Research Document - 2007/072
A progress report on the development of a new fishery for inshore Tanner Crabs (Chionoecetes bairdi Rathbun, 1924) in Rivers Inlet, British Columbia
By Fong, K.H., and J.S. Dunham
Abstract
Fisheries and Oceans Canada (DFO) and the Wuikinuxv Nation have undertaken a cooperative Phase 1 assessment of inshore Tanner crab, Chionoecetes bairdi, in support of developing a fishery for Tanner crabs in Rivers Inlet, B.C. This follows the Phase 0 information review on C. bairdi by Krause et al. (2001). This paper presents new information regarding distribution, abundance, and biological characteristics of C. bairdi in Rivers Inlet.
The results from exploratory trap and trawl surveys and a mark-recapture program carried out between January 2004 through March 2005 by DFO and the Wuikinuxv Nation are presented. Trappings captured more male than female C. bairdi (5.27:1). Trawling proved unsuccessful at capturing Tanner crabs. C. bairdi were found distributed throughout Rivers Inlet (except Fitz Hugh Sound) but were not overly abundant. Some areas supported more Tanner crabs than others; crabs were more abundant in Draney Inlet and Darby Channel. C. bairdi were found at depths ranging from 36-340 m with greatest abundance of males at 50 to 150 m and females 100 to 200 m.
Male C. bairdi captured in Rivers Inlet ranged in size from 4 to 137 mm carapace width (CW). The mean size of 50% maturity for males is 94 mm CW and 81 mm CW for females. A 20% growth rate was estimated for mature males. Based on this information a minimum size limit of 113 mm CW was determined for males taking into account one moult increment past size of maturity to allow a season of breeding. The presence of a terminal moult after maturity remains inconclusive.
Our data show a small proportion of mature males moult throughout the year with a moult event possibly taking place in the spring or summer. Breeding in Rivers Inlet occurs in the fall with an egg-release period in the spring. Natural mortality (M) for male Tanner crabs was estimated at 0.69 and 1.12. Tagging showed that Tanner crab movements ranged between 0 and 4,592 m over a period of 4 to 422 days. Tanner crabs did not move between major areas within Rivers Inlet.
A multiple census mark-recapture technique was used to estimate the population size of male Tanner crabs in Rivers Inlet. A total of 1,315 crabs (1,055 males and 260 females) were tagged and released during six research surveys in Rivers Inlet of which 20 were recovered by surveys and 6 by the public. The population estimate for male Tanner crabs greater than 55 mm CW in Rivers Inlet is 37,907 (28,483 to 56,650). The total number and biomass of legal size males (≥ 113 mm CW) was estimated to be 7,178 to 9,553 (lower 95% CI and mean) or approximately 4,163 to 5,541 kg, respectively.
The relatively small size of C. bairdi found in Rivers Inlet compared to those in Alaska and the lack of significant abundance of legal males raises doubt whether a future fishery for Tanner crabs in Rivers Inlet would be economically viable.
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