Synchronous Rise and Fall of Cod Stocks Points to Environmental Factors in Decline

A study of decades of population estimates for the various cod stocks off Canada and New England shows that the stocks grew and declined at about the same time, revealing that environmental factors played a stronger role than previously thought in the collapse of the cod fishery. In an article in the latest Transactions of the American Fisheries Society, Brian Rothschild of the University of Massachusetts Dartmouth points to a strong negative environmental signal, possibly associated with plankton dynamics, as a leading suspect in the cod’s disappearance. 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 and then the steep decline beginning in the mid-1980s when fishing pressure was still 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. 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, complexes of cod stocks must have been responding to environmental factors operating over a wide area. 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 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.” 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 overfishing. In particular, it needs to be recognized that rebuilding stocks in a mandated finite 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. For example, are the observations on cod populations over the last several decades the harbinger of permanent changes in the ocean ecosystem that result from climate change signals in the North Atlantic Ocean? “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 Washongton. “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.” Avenues for future research focus upon developing capabilities to separate the influence of fishing from the influence of the ocean environment on fish stock variability. This capability must relate in a significant way to the as-yet-unresolved problem of understanding the variability in recruitment. What would be new in recruitment research would be the development of an observation system that could statistically resolve events in the ocean on scales relevant to a larval fish: hours and meters. 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. The views expressed in this article do not necessarily represent the views of the Society. 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 [email protected].