in Orono and GMRI, suspect that the meltwater caused an algal bloom.
Weighing less than the surrounding ocean, the low-salinity water formed a distinct layer at the ocean's surface. Usually limited by churning water, the algae had increased access to sunlight in this relatively still layer. Small shrimp-like creatures called copepods then gorged on the algae, and grew in number. Herring fed on the copious copepods, and multiplied as well. But a decade later, "there are so many adult [herring] now that they're reducing their own food," says Dr. Pershing.
Another possibility: More arctic meltwater knocked the herring-copepod feeding cycle out of synch. Copepods have several stages of development. Not coincidentally, herring spawn when copepods are most fatty – or at least they used to. The freshwater pulses, and perhaps changing wind patterns, may have reduced the number of fat copepods when the herring arrive to feed.
"In the past few years, the abundance of these [fatty] copepods has been low," says Runge. "It may be [that] the numbers ... that are present when the herring need them is changing."
Runge has begun higher frequency plankton sampling from a greater number of sites in the hope of better gauging the climate-plankton link. He's also looking at how copepod and herring abundance relate. Pershing hopes to determine if the changes observed in the Gulf of Maine hold across a greater area. And Golet plans to analyze herring logs the way he did for tuna. Results indicating a decline in herring quality that jibes with other changes in the North Atlantic would greatly solidify these hypotheses.
But the greater story is one of what Perkins calls "a transition to a more systems approach to ecosystem management."
Take everything, from shifting currents and plankton blooms to tuna, sharks, and humans, into account when setting quotas, he says. Otherwise, "When you look at a species in isolation, you miss really important factors," he says. "You could miss the problem or construct a solution that hurts rather than helps."
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