PORTLAND — It’s no secret that cod stocks in the Gulf of Maine have declined precipitously over the past 25 years.
According to the most recent NOAA Fisheries stock assessment update for cod, the spawning biomass of Gulf of Maine cod has dropped from a little over 20,000 metric tons in 1990 to about 3,000 metric tons in 2013. During most of that time, scientists and fisheries managers placed much of the blame for the cod population crash on overfishing.
In recent years, the New England Fisheries Management Council adopted a number of increasingly stringent restrictions on fishing in the Gulf of Maine. The number of days that fishermen are allowed to seek groundfish such as cod were limited and repeatedly cut. Some kinds of fishing gear are prohibited and some areas are closed entirely to fishing. Most recently, regulators have used landings quotas to reduce fishing at levels that would allow depleted stocks of cod to rebound to sustainable levels.
But maybe it’s not the fishermen’s fault after all.
Whether or not the climate is changing, and whether or not humans are to blame if it is, there is no question that sea temperatures are rising. Recent studies show that, over the last decade, sea surface temperatures in the Gulf of Maine have risen faster than 99 percent of the world’s oceans.
According to a report published last week in the journal Science, those management efforts were doomed to fail because they didn’t account for the impact of rising sea temperatures on the gulf’s cod population.
“Managers kept reducing quotas, but the cod population kept declining,” Andrew Pershing, chief scientific officer of the Gulf of Maine Research Institute and lead author of the study, said in a statement announcing its publication. “It turns out that warming waters were making the Gulf of Maine less hospitable for cod, and the management response was too slow to keep up with the changes.”
Pershing and colleagues from the Gulf of Maine Research Institute, the University of Maine, Stony Brook University, the Department of Marine Resources Bigelow Laboratory and NOAA’s Earth System Research Laboratory found that increasing water temperatures affect both reproduction and the survival of young fish. This study was funded by the Lenfest Ocean Program and the U.S. National Science Foundation.
According to Pershing and his colleagues, high water temperature appears to reduce the number of juvenile cod produced by spawning females. The study also suggests that warming waters led to fewer young fish surviving to adulthood.
In some ways, the study’s conclusions are unsurprising. Cod are a cold water fish species and the Gulf of Maine has always been near the southern end of their range.
Coincidentally, Canadian scientists recently reported that the cod fishery off Newfoundland, which collapsed in the early 1990s, is showing signs of a rebound. In a paper published last week in the Canadian Journal of Fisheries and Aquatic Sciences, marine biologists at Memorial University of Newfoundland reported that, over the past several years, large numbers of cod have been spotted during spawning season off Labrador and Newfoundland, where the water is considerably colder than in the Gulf of Maine.