Climate change threatens one of world’s biggest fish harvests | Science

The Peruvian anchovy is a small fish with a big impact. Only about the size of an index finger, they make up the single largest fish catch in the world—sometimes up to 15% of the global haul. Nearly all the highly nutritious fish are ground up to feed salmon and other farm-raised species that are worth billions of dollars. Now, scientists studying ancient sediments and fossils have shown warming waters once nearly eliminated this valuable resource, raising fears that today’s climate change could repeat the disaster.

“The finding is really concerning,” says Becca Selden, a marine ecologist at Wellesley College who was not involved with the research. The new record of climate change’s impact on fish shows “a complete shift in what that ecosystem looked like,” she says.

The research portends potentially devastating impacts across the globe, says William Cheung, a marine ecologist at the University of British Columbia, Vancouver, also not involved with the work. Predators from seabirds to marine mammals would have a tough time without anchovies, he notes, especially as the oceans continue to warm. “It will substantially increase the extinction risk.”

Researchers have long been worried about the impact of climate change on wild fish populations. One reason is that some species require a narrow range of temperatures in order to reproduce. A broader issue is that when water gets warmer, it contains less oxygen. This is a particular handicap for bigger fish, which require proportionally more oxygen than smaller fish. If species can’t migrate easily to cooler waters, researchers predict that their ecological communities will end up dominated by small species, which could be a dire problem for the local fishing industry.

But it’s difficult to know whether climate change actually forces this change, because of a confounding factor: The act of intense fishing can also drive communities to be dominated by smaller fish, which more easily avoid nets.

Renato Salvatteci, a fisheries biologist at the Christian-Albrecht University of Kiel, decided to go back in time, to a warm period before the advent of intense fishing. The waters off Peru seemed like an ideal place to study the impact of climate change on fish populations. Researchers believed the sea floor there was likely to hold a detailed fossil record because of the high rate of sediment settling in the ocean and the abundant fish.

Salvatteci studied a 14-meter-long core that was collected by a research vessel in 2008. It contained a section with sediment deposited between 116,000 and 130,000 years ago, when Earth was experiencing a warmer climate than today. To figure out the temperature and oxygen concentration of seawater when the sediments formed, his colleagues studied biomarkers produced by fossils of tiny marine organisms called coccolithophorids and measured nitrogen isotopes. These records indicated the water was about 2°C warmer than today and contained less oxygen.  

Taking on a herculean task, Salvatteci figured out which kinds of fish had lived in the waters—an effort that involved 2 years of picking through the sediment and counting more than 100,000 vertebrae and other fish remains. “There are very, very few people in the world that are interested in looking at fish vertebrae in sediments,” he admits.

Sediment deposited over the past century is dominated by anchovy bones. But during the earlier, warmer period, about 60% of the fish were other, smaller species, the team reports today in Science. These include gobylike species that are about half the size of anchovies and better adapted to low oxygen conditions. Also common were species typical of deeper water, such as the Panama lightfish (Vinciguerria lucetia), that can also thrive in low-oxygen conditions.

Compared with anchovies, these species would cause problems for the fishing industry. Their small size makes them harder to catch, requiring nets with smaller holes that are difficult to clean. And because they don’t aggregate in dense schools, trawling vessels would need to take longer voyages, burning more fuel, to catch large numbers. The fish are also less nutritious than anchovy, which contain fats rich in healthful fatty acids. Fewer anchovies would raise the cost of fishmeal, which might mean more expensive—and less nutritious—salmon and shrimp, for example.

Abundant anchovy populations also support mackerel and other wild species that are desirable catches. A declining population of anchovies could mean these species would become scarcer and more challenging to catch, potentially raising prices for consumers.

In the past, when the water became uncomfortably warm, some anchovies may have swum south to the cooler water that they need to reproduce. But these southern waters can’t sustain the massive populations off the coast of Peru, which are nourished by local currents that provide abundant food from the deep.

When a fish population is threatened by climate change, one approach to helping the species cope is to make fishing management more conservative and lower the allowable catch, Cheung says. The anchovy population in Peru is already well managed, Salvatteci notes, but an improvement would be for people to eat more anchovy directly, rather than feeding it to farmed fish, he says. That would help maintain the benefit of the species for human nutrition. “Instead of preparing fish meal,” he says, “we need to prepare a meal of fish.”