Science Advisory Report  2015/047

Evaluation of soft shell data for legal-sized male Dungeness Crab (Metacarcinus magister) in Crab Management Areas E, G, and H in British Columbia, 2009-2013

Summary

• DFO crab fishery managers requested information on the soft-shell timing of legal-sized male Dungeness Crabs (Metacarcinus magister)¹ in Crab Management Areas (CMAs) E, G, and H.
• Crab biological data collected from ongoing collaborative sampling programs between DFO, the commercial crab harvesters and their service provider, from 2009 to 2013, at index areas in CMAs E, G, and H, were used to evaluate when legal-sized male crabs were in soft-shell condition.
• Data were collected from two sampling programs:
• a fishery-independent (FI), using standardized trap gear fished independently of the commercial fishery; and,
• a fishery-dependent (FD) program, where sampling was conducted on commercial vessels while actively fishing.
• Sampling included 19,742 traps hauled during 820 sampling events. Approximately 157,000 Dungeness Crabs were sampled, of which about 18,000 were soft and 28,000 were hard-shell legal-sized males.
• Two analytical methods were used:
• The proportion method evaluated the proportion of soft-shell legal-sized males to all legal-sized male crabs sampled per sampling event; and,
• The CPUE method evaluated the catch per unit of effort (CPUE) of soft-shell legal-sized male crabs, defined as the total catch of soft-shell legal-sized males divided by the total number of traps in a sampling event.

Two Bayesian models, a proportion model and a CPUE model, were fitted to sample data to estimate the timing of peak proportion and peak relative abundance, respectively, of soft-shell legal-sized male crabs (CPUE is used as a proxy indicator of relative abundance of soft-shell legal-sized male crabs in the population). Time periods (including estimates of uncertainty) when the proportion and relative abundance of soft-shell legal-sized males were above various levels, ranging from 0.50 to 0.95 of peak soft-shell, were estimated.

• The proportion model produced longer estimated soft-shell periods than the CPUE model, and results regarding start/end and peak dates, and their 95% credible intervals, were generally less variable and more precise.
• Although soft-shell legal-sized males were observed throughout the year, the estimated peak of soft-shell legal-sized male crabs generally occurred in March.
• High numbers of soft-shell legal-sized male crabs were observed in other seasons, for example, in 2009 a secondary summer peak was observed in some areas.
• Results from the FD and FI sampling programs showed similar trends in peak timing of soft-shell legal-sized male crabs.

¹Metacarcinus magister, formally Cancer magister, is accepted as the current taxonomic name for Dungeness Crab (Davie 2015, Schram and Ng 2012).

This Science Advisory Report is from the April 21-22, 2015 regional peer review on Evaluation of soft shell data for legal-sized male Dungeness Crab (Metacarcinus magister) in Crab Management Areas E, G, and H in British Columbia, 2009-2013. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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