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Oceanographic influences on Pacific hake (Merluccius productus) distribution and biology Benson, Ashleen Julia


There are persistent decadal scale states in atmosphere-ocean conditions that are called regimes, and a shift from one set of stable conditions to another is called a regime shift. Regime shifts have been identified in 1925, 1947, 1977, 1989, and possibly 1998. In recent years, a substantial amount of effort has been directed at understanding the effects of regime shifts on fish populations. A synthesis of the available information for the eastern North Pacific and Bering Sea is presented in Chapter 1. What is most evident from the review is that pelagic fish exhibit the most dramatic responses to changes in climate-ocean conditions. Pacific hake (Merluccius productus) is the most abundant pelagic fish species in the California Current ecosystem, and is important both commercially and in terms of its large predatory biomass. A review of the life history and ecology of hake is presented in Chapter 2. Hake is a widely distributed species that migrates seasonally between winter spawning grounds off California and summer feeding grounds off the coast of British Columbia. The feeding and spawning distribution of hake shifted northwards following the 1989 regime shift and the biomass of hake present in Canadian waters increased by a factor of two. These changes resulted in disputes on catch allocation between Canadian and United States fisheries management agencies. A method of forecasting would aid in negotiations. Early studies established a link between hake distribution and physical oceanographic conditions, but the exact mechanisms remained to be identified. The objectives of this study are to develop a predictive index of the proportion of the stock that migrates into Canadian waters, and to examine the possibility that the change in distribution coincided with a change in hake growth. The key findings of this study are that the best predictor of hake distribution is the Bakun upwelling index off the coast of California, and that juveniles produced in northern waters off Canada during the 1990s grew faster than those produced in the south. These changes in distribution and juvenile growth are probably related to biomass and/or availability of euphausiids, which are the key prey item of hake.

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