Publication Date

10-2007

Series Number

165

Publisher

Department of Fisheries

City

Perth

ISBN

1 877098 91 4

ISSN

1035 - 4549

Abstract

The crystal crab fishery on the west coast of Western Australia has only been commercially fished since the late 1990s. When this project was initiated in 2001, only compulsory monthly catch and effort data were being collected. This project has successfully set up the methods and means to collect and analyse more appropriate commercial catch and effort data for the fishery. Fishers now record detailed daily catch and effort data for legal as well as undersize and berried crabs and the Department of Fisheries, WA, has a regular monitoring programme to record population size structure information.

This project has also allowed for the collection, analysis and interpretation of biological data for the species. An extensive tagging programme has been undertaken over the five-year duration of this project. Tag recaptures have provided information on the growth rates and movement patterns of crystal crabs. These data have shown that crystal crabs grow slowly; best estimates are that legal size male crabs are 13 to 15 years old and that some large crabs in the population are around 30 years old. The size range of female crabs returned was insufficient to attempt an estimate of size at age for that sex.

Tag recaptures have also shown that neither sex, nor particular size class over the size range tagged, showed strong trends in their direction of movement after release. The majority moved less than 50 km, even after three and four years at large. Only 2% of recaptured crabs moved more than 100 km while at large.

The efficacy of escape gaps currently in use in the fishery has been examined. The gap is effective for females, but not for males. Pots fitted with two escape gaps lost approximately 16% of the legal size male catch.

In addition to researching the biology of the species, an assessment has also been made of the status of the stock. The fishery is largely focused on the coast between North West Cape and Fremantle and most of catch has been taken between 500-800 m. Fishing outside of those depths has contributed less than 5% and in some years less than 1% of the total landings.

Annual catches have hovered around 200 t since 2001 and peaked at 233 t in 2004. Effort has increased in each year except 2005. Catch rates declined from 2000, but soon stabilized and in recent years have shown an upward trend. Undersize crabs showed a continual decline in catch rates from 2000 to 2004, but in 2005 increased sharply. Berried crabs have shown a continuous increase in CPUE since 2001. Catch length frequency monitoring has shown a decrease in mean size for both sexes, with the decline being more marked for males than females, presumably because of their larger maximum size. This decline is expected at the start of a fishery. Mean size frequencies appear to have stabilized for both sexes in recent years.

Fisheries Research Report [Western Australia] No. 165, 2007 Although crystal crabs cover an extensive area they would not be expected to sustain intense fishing pressure. The reason for this is that at the depths at which they are found (~500-800 m), temperatures are low and food is probably limited. In addition, the species has been shown to be slow growing and long lived, a fact that has already been mentioned. Given that there is uncertainty surrounding any TAC recommendation and that recent catches have been maintained at about 200 t, it has been decided that an adaptive management approach will be adopted. It is proposed that a TAC of 140 t will be set for the fishery in the future and that its performance will be monitored, primarily against cpue. It is clear that a fishery of 140 t is unlikely to support seven full time fishers, but under future management arrangements each licence holder will have an even share of quota, which will be transferable.

The success of this project is that it has established the systems, databases, sampling regimes and basic biological information that have enabled a stock assessment to be undertaken for the fishery. It has also established performance indicators to assess the status of the stock. This fishery is undergoing major changes, in moving from an interim to full management plan. The research from this project will be crucial in enabling future decisions in this fishery to be made on a scientifically defensible basis.

Number of Pages

248

Keywords

Chaceon; crab; deep-sea; movement; growth; escape gap; CPUE

Disciplines

Aquaculture and Fisheries | Marine Biology

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