Research Document - 2002/003
Exploitation rates and movements of Atlantic cod (Gadus morhua) in NAFO Subdivision 3Ps based on tagging experiments conducted during 1997-2001
By J. Brattey, D.R. Porter, and C.W. George
Abstract
large-scale mark-recapture study of adult (>45 cm) Atlantic cod (Gadus morhua), initiated in 3Ps in spring 1997, was continued. During 1997-2001 a total of 42,000 live cod were tagged with single, double, or high-reward t-bar anchor tags and released at various inshore and offshore locations off the south coast of Newfoundland. Approximately 6,500 tagged cod have been reported as recaptured up to September 2001. Tag returns were adjusted to account for region-specific reporting rates, and the number of tagged cod available for capture was adjusted to account for tagging mortality, tag loss and assumed natural mortality. These data were used to estimate annual exploitation rates for each batch of tagged cod. The spatial distribution of recaptures from tagging in each area was summarized annually with numbers recaptured adjusted to account for region-specific reporting rates. Cod tagged offshore were often recaptured considerable distances from the tagging area; those tagged on Burgeo Bank moved into 3Pn-4RS or eastward along the inshore of 3Ps and the proportions in each region differed considerably between years. Those tagged in Halibut Channel remained offshore or migrated inshore to Placentia Bay and the Avalon Peninsula. Cod tagged inshore in Placentia Bay were mostly recaptured within Placentia Bay, but small proportions (~5%) were also recaptured in southern 3L, Fortune Bay, or in offshore 3Ps. Cod tagged in Fortune Bay were mostly recaptured inshore in Fortune Bay or Placentia Bay, or more rarely westward into Hermitage Bay. Exploitation rates estimated from tag returns differed markedly across the stock area. Annual estimates of exploitation for cod tagged offshore in Halibut Channel in three consecutive years (1998-2000) were consistently low (0-0.06), as were those for cod tagged offshore in the Burgeo Bank-Hermitage Channel area (0.05-0.13). Exploitation of cod tagged in Fortune Bay ranged from 0.11 to 0.22. Estimates of exploitation rate for cod tagged in Placentia Bay have been much higher, with many estimates exceeding 0.30 in both 1999 and 2000; results from the tagging analyses indicate that measures are required to lower the exploitation rate in this portion of the stock area.
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