Document Type

SCU access only

Publication details

Johnson, D 2008, 'Fisheries and biological data for management of the blue swimmer crab fisheries of New South Wales', MSc thesis, Southern Cross University, Lismore, NSW.

Copyright D Johnson 2008

Abstract

This research describes the commercial trap fishery and biology of the blue swimmer crab, Portunus pelagicus, in Wallis Lake (NSW). It also reports on preliminary trials for appropriate sampling of P. pelagicus populations for fisheries and biological studies. It reports on the status of the NSW commercial fisheries.

Overall 81% of male and 79% of female P. pelagicus captured in the Wallis lake crab trap fishery were larger than the current minimum legal carapace length (CL)(i.e. 60 mm) and therefore retained. It was estimated that approximately 385,988 (± 37,551) P. pelagicus were retained and 183,653 (± 29,716) discarded from the trap fishery during the 2002/03 season. The quantities of non-target species captured were generally very low, and significantly less than that generally reported for other crab-trap fisheries. Acanthopagrus australis was the most abundant bycatch species, with an estimated total of 2,445 (± 1,564) caught, representing 43% of the total bycatch, during the 2002/03 season. Discarded organisms were in good condition when returned to the wild. This study demonstrates that the Wallis Lake trap fishery for P. pelagicus is relatively selective and has minimal direct impact on non-target species.

The fecundity of Portunus. pelagicus in Wallis Lake increased in a positive linear relationship with CL. The mean number of eggs carried per brood in this study was greater than that reported throughout P.pelagicus distribution. Examination of gonads in overvigorous females showed that female P. pelagicus from Wallis Lake can produce several batches of eggs from a single mating. The estimated number of egg broods produced by female crabs in the different size classes ranged from one in crabs (50 - 55 mm CL) to three in crabs (75 - 80 mm CL). The size at which females reach maturity (CL50) is below the current minimum legal size limit of 60 mm carapace length applied in NSW waters. Yield per recruit analysis indicated that lowering the minimum legal size to 52 mm would increase yield provided there was no change in exploitation rates. However, by exposing female crabs in the 55-60 mm size class to fishing pressure, egg production in Wallis Lake could potentially decline.

Tag-based studies have been of limited success in estimating growth and describing the movement patterns of the blue swimmer crabs in Australia. In an attempt to improve survival rates of tagged individuals, this study investigated alternative tag designs. However, results indicate that tagging P. pelagicus using polyethylene streamer tags may not be viable due to the high levels of initial mortality observed after tagging (Male 62.5%, female 25%).

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