Warming oceans are changing the mix of species in the world's fisheries as fish try to remain in waters in their preferred temperature range, according to a new study.
The movement to keep pace with preferred temperatures shows up most starkly in the northeastern Pacific Ocean and the northeastern Atlantic Ocean, as fish migrate out of the subtropics to beat the heat.
The changes have particular implication for people living in the coastal tropics who either subsist on fishing or fish commercially, the research team says. If ocean temperatures continue to warm there, the heat could top a level that even tropical species find intolerable, reducing their abundance, the researchers say.
This raises the urgency of adopting approaches that minimize other stresses on fisheries, such as pollution and overfishing, the team says.
Marine-ecosystem models have indicated that global warming's impact on ocean temperatures would trigger such a migration. And studies of individual regions have documented the arrival of species from warmer aquatic climes.
This latest effort represents the first attempt at documenting the changes for the planet as a whole, says William Cheung, a scientist with the Fisheries Centre at the University of British Columbia in Vancouver, who led the team. The techniques that the team used, along with the results, appear in Thursday's issue of the journal Nature.
The general pattern reported in the study is "very similar" to results from studies that have focused on the US Northeast's fisheries, says Michael Fogarty, who heads the ecosystem assessment program at the National Oceanic and Atmospheric Administration's Northeast Fisheries Science Center in Woods Hole., Mass.
Off the New England coast, for instance, marine scientists tracked migration trends for 36 fish species and found that 75 percent had moved north or into deeper water or both to keep their cool, Dr. Fogarty says.
At the same time, "the Atlantic croaker, a subtropical species, is moving north and is having higher reproductive success as well" in northern waters, he says.
Meanwhile, fishermen in the Gulf of Maine are reporting highly unusual species for the area: black sea bass, which could earn them a tidy sum; new species of squid; and blue crabs, Fogarty adds.
The work by Dr. Cheung and colleagues "is a very interesting study, and its global reach makes it quite important," he says.
The study covers a period spanning 1970 to 2006. The team examined catch records compiled by the United Nations Food and Agriculture Organization, as well as from regional and national fisheries groups. The researchers divvied the catch data among 52 large marine ecosystems – for example, the US Northeast's continental shelf, the North Sea, or ecosystems defined by currents such as the Canary Current (a segment of a much larger North Atlantic surface current that skirts the Canary Islands).
The researchers used the catch information, which collectively covered 990 fish species, to determine the relative abundance of species in each of these regions for each year the study covered. They also determined each species' preferred temperature range.
From these data, they calculated a "catch temperature" – the average of the preferred temperature ranges of all the species in each of the large marine ecosystems they identified. Changes in the average catch temperature became a stand-in for changes in the mix of species.
The team then compared changes in catch temperature with changes in sea-surface temperature. When the researchers did that, they found a strong correlation between rising catch temperatures and rising sea-surface temperatures in the same region.
Between 1970 and 2006, the global-average catch temperature increased by 0.19 degrees C per decade. As global averages do, the figure masks significant regional differences. For the northeastern Pacific and the northeastern Atlantic, the catch temperature increased by a whisker under 0.5 degrees C per decade. The increases coincided with sea-surface temperatures that were increasing by 0.2 degrees C per decade in the northeastern Pacific and 0.26 degrees C per decade in the northeastern Atlantic.
Ocean temperatures in the 14 ecosystems in the tropics increased at a pace of about 0.14 degrees per decade. The catch temperature in the tropics, the researchers note, rose by 0.6 degrees between 1970 and 1980 to 26 degrees C, then leveled off. This suggested to the team that things had gotten too hot for the subtropical species that once shared these waters with the tropical fish. The subtropical species voted with their fins and headed for cooler aquatic climes.
The use of catch data for studies like this has its limits, Fogarty notes. The quality of catch records can vary widely in different parts of the world. Apart from the rigor people bring to recording their catches, catch records can change just because of different levels of effort people exert to catch fish, he says.
The researchers appear to have been aware of these and other shortcomings of catch data, he adds. But for the kind of global question the team was asking, catch data represent the most comprehensive source of information available.
Indeed, the approach could lend itself to regular updates – tracking changes in fisheries as they happen, in much the same way systematic temperature records track temperature trends, some researchers say.
The evidence of climate change's global impact on fisheries is "startling," notes Mark Payne, a researcher with the National Institute of Aquatic Resources at the Technical University of Denmark in Lyngby, outside Copenhagen.
The changes present adaptation challenges for local fishing interests in the developed and developing world, he notes in an e-mail.
For some regions, especially in the developed world, the changes may not portend effects as dramatic as California's loss of "Cannery Row" in the 1950s or the collapse of the Northeast's cod fisheries in the 1990s, which shuttered fishing villages in Newfoundland. But fishermen will have to retrace a learning curve to get to know the habits of new species that move into a fishery.
Pursuit of the traditional species into new waters could set the stage for international disputes over fishing.
The largest adjustment may be required in the tropics, the research team posits, because as subtropical fish leave for cooler waters, they aren't being replaced by fish seeking relief from still-warmer water elsewhere.
Adaptation measures could include adding other sources of income "or changing their fishing practices," Cheung says.