Can the world act fast enough to save the disappearing tuna?
Scientists say drastic measures need to be taken to restore the bluefin.
(Page 4 of 4)
Climate change may shrink stocksSkip to next paragraph
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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 â€¦ 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."