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Blue crabs in Maine? Something fishy about global warming.

Warming oceans are changing the mix of species in the world's fisheries, according to a new study. Marine-ecosystem models have indicated that this could be an effect from global warming.

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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.

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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.

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