Natal source contributions of Pacific cod (Gadus macrocephalus) recruits in the southeastern Bering Sea Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/08612s22f

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  • Effective and sustainable fisheries management not only depends on identifying and defining stocks (fundamental unit of management), but also on having knowledge of factors influencing the abundance, distribution and connectivity of stocks. Population structure of Pacific cod (Gadus macrocephalus) in the southeastern Bering Sea (EBS) remains unresolved although genetic data indicate isolation-by-distance throughout the species range across the North Pacific Ocean. Pacific cod are fished on their spawning grounds, therefore, it is important to evaluate whether particular spawning sources are more critical than others in sustaining the productivity of fished populations. Chemical analysis of otoliths can provide information on mixing among groups of fish on ecological time scales and at finer spatial scales than genetic analyses. Therefore, I completed laboratory and field research on the otolith elemental composition of larval and juvenile cod to: (1) investigate temperature and growth-rate effects on otolith elemental incorporation to aid interpretations of field data; (2) evaluate the potential to detect larval exchange between the Gulf of Alaska (GOA) and the EBS; (3) quantify spatial scales at which there was significant variation in otolith elemental composition; and (4) quantify the number of larval sources (chemically distinct groups) contributing to juvenile recruits in the EBS. In the laboratory study, I observed higher rates of incorporation at lower temperatures for both Sr and Ba but similar ranges of Mg incorporation at 2°C, 6°C and 8°C. There was no significant effect of somatic growth or otolith precipitation rate on elemental incorporation detected, indicating that variation in individual growth rates should not confound interpretations of field data. In two years of field collections (2006 & 2008), otolith elemental signatures (Mg:Ca, Mn:Ca, Zn:Ca, Sr:Ca, and Ba:Ca) differed between larvae collected in the EBS and GOA, and over 70% of the larvae were correctly classified to source. Furthermore, I examined the chemical signatures of the otolith edge (representing approximately the last 10 d of life) for juveniles collected at six locations throughout the EBS. Seventy-nine and 71% of the juveniles were correctly classified to their collection location in 2006 and 2008, respectively. These results demonstrate that there is sufficient spatial variation in otolith composition to examine spatial structure in Pacific cod within the EBS and to identify GOA larval contribution to the EBS population. Finally, larval source signatures, represented by otolith core chemistry, of juveniles collected throughout the EBS were classified into chemically distinct groups using hierarchical cluster analysis. In both years we identified two dominant sources representing 56% and 30% of the cohort in 2006, and 53% and 40% in 2008. Most collection sites contained a mixture of larval sources although one or two sites per year were comprised of a single source. These chemically unique sources likely represent different spawning locations with distinct environmental characteristics, although the exact locations are unknown. These sources potentially represent contributions from known spawning areas, which include the Alaskan Peninsula, Aleutian Islands, and Pribilof Islands. Our results highlight the potential of otolith chemical analyses to identify source contributions of Pacific cod in the EBS but larval collections throughout the spawning season are needed to provide additional detail on the spatial distribution of those sources.
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