A few years ago, Dr. Cheung [a professor at the University of British Columbia Fisheries Centre] and his scientific colleagues asked “is climate change affecting global fisheries catches?”
For more than a decade, scientists have been saying that global warming will cause many marine fish and invertebrate species to move to higher latitudes; tropical fish will move to temperate waters and temperate fish will move to arctic regions. Consequently, the species composition of fisheries catches will change ̶ Warm-water species will replace cool-water species. Except in the tropics, of course, because tropical areas already contain the hottest-adapted species. In this region as temperatures become too warm for many species, the number of species present in fisheries catches will likely decline.
Scientists have already shown that rising ocean temperatures have affected fish distributions and fisheries catches in some regions, like the North Sea and Northeastern U.S. But Dr. Cheung and his colleagues wanted to know if large-scale, global fisheries changes have occurred.
To test this, they took a very unique approach. They analyzed the composition of fisheries catches overtime in terms of “Mean Temperature of the Catch” – which is really the mean temperature preference of all species in the catch. An increase in the Mean Temperature of the Catch would indicate an increase in the amount of warm-water species in the catch and/or a decrease in the amount of cool-water species.
First the scientists gathered fisheries catch information and determined the temperature preference of each species in the catch [they evaluated 990 species in total]. Then they calculated the Mean Temperature of the Catch for 52 large marine ecosystems for the years 1970 to 2006. Finally they examined the relationship between rising ocean temperatures and changes in the Mean Temperature of the Catch using a computer model.
The scientists found that as ocean temperatures have risen the Mean Temperature of the Catch has increased. It has increased at a rate of 0.23 ᴼC per decade in non-tropical regions. This means warm-water species have indeed been replacing cool-water species in fisheries catches!
In tropical regions, the Mean Temperature of the Catch initially increased between 1970 and 1980 [likely due to a decline in sub-tropical species in the catch]. But after 1980 the Mean Temperature of the Catch has remained the same. This suggests that tropical areas have already become too warm for all but the hottest-adapted species.
This has major implications for the world’s fisheries. It shows that over the last four decades rising ocean temperatures have already significantly affected global fisheries. And in the future fishermen and seafood consumers can expect to see even more warm-water species and less traditional cool-water species, like cod and haddock, in their nets and on their dinner plates.
The scientists’ findings also suggest that tropical fisheries are in real trouble. If ocean temperatures continue to rise, catches in tropical regions may decline, if they haven’t already. This is a major concern for the many developing nations found in tropical regions that rely on fisheries for jobs, revenue, and food.
Global warming’s effects on our fisheries is not a problem of the future; it is a problem NOW! We must start creating adaptation plans to deal with changing fisheries, particularly for vulnerable nations in tropical regions.
To learn more about how climate change affects our oceans please visit our Climate Change page.
Elizabeth Brown is a research scientist at Blue Ocean Institute.