Research Document - 1999/038
Estimation of coarse exploitation and migration rates of Atlantic cod (Gadus morhua) in NAFO subdivisions 3Psc and 3KL during 1997 and 1998 based on tagging experiments.
By N.G. Cadigan and J. Brattey
Abstract
Mark-recapture experiments can be used to estimate the abundance of fish inhabiting inshore regions where rough bottom and steep bathymetry preclude use of other common survey methods such as trawling and acoustics. Recaptures of tagged fish can be used to estimate the exploitation rate of the fishery and the exploitation rate can, in conjunction with estimates of the total landings by the commercial fishery, then be used to estimate stock size. Estimation of exploitation rates requires estimates of the number of tagged fish available for capture. This number changes with time-since-release because of tag loss and migration of tagged fish out of the fishing region, as well as natural mortality and removals by the fishery. In this paper we use a simple cyclical-seasonal migration model to estimate migration rates of cod between Placentia Bay off the south coast of Newfoundland (3Psc), and the inshore off the northeast coast of Newfoundland (3KL). We also use the model to estimate exploitation rates for the 1997 and 1998 fisheries in these regions.
Our results indicate that exploitation of cod in Placentia Bay was 7% during January-June 1997, and 4% in the remainder of the year. Exploitation was also very low during the first half of 1998 in Placentia Bay, but increased to 10% in the second half of the year. Exploitation in 3KL was very low in 1997 and the first half of 1998 due to lack of fishing activity, but in the second half of 1998 exploitation was estimated to be 6% for cod in northern 3L and 3K, and 12% for cod in southern 3L.
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