Wednesday, June 13, 2007

Declining Cod Stocks and Climate Change

From the inbox:

Evidence and research on declining fish stocks bringing together 2 different elements of taught Environmental Economics.


June 13, 2007

UMass Dartmouth study links environmental factors such as climate change to the collapse of North Atlantic cod fishery. Fishing previously cited as primary cause.

A recently published study of cod stocks off Canada and New England shows that the cod stocks grew and declined at about the same time, revealing that environmental factors played a stronger role in the collapse of the cod fishery than previously thought.

In an article in the latest Transactions of the American Fisheries Society, Dr. Brian Rothschild of the University of Massachusetts Dartmouth argues that an interruption in the food chain, possibly caused by climate change, was a key factor in the cod's disappearance.

"These environmental changes were probably as important in influencing declines in cod abundance as the effects of fishing," said Rothschild. "The standing stock biomass and weight-at-age statistics for various stocks tend to follow the same pattern. However, when fishing is superimposed on top of an unfavorable environment, it appears to accelerate the negative effects of the environment in bringing about a decline."

Since colonial times, cod has been the mainstay of New England and Canadian Maritime fishing fleets. The collapse of cod fisheries in the northwest Atlantic in the early 1990s hit both the industry and fishing towns hard, but the cod population still hasn't recovered despite radically reduced fishing.

The cod's decline has been an intriguing scientific mystery filled with dozens of sometimes apparently conflicting clues. First came the cod population declines of the 1970s. This was followed by a strong upswing in the 1980s and then the steep decline beginning in the mid-1980s when fishing pressure was low. Another clue was that cod were not only waning in numbers but were experiencing significant decreases in growth rate. Meanwhile, scientists painstakingly worked to tease apart the various stocks of cod, only to find that the extent to which the stocks mingle is still its own mystery and has unknown effects on the population as a whole. Finally, changing water temperatures seemed to be associated with salinity decreases and changing cod diets.

In his study, Dr. Rothschild assembles the various pieces of this puzzle into a coherent picture to answer the question of what happened to the cod. Since the abundance of various stocks from southern Newfoundland all the way to the Gulf of Maine rose and fell at the same time, cod stocks must have been responding to environmental factors operating over a wide area, Rothschild contends. The dramatically reduced slower growth of cod and their changing stomach contents support the concept that the supply of plankton may have been disrupted, hence affecting the availability of cod forage like capelin and herring that feed on plankton.

These observations have important implications for fishery management. All of the "rules" used in fishery management-production, yield-per-recruit, and stock-and-recruitment-relate to the effects of fishing and ignore the effects of the environment. The known strong influence of the environment on stock abundance suggests reevaluating definitions and remedies for over-fishing. In particular, Rothschild says, rebuilding stocks in a mandated, specific period of time may not be feasible.

These observations are also critically important to the fishing industry. The industry needs to know whether decreases or increases in stock abundance are the result of fishing or environmental change. Causes associated with fishing suggest modifying the intensity of fishing, but causes associated with multi-annual environmental variability suggest longer term strategies that might involve changing target species or investment strategies.

Of greatest concern to the industry are questions related to longer term changes. Rothschild said scientists need to fully explore whether climate change is affecting the North Atlantic Ocean ecosystem.

"I think the most important point is that a decline in the cod populations was inevitable, and fishing simply aggravated it," said noted fisheries biologist Ray Hilborn, the Richard C. and Lois M. Worthington Professor of Fisheries Management at the University of Washington." Fishing pressure should have been reduced sooner on the Canadian stocks, but they were going to decline regardless. The decline of the Gulf of St. Lawrence stocks in particular began at a time of high abundance and low fishing pressure."

Future research will focus separating the influence of fishing from the influence of the ocean environment on changes in fish stocks.

Transactions of the American Fisheries Society is a bimonthly journal of the American Fisheries Society now in its 136th year of continuous publication. Abstracts can be searched online for free at The American Fisheries Society is a professional scientific organization of almost 10,000 fisheries biologists and managers from around the world. Its mission is to improve the conservation and sustainability of fishery resources and aquatic ecosystems by advancing fisheries and aquatic science and promoting the development of fisheries professionals.
Coherence of Cod Stock Dynamics in the Northwest Atlantic, by Brian J. Rothschild of the School for Marine Science and Technology, University of Massachusetts Dartmouth, New Bedford, Massachusetts. Transactions of the American Fisheries Society 136:858-874. Rothschild can be contacted at 508/910-6382 or

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