Ocean heat puts pressure on poorest fisheries

Warm water Red Mullet catches in the UK have increased as sea temperatures have warmed, which William Cheung has linked to global warming. Credit: Nate Gray: A Culinary (Photo) Journal via Flickr Creative Commons License

Since 1970, our warming seas have driven fish across the world into cooler, deeper waters, potentially threatening fishing in Earth’s hottest seas. By analysing worldwide fish catches, Canadian and Australian scientists have found that the proportion of warmer-water fish caught has steadily grown. And in future, the warmest waters are set to become too hot for some of the fish that might previously have been caught there.

“Tropical fisheries are likely to be most impacted by ocean warming,” William Cheung from the University of British Columbia in Vancouver, Canada, told me. “We expect that the current trend would continue, and will reduce the catch for tropical fisheries. Many tropical fishing communities are in developing countries with limited socio-economic scope to deal with changes in resource abundance. Thus, these communities are most vulnerable to ocean warming.”

Like all living creatures, fish have a range of temperatures that they can comfortably live in. Sea temperatures are rising, with the US coast from North Carolina to Maine reaching the warmest level in 150 years last year, for example. Changing climate has already been linked to fish catches in some places, with William previously suggesting it’s behind rapid increases in warm-water red mullet catches around the UK. “However, there was no study that assessed the linkages between ocean warming and fisheries changes in the global scale,” he said.

So William and his teammates set about bringing together fish catch information from 52 ecosystems, including most of the world’s fisheries. That included data on 990 species, which the scientists analysed using a new measure, the ‘mean temperature of the catch’ or MTC, which William also calls a ‘fish thermometer’. To find the MTC, the scientists start by working out the preferred temperature of each species, based on the sea water temperatures in the areas that they used to live in. “For example, fishes that live in colder area, such as cod, will have a lower preferred temperature than a tropical fish, such as a tropical grouper,” William explained.

Fishing weight

Groupers, such as the one shown here in front of a lemon shark, are tropical fish that William’s team give a high preferred temperature value. Credit: AlKok via Flickr Creative Commons License

William’s team then combined these preferred temperatures into a single average value for each of the 52 ecosystems, MTC, for each year between 1970 and 2006. They weight the average according to the annual catch of each fish, so that the preferred temperature of the most-caught fish has a much bigger impact on MTC than the least caught. If the catches contain more warm water species, MTC increases.

But factors other than temperature could also affect MTC, including changes in fishing practices, and other natural processes in the ocean. To untangle those effects, the scientists also collected statistics about how fishing was done in each area. William and his teammates fed those numbers, their MTCs, and worldwide set temperatures into a statistical model, giving many possible scenarios testing the impact of each factor.

Writing in a paper published in top research journal Nature on Wednesday, the scientists found that the makeup of fishing catches is significantly related to ocean temperature changes. And that link remained even if they changed how they calculated MTCs, underlining its strength. On average across the whole world, they found that MTC had increased by 0.19°C per decade. Outside the tropics, MTC increased faster on average, increasing almost 0.5°C per decade in the northeast Pacific and northeast Atlantic Oceans, as warmer waters species become more common.

Warming dinner

Changes in mean temperature of the catch (MTC, red) and sea surface temperature (SST, grey) in tropical ecosystems. MTC changed rapidly at first, as cold water fish were found less often in warm water areas, and then flattened as the balance between species changed less. Image copyright Nature, used with permission, see citation below.

Meanwhile, in the tropics, MTC grew rapidly between the 1970s and 1980s, by around 0.6°C with the proportion of the catch from colder-water species falling. Since the 1980s MTC in these hot areas has flattened off, showing little further change in the balance of species living there. That shows these areas are now too hot for all but the warmest fish. And while MTC may be stable, numbers of fish in tropical areas may fall if warming continues as those seas become too warm for them. That could have serious effects on the food supply in surrounding countries.

Writing separately in the same issue of Nature, Technical University of Denmark’s Mark Payne called these results ‘startling’. “Climate change is suddenly an unexpected guest at dinner,” he wrote. He pointed out that the paper only reflects fish caught, not the total population in the sea. “Nevertheless, the changes in the make-up of the catch alone are startling and cause for concern, particularly in tropical countries. The countries that border tropical waters are also those that are the most dependent on fisheries as sources of employment, foreign revenue and food, and are least able to adapt to such changes.”

The next step for William and his teammates will be to combine this knowledge with climate models to make predictions of how further warming will influence MTC. However he stressed that what they’ve already found is reason enough for action. “The effects of ocean warming on fisheries are global and have already been happening in the last four decades,” he said. “We need to urgently make decisions to mitigate and adapt to these effects, through reducing greenhouse gas emissions, managing ocean ecosystems better and developing policies that could help fishing communities and industries adapt to these changes.”

The relationship between rate of change of the worldwide mean temperature of the catch (MTC) and sea surface temperature (SST). The graph shows that fish are moving to cooler waters as the world warms. The black line shows the average value output by William’s team’s statistical model, and the grey lines are the range that falls within the ‘95% confidence interval’. Grey bubbles are the MTC for each of the 52 ecosystems, and the dotted line shows the slope of the line if MTC increased 1°C/year as SST increased 1°C/year. Image copyright Nature, used with permission, see citation below

Journal references:
Cheung, W., Watson, R., & Pauly, D. (2013). Signature of ocean warming in global fisheries catch Nature, 497 (7449), 365-368 DOI: 10.1038/nature12156
Payne, M. (2013). Fisheries: Climate change at the dinner table Nature, 497 (7449), 320-321 DOI: 10.1038/497320a