Research Document - 2000/074

Estimation of cod (Gadus morhua) growth in NAFO Subdiv. 3Ps and Divs. 3KL in 1997-1999 from tagging experiments.

By N. Cadigan and J. Brattey

Abstract

We use tag-returns from the commercial, sentinel, and food fisheries in these regions during 1997-1999. The returns are from numerous tagging experiments conducted in 3Ps and 3KL during 1997-1999. Many of the tags that are returned supply the length of the fish at capture, and we use this information to estimate growth based on known lengths-at-release and times-at-liberty. We use the Von Bertalanffy growth model, modified to accommodate seasonal variations in growth. We also incorporate a measurement bias component in our estimation because the tag-return data suggest that there is some bias in the lengths-at-capture reported by fishermen. The basic approach we use has been studied fairly extensively in the fisheries literature, and we present a review of this literature. Important biases in estimating growth from tagging data have been identified in the literature, and we assess the magnitude of the bias in our analysis using a simple simulation. The conclusion from our simulation is that the bias may not be substantial for our population. We use the estimated model to predict cod growth for times-at-liberty ranging from 0-3 years, and for fish lengths ranging between 40 and 90 cm. For example, we estimate that a 40 cm fish will grow 22 cm during 1997-1999, but in the same period a 90 cm fish will grow only 10 cm.

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