Fisheries long term monitoring program : summary of spanner crab (Ranina ranina) survey results: 2000–2005

Abstract

The Department of Primary Industries and Fisheries manages the harvest of Queensland’s fish, mollusc and crustacean species and the habitats they live in. Inherent in this responsibility is a commitment to monitoring the condition and trends in fish populations and their associated habitats. This information is used to assess the effectiveness of fisheries management strategies and contributes to ensuring that the fisheries remain ecologically sustainable. The spanner crab, Ranina ranina, inhabits sandy bottoms on the continental shelf off Australia’s east coast, from southern New South Wales, north to the southern Great Barrier Reef. The annual commercial harvest of spanner crabs in Queensland increased rapidly from 1988 to 1994 with fishing effort and catch rates increasing as the fishery expanded into previously unexploited areas. Since 1999, the fishery has been subject to a Total Allowable Commercial Catch (TACC) in Management Area A, divided between the licensed operators by way of Individual Transferable Quota units (ITQ). The current TACC of 1727 tonnes has been in effect since June 2002.
The present TACC setting decision rules are based on performance criteria derived from commercial catch rates. However, there is concern about the reliability of fishery dependent catch rates as indicators of stock abundance, partly because the stock is not uniformly distributed, resulting in fishers targeting aggregations of crabs. In response to these management concerns and the high value of the fishery (approximately $10 m), spanner crabs were included in the Department’s Long Term Monitoring Program (LTMP).
The objectives of the spanner crab monitoring component of this program are to obtain fishery independent catch per unit effort (CPUE) data, record length frequency and sexratio data, identify interactions with species of conservation interest (SOCI), and monitor bycatch composition.
The sampling design includes five assessment regions (Regions 2 to 6) comprising of the Queensland Commercial Fishery (Managed Area A), which is subject to the TACC. For the 2005 survey an extra region (Region 7) was added in New South Wales. Fifteen sets per subgrids were conducted, with each set consisting of ten standard commercial spanner crab dillies connected by a ‘trot-line’ (or single string). Five subgrids were sampled in Regions 2 to 6, and four subgrids in Region 7. This report presents survey results from five years, 2000 to 2003 and 2005.
A total of 19 290 individual dilly lifts have been undertaken by the LTMP fishery independent spanner crab survey. In the Queensland regions observations of fishery bycatch during 2002, 2003 and 2005 show very low catch rates. Most of the bycatch species captured by spanner crab fishing gear are alive, and expected survival rates after release would be extremely high. To date there is not a single record of physical interaction between sampling gear and marine protected species.
Differences in mean carapace lengths were observed between male and female crabs over all regions. Overall, 68.1% of the 23 842 males and 13.0% of the 4193 females captured over the five year survey period were larger than the minimum legal carapace length of 100 mm. Carapace lengths also varied greatly between all regions sampled (Regions 2 to 7); male crabs were smaller in Region 5 compared to other regions and female crabs were largest in Regions 6 and 7.
Overall, catches were dominated by males, which represented 85.0% of the overall catch. Distinct trends were evident in the proportion of the catch represented by males and females in each region and these were relatively consistent across the five year study period. In particular, Regions 6 and 7 had a much higher percentage of females in the catch, 25.5% and 50.9% respectively compared to Regions 2, 3, 4 and 5 (9.6% to 17.6%).
Catch rates of male and female crabs varied between all regions. While catch rates differed among years within regions, differences among regions were generally consistent over the five year study period. Catch rates over the five year period were much higher in Region 4 than all other regions, reflecting the commercial catch data for the same period, which indicated highest catch rates in Region 4.
Analysis of catch rates with respect to the fishing depth and average bottom water temperature indicated that catches were highest at depths of 60 to 69 m for both male and female crabs. Catch rates of male and female crabs were variable with regards to bottom water temperature. However, these relationships were confounded by region to a large extent and were not consistently observed across all regions.
The LTMP spanner crab component, has delivered a time series of size and sex-ratio data coupled with fishery independent catch rate data for the five Queensland assessment regions in Commercial Fishery (Managed Area A). Similar data were collected from NSW in 2005 only, although only for a single year. Further standardization of the fishery independent catch rate with associated biological and physical characteristics will greatly enhance the importance of this data set for future analysis and regional population assessments.