Research Document 1998/55
Stock composition of cod aggregations near the mouth of the Gulf of St. Lawrence in January 1996 based on an analysis of otolith elemental fingerprints
By S.E. Campana, G. Chouinard, M. Hanson, A. Fréchet, and J. Brattey
Dense aggregations of cod were observed in and around the mouth of the Gulf of St. Lawrence during a synoptic survey in January 1996, in keeping with previous winter RV surveys of the Gulf since 1994. The results of previously-reported tagging studies suggested that the 3Pn4RS, 3Ps, 4T, 4Vn and 4VsW stocks probably contributed to the aggregations, but the relative contribution, distribution and biomass of each of the stocks has never been determined. As part of the High Priority Project on the Identification of Mixed Cod Stocks in the Gulf of St. Lawrence and Approaches, the various aggregations were surveyed and sampled in January 1996, and 781 adult cod analyzed for stock identity using otolith elemental fingerprints, microsatellite DNA and vertebral counts. Spring spawning aggregations (used as baseline or reference collections) indicated that there were large differences in otolith elemental fingerprints among stocks for all but the 4Vn stock. Maximum likelihood-based stock composition analysis of independent collections of 420 pre-migratory cod in the northern and southern Gulf indicated that the accuracy of the elemental fingerprints as stock identifiers for 3Pn4RS and 4T cod exceeded 95%. Stock composition analysis of the Jan 1996 samples indicated that all stocks were present near the mouth of the Gulf, but that considerable stock separation was maintained. 4T cod dominated the stock composition of catches on the southern edge of the Laurentian Channel from the southern Gulf down to and including the northern edge of Banquereau Bank (4Vsb). Cod from 3Pn4Rs comprised most of the catch in the Laurentian Channel, as well as along its northern edge to St. Pierre Bank. 3Ps cod were concentrated most heavily to the west of St. Pierre Bank, where they mixed extensively with 3Pn4RS cod. 4VsW cod were virtually absent from all areas, including the northern edge of Banquereau Bank (4Vs). When calculated on a stock-specific basis, the 3Pn4Rs and 4T stocks together comprised about 94% of the survey biomass of 102,786 t, with the remainder being made up of 3Ps and 4VsW cod. Most of the 4T, and in particular the 3Pn4RS stock biomass, appeared to be represented in the survey area. The distribution of the 3Pn4RS biomass was not in keeping with current management units, indicating that management of a winter fishery would require different boundaries.
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