As our planet warms from excess carbon in the atmosphere, some of that heat is absorbed by the ocean. Fish and invertebrates are responding to warming waters by moving to higher latitudes or deeper waters where the water is cooler, and it was expected that these shifts would eventually affect availability of some commercially harvested species.

"Eventually" may be now. Ocean warming has already affected global fisheries in the past four decades, according to a new study published in Nature. By looking at catch statistics, scientists discovered that the composition of species in fisheries around the world is already shifting and changing our menu.

Scientists compared the temperature preferences of 698 commercial fish species with the size of catches to develop an index known as the “mean temperature of the catch," and this index was used to evaluate the potential effects of climate change on fisheries. They found that water temperatures rose steadily every decade between 1970 and 2006 and that the mean temperature of the catch rose significantly in 52 large marine ecosystems, which cover the majority of the world’s coastal and shelf areas.

The United Kingdom has already seen a rapid increase in catches of red mullet, a warm-water species previously native to the Mediterranean but now found in waters as far north as Norway. While cold waters are now seeing more species from the tropics, there aren't any fish to replace those which are leaving. As waters become too hot for tropical species to tolerate, they will decrease in abundance due to their stunted aerobic capacity, which hinders their ability to grow and reproduce.

Changes in catch composition will have direct implications for coastal fishing communities, where the economy and food security often relies on fisheries. Subtropical developing counties will be particularly vulnerable to the effects of climate change. The study authors conclude that adaptation plans must be developed immediately to minimize the risk of ocean warming on the economy and food security.

Nature, 2013. DOI: 10.1038/nature12156 (About DOIs.)