Research Document - 2002/118

Evaluation of an Intensive Fishery on Dungeness Crab, Cancer magister, in Fraser Delta, British Columbia

By Z. Zhang, W. Hajas, A. Phillips, J.A. Boutillier

Abstract

The Dungeness crab fishery in British Columbia has been passively managed through sex and size limits. Only male crabs larger than or equal to 155 mm in carapace width may be harvested. The fishery is intensive with exploitation rates well over 90% in the Fraser Delta. The paper attempts to address three specific concerns: (1) is there a recruitment problem? (2) what is the impact of intensive fishing on yield and profit? (3) what scientific criteria could managers use to close the fishery?

To address the first concern, we examined the time-series (1995-2000) data on catch rates for sub-legal crabs. There is no strong evidence that small crabs are becoming less abundant. However, the time-series is short and we (the authors) don't know the recruitment status before the intensive fishing, which has existed for a long time. Thus, it is unknown whether recruitment will increase, if exploitation rate decreases. The impact of intensive fishing on recruitment is yet to be determined.

To address the second concern, we used a length-based and an instar-based models to generate biological reference points to be used for managing the fishery. We calculated yield, revenue and profit per recruit, after some important biological parameters, such as natural mortality rate, vulnerability of different sized crabs to traps, probability of moulting, survival rate for newly moulted crabs, were estimated based on scientific surveys in the Vancouver Harbour. An intensive fishing also results in a great deal of catch-and-release of sub-legal sized crabs. Continuing fishing at a high ratio of sub-legal to legal sized crabs in the catch will result in a net loss in yield in the long-term, as some sub-legal sized crabs will die of handling mortality and could not contribute to the future yield. We conducted analyses on gain-or-loss in yield for continuing fishing at different ratios of sub-legal to legal crabs in the catch to determine threshold points, at which gain is balanced with loss in yield in the long term. To avoid losing yield in the long term, the ratio of sub-legal to legal crabs in the catch should not be allowed to rise above 19:1, 9.5:1, 6.5:1 or 5:1, if the handling mortality rate is, respectively, 5%, 10%, 15%, or 20%.

We provide the following recommendations to the managers based on these analyses:

1. The current exploitation rate (> 90%) should be reduced to 65-75%. A level of reduction in CPUE relative to the CPUE at the beginning of the fishery could be used to determine when to close the fishery.
2. A ratio of retained crabs to discards should be used as a means of limiting effort and protecting stocks in conjunction with using CPUE measures, or in fisheries where it is difficult to use CPUE because of protracted moulting seasons.
3. Efforts should be made to reduce the negative handling impacts. Tools may include longer soak times, earlier closure of the fishery or adjustment of the fishing season.
4. Industry, management and science should use these models to assist in assessing the impact of intensive fishing on population dynamics, economic and social benefits for each fishery and in finding optimal management and assessment schemes.

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