Date of Award

Winter 2008

Project Type

Thesis

Program or Major

Zoology

Degree Name

Master of Science

First Advisor

Adrienne I Kovach

Second Advisor

David L Berlinsky

Abstract

Most Atlantic cod (Gadus morhua) stocks within U.S. waters are currently in decline as a result of overexploitation and fishing pressure from commercial fisheries. A better understanding of the genetic structure of cod populations is essential to identify stocks for successful fisheries management. In this study, the genetic structure of cod from major temporally and geographically distinct spawning grounds in U.S. waters was investigated. Adult and juvenile cod were sampled from aggregations within the Gulf of Maine, Georges Bank, and southern New England waters in cooperation with commercial fishermen and state fishery biologists. Caudal fin clips were collected and analyzed using 11 microsatellite markers and the Pan I locus. Two spawning complexes of cod were identified. The northern spring complex was comprised of cod from coastal Gulf of Maine regions during the spring and summer seasons. The southern complex was comprised of cod collected during the winter months in the Gulf of Maine, on Stellwagen Bank in early spring, and at all southern locations. Georges Bank spawners were identified as a possible intermediate population between the complexes; they exhibited significant divergence from southern New England spawning aggregations, but not from cod within the Gulf of Maine. Differentiation among these populations was stable over a two year study period and was consistent with previous results for Atlantic cod reported by Lage et al. 2004 and Wirgin et al. 2007. Juveniles collected from within the Gulf of Maine could be assigned as mixtures to parental spawning complexes. Divergence among adult and juvenile cod was primarily dependent on differentiation at the Gmo132 and Pan I markers, which are suspected to be under natural selection pressures. Local adaptation to environmental factors such as water temperature and salinity may therefore be the driving force of population differentiation. Natal homing and water currents that limit larval dispersal may also influence the stock structure, but cod migrations and dispersions among spawning aggregations likely limit reproductive isolation and neutral genetic drift of populations within US waters.

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