Research Document 1998/20

Exploitation rates and population size of cod in Placentia Bay (Subdivision 3Ps): estimates from a new mark-recapture study

By John Brattey and Noel Cadigan

Abstract

Approximately 6,000 commercial sized (>40 cm) cod were tagged with t-bar anchor tags and released at various locations in Placentia Bay and adjacent areas prior to and during the 1997 commercial fishery. The tagging program and associated reward scheme were advertised widely among those participating in the fishery. Cod were tagged with one yellow ($10 reward), two yellow ($20 reward for both), or one pink tag ($100 reward) in approximate proportions 0.45:0.45:0.1. The high-reward tags were used to estimate the reporting rate of standard ($10 reward) tags; double tags were applied to investigate the rate of long-term tag loss. Experiments involving retention of tagged cod in submersible enclosures were conducted to provide estimates of initial tag shedding and short-term tagging mortality. Exploitation rates were estimated for each stock component that was tagged; the estimates were based on numbers of single-tagged cod reported as recaptured during the fishery, adjusted to account for reporting rate, natural mortality, and various sources of tag loss and tagging mortality. Estimates of exploitation rate on inshore spawning aggregations tagged before the fishery began averaged 0.170 and ranged from 0.126 to 0.213; the corresponding estimate for a large spawning aggregation tagged 5 - 9 days after the fishery began was 0.078. The 1997 commercial fishery consisted mainly of three short periods of fishing activity. A model that used information on tag returns and commercial catch was developed to provide estimates of the overall exploitation rate associated with each period of fishing and to back-calculate a population size for cod in Placentia Bay prior to reopening of the commercial fishery. The model assumed a closed population from April-October, and constant exploitation rates throughout Placentia Bay. The model gave estimated exploitation rates of 0.042, 0.041 and 0.023 for the first, second, and third periods of fishing. Initial population size was estimated at about 26 million fish, or approximately 52,000 mT based on an average commercial weight of 2.0 kg per fish from the commercial catch.

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