Date of Award
Level of Access Assigned by Author
Master of Science (MS)
Second Committee Member
Third Committee Member
Overfishing is widely recognized as a major impediment to the long term sustainability of seafood harvests. Overfishing is evident in the Gulf of Maine off the northeastern coast of North America where large predators experienced stock collapses in the late twentieth century. Here, Atlantic cod and haddock stocks remain far below their historical levels. One hypothesis explaining the decline and subsequent lack of recovery is a mismatch of spatial scales between fisheries management and the fishes' ecological boundaries. Although such a mismatch may be unavoidable, it is important to identify when one occurs and implement policies or incentives to prevent overfishing. This project explores the conditions that may prevent overfishing when a mismatch occurs between the scale at which the stock is managed and the scale at which the stock migrates, breeds and feeds. To evaluate stock structure, the spatial variation in the life history parameters of three groundfish species was examined in the inshore waters of the Gulf of Maine using an eight-year dataset from the Maine Department of Marine Resources Inshore Trawl Survey. The parameters examined included relative abundance, maturity ratio, size structure, sex ratio, and condition factors, which were analyzed for small-scale variation across depth and alongshore gradients in the inshore Gulf of Maine. Although evidence of small-scale variation was found, many challenges to using trawl survey data to assess stock structure were identified. A theoretical computer model was designed to evaluate policy implications of a mismatch between the scale of a fish stock's movements and the management of that stock. The model examined a fishery with stationary harvesters and a migrating stock, a case in which the scale of management is smaller than the scale of the stock's movements. Observations were made of how harvesters in different areas respond to varying market regimes, recruitment patterns, and costs of harvesting in order to determine what economic and environmental conditions would promote or restrain an intercept fishery in the model. Overall, the economic conditions interplay with the environmental conditions to promote or restrain an intercept fishery. Specifically, an intercept fishery was restrained when recruitment levels were low and prices were constant, as might happen in a competitive world market, whereas when recruitment levels were high, a price that was sensitive to local supplies was necessary to restrain the development of an intercept fishery. This thesis highlights some techniques to describe the biological scale of a stock and tools to manage a stock sustainably despite a mismatch. The groundfish survey data showed evidence of small-scale variation in stock structure. While this small-scale variation suggests that the scale at which these stocks are currently managed should be adjusted to smaller scales, the model results emphasize that in the face of a mismatch, overfishing may be prevented with wise use of economic incentives and management policies. However, a sound understanding of biological parameters, such as recruitment levels, is critical to determine appropriate management strategies, and some control of the economic system surrounding fish harvests, such as prices, may be necessary to prevent overfishing. Although mismatches between fish stocks and management scales may be inevitable, an adequate understanding of the fisheries' biology and economics can provide powerful tools for preventing overfishing.
Al-Humaidhi, Alia Waleed, "Identifying Spatial Scales of Fisheries Stock Structure and Conditions That Improve Their Management" (2012). Electronic Theses and Dissertations. 1664.