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STONINGTON - Restoring and maintaining robust population components of Atlantic cod to achieve sustainability is made difficult when their distribution and character are unknown. A 2004 study clarifies the structure of the Gulf of Maine cod grouping by deriving the distribution, movements, and behavior of population components from 1920s data and surveys of retired fishermen. The study was conducted by Ted Ames (367-2473, ames@hypernet.com), a commercial fisherman with the Stonington Fisheries Alliance and Penobscot East Resource Center, as well as the recipient of a MacArthur genius- grant. His work was supported by the Davis Conservation Foundation, Island Foundation and Gulf of Maine Research Institute. In 1998, the National Marine Fisheries Service reported cod stocks in the Gulf of Maine were overexploited and at extremely low biomass levels. To improve management efforts, Ames evaluated the distribution and dynamics of GOM cod during the 1920s, a period when cod was abundant. Subpopulations and spawning components were tentatively identified; their distribution and spawning areas were compared to recent tagging studies and egg distribution surveys. The GOM stock within the study area is composed of three groupings of cod in the northeast U.S.; groupings were reported to have limited exchange with others, had different migration routes and their abundance varied independently of the others. These were tentatively identified as sub-populations. In the study area—which included the GOM north of northern Massachusetts Bay to Wrights Swell, thence northeast to Yarmouth, Nova Scotia—coastal spawning occurs in both the spring and fall. With the approach of spawning season, cod start migrating toward their respective spawning grounds. After arriving, cod often gather into large schools. They broadcast eggs into the water column until spent and often involves more than a month. Once spawning is completed, cod leave their spawning areas to pursue forage stocks and initiate the next annual cycle. The 1920s database was gathered through direct interviews at that time, with a large number of captains, most using hook-and-line techniques, according to Ames. Ames collected additional information from interviews with retired fishing captains, many of whom operated otter trawlers from 1930-1960. In all, about 260 fishing grounds and 91 spawning grounds were found to be distributed between Gloucester in the western GOM and the Lurcher Shoal in eastern GOM. Coastal spawning grounds were found to be either contiguous with or often closer to shore than fishing grounds of the period. Spawning occurred in channels and basins bordering the rocky, shallow historical fishing grounds, generally on bottoms of muddy gravel, sand or mud with borders of gravel, varying in depth from less than 10 meters to 100 meters. Because hook fishing is selective for foraging fish, it was possible to identify which historical fishing grounds were the preferred foraging habitats of GOM cod for each season of the year. Even though it wasn’t possible to tell what forage species had once attracted cod to feed at these locations were, the fishing grounds having large feeding areas that cod used at certain seasons of the year allows thousands of square kilometers of Atlantic cod habitat to be identified, Ames writes. In similar fashion, captains of early otter trawlers were able to catch cod regardless of whether or not the cod were feeding. The result of this was that they could identify a number of cod grounds that were spawning areas. In spite of their wide distribution, cod stayed on most of the 1920s fishing grounds for only a few months of the year. The reason for this was unclear, but may have been related to cod co-migrating with herring and other forage species. Most grounds held cod for two seasons of the year or less. A number of reports noted that significant numbers of cod remained on offshore grounds during spawning season, even though no spawning activity occurred there. Cod were present in large numbers all through the year on the grounds of the Cashes Ledge complex and the rises and swells rimming the gulfs major basins. Several features were common to GOM subpopulations, the study says. In winter, cod gathered into major concentrations, via separate movement patterns. Each subdivision occupied a relatively distinct region on the coastal shelf that appeared to be partially separated from the others, usually by deeper basins. Subpopulations were composed of several spawning components using separate spawning grounds, nursery areas, and a seasonal migration corridor that abutted their winter grounds. The study has presented evidence that the following observations are valid: • Inferred movements of cod stocks can be used to make conclusions about stock structure. • Subdivisions of Atlantic cod in the Gulf of Maine identified as groupings can be described as subpopulations. • Accuracy of charts and navigation techniques of the period were adequate to allow fishing grounds of the period to be used in the study. • Accuracy of historical fishery information was from peer reviewed publications and logs. • Migration patterns of cod during the 1920s were not transient events but follow the same routes today. • That many of the same spawning areas used by cod in the 1920s are still active today, a period of nearly a century and supports the idea that cod return to the same spawning area. Even so, a number of the innermost coastal spawning components of Atlantic cod have disappeared. • Depletion and recovery patterns of spawning components. The study concludes that active spawning grounds in the study area fell from 90 in the 1920s to no more than 46 in the 1980s, with many of the remaining grounds exhibiting only sporadic activity, and similar losses in reproductive capacity. At the same time, the study says system-wide assessments, the basis of current management strategies, have been unable to detect coastal depletion and the gradual erosion of spawning components. Thus, the study says, because the confidence limits of NMFS surveys is +/- 25%, the collapse of a single one of the four subpopulations would be undetected and would not detect a difference until two or more subpopulations. This, however, would have reduced annual landings in the GOM by more than half. It is, sadly what appears to have happened. This simple calculation points out that if cod stocks are to be restored as mandated, the particular areas occupied by subpopulations and spawning components have to be managed separately. The study says effective area management is pivotal to rebuilding GOM cod stocks and maintaining them at high, sustainable levels, because it addresses the geographical character of cod populations. It also points out the consequences when management relies primarily on system-wide assessments. Managing subpopulations and spawning components on the basis of their geographical character would address such losses, the study concludes. The study cites the work of University of Maine professor Jim Wilson, who stated that decentralized, hierarchical management units equivalent to the scale of the Atlantic cod’s population structure would be more effective. He concluded that organizing a decentralized fisheries management system and creating individual incentives that are consistent with the goal of sustainability is best achieved through local authority over ecological events whose impacts are strictly local. |
