Research Document - 2000/073

Lower bounds on the exploitation of cod (Gadus morhua) in NAFO Subdiv. 3Ps and Divs. 3KL in 1997-1999 from tagging experiments.

By N. Cadigan and J. Brattey

Abstract

We develop a model to estimate exploitation rates (fraction of stock removed by the fishery) using tags returned from the commercial fisheries for Atlantic cod in NAFO Divisions 3L and 3K (3KL) and Subdiv. 3Ps and adjacent areas during 1997-1999. The tag-returns are from numerous tagging experiments conducted in these areas during 1997-1999. Over 36 000 cod in total have been tagged and released during 1997-1999 and 4007 of these have been reported as recaptured. Our model involves length- and gear-based exploitation rates and incorporates methods to estimate tagging mortality, tag loss, tag reporting rates, and fish growth. We use this model to estimate weekly exploitation rates in 3KL and 3Ps; however, we focus on exploitation in 3KL during the two time periods in which most of the commercial catch in 1999 was taken: July, and September 5 to October 10.

Emigration of tagged fish complicates the analysis of tag-returns, and can result in underestimation of exploitation rates. The fraction of tagged fish that were released and subsequently returned by the fishery will represent only part of the total exploitation when tagged fish emigrate outside the fishing region. This is because the size of the tagged population available to the fishery is less than the number of tagged fish that were released. At present we have not been able to account for migration in our tagging model. Our estimates represent the fraction of the tagged population exploited by the fishery if there was no migration. The actual exploitation of the fishery is higher than the quantity we estimate. Nonetheless, we feel our results are reasonably accurate for 3K and the northern part of 3L where migration appears low. Our results suggest that exploitation rates in 3K and northern 3L during July of 1999 were 19.4% and 2.3%, respectively. During September 5 to October 10 the exploitation rate estimated in 3K was 13.5%, and in northern 3L the estimate was 3.8%. Using the reported landings for these two periods, we estimate that the biomass of fish in 3K was 9 000 and 11 000 tonnes in July and September 5 to October 10, respectively. The biomass in northern 3L was estimated to be 50 000 and 42 000 tonnes in July and September 5 to October 10. Our results for southern 3L were considered too unreliable because of the extensive movements of fish between this region and 3Ps.

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