Stock assessment of Moreton Bay bugs (Thenus australiensis and Thenus parindicus) in Queensland, Australia with data to December 2021

Abstract

Moreton Bay bugs are distributed throughout tropical and subtropical coastal waters of Australia from northern New South Wales to Shark Bay in Western Australia. The Moreton Bay bug population on the east coast of Queensland is comprised of two species—Thenus australiensis, also known as the sand or reef bug, and Thenus parindicus, also known as the mud bug. Sand bug females reach 50% maturity at 82 mm carapace width (CW) or 59 mm carapace length (CL). Mud bug females reach 50% maturity at 75 mm carapace width or 53 mm carapace length. Both species spawn year-round with spawning peaks during the period between spring and mid-summer.

This is the first stock assessment conducted on Queensland Moreton Bay bugs.

This stock assessment includes input data through to December 2021. All assessment inputs and outputs were referenced on a calendar year basis (that is, ‘2021’ means January 2021–December 2021).

For all stocks analysed, the assessment used a one-sex monthly delay-difference population model,
fitted to catch rates. Age-structured models were also trialed, however these did not lead to outcomes that were considered plausible by the project team.

For sand bugs, the data from 1968 to 2021 comprised of commercial catch and effort (1988—2021), historical commercial catch (1968–1981, 1974–1987), fishery independent survey data (2017-2021) and licence numbers (1968–2003). For mud bugs, the data from 1948 to 2021 comprised of commercial catch and effort (1988-–2021), historical commercial catch (1948–1981, 1974–1987) and licence numbers (1968–2003). The model split the fishery into two fleets to account for the rezoning of the Great Barrier Reef (GBR) in 2004—one for the commercial sector pre-July 2004, and one for commercial sector post-July 2004.

The stock assessment was guided by a project team consisting of scientists, managers, and industry representatives.

Thirteen model scenarios were run for the sand bug stock, covering a range of modelling assumptions and sensitivity tests. All scenarios were optimised using Markov chain Monte Carlo (MCMC) to better explore the robustness of the models. Project team preferred scenario results suggested that the sand bug stock experienced a decline in the period 1968 to 2000 to reach 67% of unfished biomass. The biomass has been generally increasing since, and in 2021 the stock level was estimated to be 78% (63—94% range across the 95% credible interval) of the unfished biomass.

Thirteen model scenarios were run for the mud bug stock, covering a range of modelling assumptions and sensitivity tests. Seven scenarios had convergence problems, or diagnostics that indicated issues. The non-target nature of the fishery combined with fishery-dependent catch rates being the primary data set for model tuning makes assessment difficult. The status of the mud bug stock is undefined. The general trajectory across the thirteen scenarios shows the biomass experienced a decline from the period of 1968 until the mid 1980s, then slowly recovered since that time.