Fishing nets left empty as fish seek cooler waters

There was a time when whiting were plentiful in the waters of Rhode Island Sound at Point Judith, Rhode Island, USA and Christopher Brown pulled the fish into his long stern trawler by the bucketful. “We used to come right here and catch two, three, four thousand pounds a day, sometimes 10,” he said, sitting at the wheel of the Proud Mary — a 44-footer boat — as it cruised out to sea. But like many other fish on the Atlantic Coast, whiting have moved north, seeking cooler waters as ocean temperatures have risen, and they are filling the nets of fishermen farther up the coast.

Studies have found that two-thirds of marine species in the Northeast United States have shifted or extended their range as a result of ocean warming, migrating northward or outward into deeper and cooler water. Lobster, once a staple in southern New England, have decamped to Maine. Black sea bass, yellowtail flounder, mackerel and monkfish, to name a few, have all moved to accommodate changing temperatures.

Convoluted workaround
Yet fishing regulations, which among other things set legal catch limits for fishermen, and are often based on where fish have been most abundant in the past, have failed to keep up with these geographical changes. The centre of the black sea bass population, for example, is now in New Jersey, USA, hundreds of miles north of where it was in the 1990s, providing the basis for regulators to distribute shares of the catch to the Atlantic states. Under those rules, North Carolina still has rights to the largest share. The result is a convoluted workaround many fishermen view as nonsensical.

Because black sea bass are harder to find in their state waters, North Carolina fishermen must steam north 10 hours, to where the fish are abundant, to even approach the state’s allocation. “Our management system assumes that the ocean has white lines drawn on it, but fish don’t see those lines,” said Malin L Pinsky, an assistant professor in the Department of Ecology, Evolution and Natural Resources at Rutgers University, USA who studies how marine species adapt to climate change. The mismatch between the location of fish and the rules for catching them has pitted recreational fishermen against commercial ones and state against state. It has heightened tensions among fishermen, government regulators and the scientists who advise them, and raised questions that have no easy answers for fishery managers.

But even in the best case, trying to estimate the size of fish populations is an uncertain proposition. And the migration of species in response to warming temperatures has made the task considerably harder. “From a scientific perspective, there are some really interesting questions,” Malin said. “Where did the fish go? Did we eat them? Or did they go somewhere else? Those are questions we haven’t really had to grapple with.”

Factoring movement patterns
A 2014 survey of butterfish — a small, silvery fish that provides food for many larger fish species — illustrated the problem with traditional assessment methods. A previous survey of butterfish had been unsuccessful at figuring out how robust the population was. Because regulators could not make a judgment about the status of the species, butterfish fishing was temporarily suspended.

But when a team of scientists began talking to fishermen, they realised that the earlier survey had not taken into account the movements of the butterfish in response to changes in water temperature. “What we learned from working with the fishermen was that the animals were probably occurring outside the survey,” said John A Manderson, a research biologist at the National Oceanic and Atmospheric Administration’s northeast fisheries science centre.

John and his colleagues developed a way to factor movement patterns and temperature shifts into models for assessing the fish. Once their work was incorporated into the next survey, which found that butterfish were still plentiful, the fishery reopened. John said that listening to fishermen, who are often in the best position to know how many fish there are and where they are, was the key to understanding what was occurring.

Yet, it remains difficult to tease apart how much of the dip in a fish population is a result of climate change and how much is a result of overfishing. A growing number of scientists and managers favour moving eventually to what they call ecosystem-based management, a system that is focused on the environmental niche a species occupies, rather than individual species.

Temperature affects fish species differently. “Climate change is going to make it hard on some of those species that are not particularly fond of warm waters,” said Christopher. “But as the impacts of climate change descend upon us, there are also species that are going to be victorious, hugely victorious.”