Mud crab (Scylla serrata) population dynamics in the Northern Territory, Australia and their relationship to the commercial fishery
Creators: name: Knuckey, Ian Andrew
Publisher: name: Charles Darwin University
Year: 1999

description: The mud crab Scylla serrata (Forskal 1775) is a large portunid crab common throughout the Indo-west Pacific region. In the Northern Territory, Australia, it is fished by traditional and recreational sectors, but the bulk of the catch is by the commercial fishery. Expansion of the commercial fishery over the last decade raised concern over the status of the mud crab resource. With little known about the fishery, the current research was initiated to provide a better understanding of mud crab population dynamics and the impact of fishing on the resource. Commercial catch and effort data from logbook records were validated and analysed. The recorded catch was found to be usually within 10% of the actual catch and "potday" was determined to be a reliable measure of effort. These data confirmed significant expansion in the fishery, which was primarily attributed to the uptake of latent effort present in the fishery since it became limited entry in 1985. Such expansion curtailed the use of annual catch-per-unit-effort (CPUE) as an index of abundance for the fishery. Analysis of monthly logbook figures revealed an extremely seasonal fishery which peaked during the middle of the year and virtually closed down between December and February in the wet season. Monthly CPUE was considered a better index of abundance, and revealed repeated seasonal declines during the wet season. Further research highlighted aspects of mud crab population biology which explained the annual cycle in the fishery. Tagging studies indicated that mud crabs grow relatively quickly and begin to recruit to the fishery (130 mm carapace width) during February to March, about 12 months after hatching. Over the next few months, effort increases as crabbers return to the fishery after their break during the relatively unproductive months of the wet season. The new recruits become increasingly vulnerable to capture by the commercial gear, which is biased towards larger crabs (~150 mm CW). Exploitation is high during the middle months of the year and significantly reduces the new cohort of 1+ year old crabs. By this time most females have reached their first mature instar and mated, but they remain vulnerable to the fishery over the latter months of the year when the proportion of females in the catch increases to around 60 - 70%. This rapidly declines to around 10 - 20% by the early months of the year because the females move offshore for the main spawning period. By the end of the year, catches and catch rates have declined and remain low until the next year's recruits enter the fishery. These population dynamics have important implications for the management of the fishery. Over the last decade, expansion of the fishery has been within the bounds of the input controls implemented in 1985. More recently there is evidence that changes in fishing practices may enable effort to continue to increase despite these effort restrictions. As a result, it is important that the minimum legal size limit provides the stocks with adequate protection from recruitment overfishing. Size at 50% maturity for females is around 135 mm CW and although males mature physiologically at smaller sizes (100 mm CW) they do not usually achieve functional maturity until about 150 mm CW. Thus, at 130 mm CW, the size limit appeared to offer minimal protection for crabs to mature and reproduce. On the other hand, commercial pots have reduced selectivity for mud crabs under 150 mm CW, which tends to override the small size limit. Nevertheless, modelling of the fishery indicates that under the current fishing regime, at least 20% of females survive to reproduce. To date, this has been sufficient to ensure adequate recruitment; in a fishery which predominantly consists of 1+ crabs, any recruitment failure would be quickly apparent. Although there is no evidence of recruitment overfishing in the fishery under the current management arrangements, exploitation rates are high. Until a stock-recruitment relationship or a fishery independent estimate of stock size can be established, a cautious approach should be adopted with respect to further expansion in the fishery. In the mean time, it would be prudent to introduce legislation to increase the minimum size limit or maintain the current pot selectivity to minimise the impact of increasing fishing effort on the spawning stock.