Research Document - 2016/012

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

By B. Waddell, J.S. Dunham, Z. Zhang and R.I. Perry

Abstract

Dungeness Crabs (Metacarcinus magister) increase in size incrementally by moulting.  In southern British Columbia, Canada, legal-sized (≥ 165 mm carapace point-to-point width) male crabs are believed to moult generally during the winter and spring, although the specific timing is unknown and can be variable.  After moulting, the new shell is soft, and gradually hardens over the next two to three months.  While in this soft-shell condition, crabs are more vulnerable to being injured and killed as a result of reduced protection from a hardened exoskeleton.  To better protect legal-sized male crabs in Crab Management Areas (CMAs) E-S, E-T, G and H, fisheries managers have requested information regarding the timing and variability when these crabs are soft-shelled.  As a result, a collaborative research program involving Fisheries and Oceans Canada (DFO) and the crab fishing industry was conducted from 2009 to 2013.  Crab biological data were collected in two ways:

1. using standardized trap gear fished independently of the commercial fishery, and
2. from commercial vessels actively fishing.

Two analytical methods were used:

1. by examining the proportion of soft-shell legal-sized males to all legal-sized male crabs sampled, and
2. by examining the numbers of soft-shell legal-sized male crabs collected per trap (CPUE).

Two models, a proportion model and a CPUE model, were developed within a Bayesian framework to estimate the timing of peak proportion and peak relative abundance, respectively, of soft-shell legal-sized male crabs based on observed data from the two sampling programs.  Analyses also determined time periods (including estimates of uncertainty) when the proportion and relative abundance of soft-shell legal-sized males were 95%, 90%, 85%, 75% and 50% of peak values. The peak proportion and peak relative abundance from analyses of soft-shell legal-sized male crabs in the four CMAs ranged from March 5 to 27.  Using proportion data from the fishery-independent sampling program, and a 10% reduction from the peak (0.90 × Peak), 48% of the legal-sized male crabs in CMA H were soft on any given day between January 23-May 7 (± 10 days), with a peak on March 15, 58% of the legal-sized male crabs in CMA E-S were soft on any given day between January 15-May 10 (± 12 days), with a peak on March 12, 43% of the legal-sized male crabs in CMA E-T were soft on any given day between January 26-May 16 (± 14 days), with a peak on March 21, and 62% of the legal-sized male crabs in CMA G were soft on any given day between February 5-May 12 (± 16 days), with a peak on March 24.

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