New cod data a reminder of environmental impact on stocks


Sean Murphy, SeafoodSource online editor

Published on
September 8, 2015

Some recently disclosed information detailing cod catch data in the North Atlantic from the first half of the 20th century may have inadvertently added fuel to the debate on just how responsible overfishing has been for harming cod stocks farther south.

The data, from U.K. survey log books ranging from the 1930s to the 1950s, are hardly a new revelation, but a recent project to digitize the data has made it easier for scientists to study the figures, and one of the conclusions they have come to is that water temperature may have played a greater role in cod stocks than previously thought.

The report, published in the scientific journal PLOS One, is behind a paywall, but just viewing the abstract is telling. “Generalized Additive Models showed that environmental, spatial and temporal variables are all valuable descriptors of cod catches, with the highest occurring … at bottom temperatures between 2 and 4°C …”

Bryony Townhill, marine climate change scientist with the Center for Environment, Fisheries & Aquaculture Science in the United Kingdom and one of the principal authors of the report, told me the study did not compare and contrast environmental factors with man-made factors such as overfishing, but that he still felt the data shows how water temperature can be a factor in cod populations.

“In this work we found that the cod distribution was affected by environmental influences, including temperature,” he said. “Other work in the area in the past and also more recently has found the same, but we have now been able to add more temperature data from that time to the picture.”

Townhill also stressed that the data covers the Barents Sea in the region of Northern Norway, and could offer no comment on the decline in cod populations off the U.S. East coast. However, the data mirrors comments by other scientists made in recent years on the possibility that environment plays more of a role in cod populations, indeed stocks of most seafood, than we might realize. I recall attending a conference in 2013 in Boston where a group of scientists and industry leaders met to discuss the factors involved in the cod stock declines that have badly hurt the livelihoods of so many New England fishermen.

Svein Sundby, with the Institute of Marine Research, said in 2013 that water temperature can control, among other things, the amount of phytoplankton in the water that cod eat, and that studies have shown the amount of phytoplankton has dropped in the same waters where the cod population has plummeted.

Regulations and ongoing fishery management programs, he said at the time, need to take into account the whole biological picture, including environmental factors.

“It’s a different science, so to say,” he said. “This is a moving target. It’s varying all the time.”

No one is suggesting that overfishing is not a factor in cod’s decline. Even industry leaders will acknowledge cod was massively overfished for a good part of the 20th century, but the New England fishery alone has gone from 1,900 boats in the 1980s to a mere 135 boats today, and there have also been serious cutbacks in cod fishing in the North Atlantic, which the recent report covers.

However, there is still a concern about cod stocks off the U.S. East coast, while in the colder northern waters there are different results. Just this week a number of sponsors announced a new effort to get cod fisheries in the North Atlantic labeled sustainable by the Marine Stewardship Council.

Since one of the biggest differences between the two regions is water temperature, it’s hard to ignore environmental factors in cod stocks both in the North Atlantic and for New England fisheries, and regulators would do well to remember this when calculating MSY and the resulting future quotas.

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