Determining the spatial distribution and abundance indices for Moreton Bay Bugs, Thenus parindicus and Thenus australiensis in Queensland to improve stock assessment and management
Department of Agriculture and Fisheries EcoScience Precinct
Tony J. Courtney
Moreton Bay bugs (Scyllarid lobsters) are a commercially important fished resource in northern Australian coastal waters. About 600 tonnes of bugs is reported in logbooks annually, valued at about $15 million (~$25/kg) nationally. About 90% of the catch is taken in the Queensland east coast otter trawl fishery (QECOTF) (https://www.fish.gov.au/report/154-MORETON-BAY-BUGS-2018). Historically, Moreton Bay bugs were retained as byproduct of prawn and scallop trawling, but over time their popularity and price have increased and they have become increasingly targeted by fishers. In recent years in the Queensland scallop fishery, the scallop catch has declined and fishers have targeted bugs to such an extent that their catch value now exceeds that of the scallops. Quantifying the fishing effort targeted at bugs and differentiating it from the prawn and scallop effort is challenging. This complicates the estimation of reliable catch rate time series that can be used as an index of abundance for each group (i.e. bugs, prawns, scallops). Despite their high value, Queensland has not previously undertaken a stock assessment of Moreton Bay Bugs, beyond yield-per-recruit analyses. This is largely because the Bug catch is composed of two species, reef bugs (Thenus australiensis) and mud bugs (Thenus parindicus) which are not differentiated by fishers in the logbook data. Mud bugs are the smaller of the two species and generally occur in depths of 10–30 m, while the larger reef bug generally occurs in depths of 30–60 m. Although there is some spatial separation of the species it is not possible to breakdown the catch of each from the logbook data alone. This project aims to examine and define the spatial distribution of the two bug species. By surveying and sampling bugs, and examining seafloor properties, we hope to predict and map the distribution of each species. Once clear distinctions are made, we plan to use all available data on catch rates and gear technology used by the fleet, to derive standardised catch rate time series for each species, which can be used as indices of abundance for improved management and assessment.
1. Implement a state-wide crew member program to obtain detailed photographic records of Moreton Bay bug catches, to assist with determining the species composition and distribution of catches.
2. Undertake a stratified survey of bug catch rates, species composition and seafloor properties in the main trawl fishing grounds off Townsville.
3. Use all available data sources to model, predict and map the spatial distribution of the two species of Moreton Bay bugs along the Queensland coast.
4. Produce long-term standardised catch rates for each bug species that can be used as an index of abundance for stock assessment and management.