Estimation Of The Exploitation Rates And Migration Rates For Cod (Gadus Morhua) In Nafo Divisions 3kl And Subdivision 3ps During 1997-2000 From Tagging Experiments

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It is possible to estimate the exploitation rate (fraction of stock removed) by a Þshery using data from Þsh tagging experiments. Exploitation rate estimates can then be used in conjunction with estimates of landings by the Þshery to estimate stock size. This is particularly useful for inshore stock components if the near-shore topography causes problems for common survey methods, e.g. trawl and acoustic surveys. A series of comprehensive Atlantic cod tagging studies have been conducted in NAFO Divisions 3L and 3K (3KL) and Subdivision 3Ps during 1997-2000. Over 49 000 cod in total have been tagged and released in 123 experiments. We use 7394 tag-returns from the cod Þsheries in 1997-2000 to estimate the exploitation and migration rates of cod in these regions. Migrations of tagged Þsh must be accounted for to get unbiased estimates of exploitation rates. In addition, knowledge of migration rates is also required to assess the total exploitation rates on stock components. We use a model that incorporates length-based exploitation rates for different gear types. The model also incorporates estimates of tagging mortality rates, tag loss rates, tag reporting rates, and Þsh growth rates. The exploitation rate estimates are used with reported landings by the Þshery to estimate weekly cod biomass and abundance in six geographic zones around the coast of Newfoundland. The difficulties caused by stock migrations will be illustrated and discussed.