Research Document - 2004/130

Assessment of the Dungeness Crab population in the Nass Estuary, 2000 and 2001

By Alexander, R.F., W.J. Gazey, I. Winther

Abstract

Dungeness crabs (Cancer magister) were captured with traps in the Nass Estuary from June 2000 to April 2002 to monitor abundance and collect biological samples. A total of 1845 traps (264 trap sets) were fished and 20,978 crabs captured. Catches of crabs included 12,164 males and 8814 females. Of the male catch, 6343 legal- (>153 mm notch width [NW]), 5297 medium- (127-153 mm NW) and 519 small- (<127 mm NW) sized crabs were caught. A total of 5976 male crabs, 2976 in 2000 and 3000 in 2001, were anchor-tagged between May and September in three defined release areas. A total of 1679 tagged crabs were later recovered during sampling sessions and in marine (Nisga’a and commercial) fisheries. Counts of marked and unmarked crabs from observed catches (sampling and commercial-fishery patrol observations), unobserved marine catches and recoveries were used to compute a population estimate in 2000 (70,092 [CI=66,912, 73,272]) and 2001 (98,701 [CI=88,816, 108,586]) for legal-sized male Dungeness crabs. Exploitation rates of the legal-sized males in the Nass Estuary were estimated at 57% and 46% for 2000 and 2001, respectively, assuming an annual instantaneous natural mortality and tag loss rate of 1.5. Of the overall exploitation rates calculated for the Nass Estuary for 2000 and 2001, the exploitation rates for the commercial-opened area were 65% and 68%, respectively, and for the commercial-closed area 25% and 12%, respectively. A catch per effort analysis was also performed for standard trap sets conducted in the open and closed commercial areas of the Nass Estuary to test for seasonal variations in abundance and to compare with the mark-recapture results. As a result of these analyses, the commercial fishery was found not to have a negative effect on the availability of legal-sized male crabs in the closed area. In addition, the level of harvest and effort observed in this study suggests that the legal-size male Dungeness crab population is not being over exploited in the Nass Estuary.

Estuary is occurring in the spring and early summer months. Consequently, the 6-wk commercial fishery in the Nass Estuary between October and mid-November is being conducted during the period of highest percentage of hard-shelled males. This study also provided information on the female and pre-recruit male populations, vertical and horizontal distribution of crabs within the Nass Estuary, incidence of injury and embrace marks of Dungeness crabs. Information regarding individual movement of legal- and medium-sized male Dungeness crabs within the Nass Estuary was also collected from the monitoring of 30 ultrasonic-tagged crabs between August and November 2000.

Complete document (PDF, 6064 KB)

Accessibility Notice

This document is available in PDF format. If the document is not accessible to you, please contact the Secretariat to obtain another appropriate format, such as regular print, large print, Braille or audio version.