Assessment of the blue swimmer crab (Portunus armatus) fishery in Queensland

Abstract

The blue swimmer crab (BSC) fishery in Queensland has undergone considerable change since its development in the middle of the 20th century. In the last 30 years the fishery has progressed to being fully fished with the expansion of crab-pot fisheries to areas outside of Moreton Bay. After the implementation of the Fisheries (East Coast Trawl) Management Plan 1999, the reported harvest from the trawl sector decreased significantly, with the commercial and recreational crab-pot sectors now accounting for the majority of the reported blue swimmer crab harvest. The commercial pot fisheries outside Moreton Bay also developed rapidly in the late 1990s. Concerns have been raised by a number of crab-pot fishers about poor economic viability under current management rules and reduced catch rates from a number of Moreton and Hervey Bay areas. This research and publication was requested by fisheries management to assess reasons for declining catch rates and profits, including evidence associated with overfishing, crab reproduction and disease, and flood events.

The analyses undertaken during the current study suggested that the blue swimmer crab population was not overfished to the point where the spawning biomass was significantly reduced. However, the current levels of population size of male legal crab and fishing effort were not suitable to produce acceptable levels of catch rates for economic profit or angling quality.

Analysis of the reproductive characters of the blue swimmer crab from fished areas of Moreton Bay has shown no significant changes in key indicators since research was conducted in the mid 1980’s. Similarly, the levels of fertilization of immediate post-moult female crabs have remained unchanged in the past 30 years. There was no evidence of declines in fertilization rate as all females assessed had >90% of eggs successfully developing. There was evidence to suggest a decline in the size of females, despite the fact that females are not legally fished.

The levels of parasitism and disease were found to be low and have not changed significantly in the last 30 years. Levels of parasitism by Sacculina granifera have declined.

Environmental factors have been shown to influence blue swimmer crab fisheries in both Cockburn Sound and Shark Bay in Western Australia, but we found no conclusive evidence to support similar effects in Queensland.

Declining catch rates appear to be related to overly competitive fishing, resulting in a smaller population size of male legal crab. Increases in the number of pots used by fishers in all regions is likely a result of escalating costs of fishing, particularly fuel, impacting on profits due to declining beach price relative to inflation. To cure this state, the data and analyses indicate that effort reduction is required in order to significantly improve catch rates and generate economic profits. A range of maximum economic yields were calculated for the Moreton and Hervey Bay regions, so that a meaningful upper limit on license numbers and total allowable fishing effort can be evaluated. Effort reductions of 50% to 70% may be required in order to maximise vessel based profit.

The evaluation of raising the minimum legal size (MLS) in order to reduce fishing pressure indicates that this would have a negative impact on fishery economics.