Graduate Thesis Or Dissertation
 

Oceanographic effects on the dynamics of food habits and growth condition of some groundfish species of the Pacific Northwest

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  • Marine fish are subject to direct and indirect oceanographic variations operating at short and long time scales. In this study feeding habits and long-term growth condition of several groundfish species of the Pacific Northwest are examined to understand the relationship between variations in the fish's biological and life history components and the ocean environment. Stomach samples of three rockfish species, yellowtail rockfish (Sebastes flavidus), widow rockfish (S. entomelas), and canary rockfish (S. pinniger), which were collected seasonally off the Pacific Northwest in 1998 and 1999, provided quantitative information on the food habits of the species during and directly after the 1997-98 El Nino event. The diets of these co-occurring species were markedly different from each other. Unusual southern zooplankton species were found in the diets of all three species possibly due to the influence of the El Niflo event. The diet of S. flavidus in 1998 summer (El Nino) was contrasted with results from a previous study based on 1980 summer samples (non-El Nino), and found to be different. An assessment of long-term changes in fish growth condition was conducted using data from Oregon market samples, in which the fish weights are generally measured for aggregations of fish rather than individuals. Non-linear regression can be applied to this type of data to estimate the length-weight parameters and to detect long-term changes in growth condition. The reliability of the estimation method was tested using Monte Carlo simulation. The results demonstrate that the estimation method provides reasonably accurate estimates and is sufficiently powerful to detect small changes in the length-weight relationship. Using Oregon market sample data collected over several decades and the non-linear regression method, growth condition indices were developed and examined for five fish species: Dover sole (Microstomus pacificus), petrale sole (Eopsetta jordani), canary rockfish, yellowtail rockfish, and widow rockfish. No particular trend or close association with environmental variables was evident in the long-term growth condition indices for any of the species, but interannual and seasonal variations in condition were clearly evident. Possible reasons for the lack of long-term trends in condition and the lack of any close association with the environmental variables are discussed.
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