Rising temperatures complicate efforts to manage cod fishery

Warming waters in the Gulf of Maine have reduced Atlantic cod populations in that region and distorted estimates of how many fish were available to catch, a new study finds.

Cod stocks have decreased even though fishing quotas should have maintained a sustainable cod fishery. But those quotas relied on historical data without considering higher water temperatures, leading to incorrect assumptions of how big cod numbers could be and how fast these fish populations could rebuild.

The new finding sends a message that past sustainability levels for the cod fishery won’t necessarily work in managing future cod populations, says Ray Hilborn, a fisheries ecologist at the University of Washington in Seattle who was not involved in the study.

Based on ocean area, the Gulf of Maine — a productive fishing area in New England — heated faster than 99.9 percent of the global ocean over the last decade, the researchers report online October 29 in Science.

From 1982 to 2013, water temperature in the Gulf of Maine increased more than most other parts of the ocean — 0.03 degreesCelsius per year compared with a global average of 0.01 degrees per year. The higher temperature in the Gulf of Maine can be traced to a period of intense warming from 2004 to 2012, when waters there reached a peak warming rate of 0.23 degrees per year, a team of U.S. researchers found. Although the rate of warming began to level off in 2013, temperatures that high weren’t expected until midway through the century. A northward shift in the Gulf Stream and a change in weather patterns might be responsible for the rapid warming trend, the scientists suspect.

“We can really see the signal of that warming in the data,” says study coauthor Andrew Pershing, an oceanographer at the Gulf of Maine Research Institute in Portland. “The human side of the fishery management couldn’t really keep up with that rate of change.”

The cod fishery collapsed in the early 1990s, a loss largely blamed on overfishing. Since then, strict management has been established to help the fish rebound. But even though fishermen have stayed under catch limits, more cod have died than can sustain a healthy fishery. The researchers think temperature is partly to blame.

Higher water temperatures limit the survival of young cod. That may be because temperature affects growth. Or predators moving into breeding grounds earlier in the year may mean fewer cod grow up to be adults, leaving fewer fish to catch.

Atlantic cod (Gadus morhua) is a cold-blooded fish, and the Gulf of Maine is near the southern extent of cod’s Atlantic range. When a cold-adapted species is living near the warmer edge of its range, “it should trigger warning bells to be on the lookout for changes,” says Michael Fogarty, a fisheries ecologist at the National Oceanic and Atmospheric Administration’s Northeast Fisheries Science Center in Woods Hole, Mass., who has also researched the relationship between cod and temperature.

Not all Atlantic cod populations have fared poorly with warming water temperatures, though. Hilborn says that even with slightly warming waters, the Norwegian stock is at record levels now. But he notes that these fish are farther north and living in areas where the water is colder than in the Gulf of Maine.

In a scenario where the Gulf of Maine warms at a rate of 0.03 degrees per year and allows only a small amount of fishing, the researchers think that the cod fishery could reach a level of productivity similar to the last decade by 2030. At a warming rate of 0.07 degrees per year, rebuilding would take several years longer.

“Recovery of this fishery depends on sound management, but the size of the stock depends on future temperature conditions,” the researchers write.