Stock assessment of Queensland east coast tiger prawns (Penaeus esculentus and Penaeus semisulcatus), Australia, with data to December 2021

Abstract

‘Tiger prawn’ is a collective term for two species: brown tiger prawn (Penaeus esculentus) and grooved tiger prawn (P. semisulcatus). Brown tiger prawns are endemic to tropical and subtropical waters of Australia, while grooved tiger prawns have a wider Indo–West Pacific distribution. This assessment focuses on tiger prawns found on the eastern coast of Queensland. The species live at least two years and have a maximum observed size of 44 mm carapace length for brown females, 35 mm carapace length for brown males, 52 mm carapace length for grooved females and 38 mm carapace length for grooved males. Sexual maturity is reached at approximately 6 months of age and around 32–39 mm carapace length. A previous assessment estimated the Northern management region was 49% of unfished in 2019, and separately estimated the Central management region was at 50% of unfished in 2019. This assessment assumes a single tiger prawn population north of 22 degrees latitude and contains significant updates to data and methodology. 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). The assessment used a one-sex monthly delay-difference population model, fitted to catch rates. An age-structured model was also trialled, however this did not lead to outcomes that were considered plausible by the project team. The model incorporated data spanning the period 1958 to 2021 including mandatory daily commercial logbook data collected by Fisheries Queensland (1988–2021), historic voluntary logbook data (1970–1988), Queensland Fish Board data (1958–1981), historic catch records (1958–2014), survey and logbook gear data collected by Fisheries Queensland (1988–2021), high resolution vessel tracking data collected by Fisheries Queensland (2000–2021) and lunar data (1958–2021). Length data collected by Fisheries Queensland (1998–2009) were also incorporated in a modelling scenario. The stock assessment was guided by a project team consisting of scientists, managers, and industry representatives. Ten scenarios were run using a delay-difference model, 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 indicated that the tiger prawn stock experienced a decline from the period 1958 to 1996 to reach 31% of unfished biomass. The biomass has been steadily rising since this time, and in 2021 the stock level was estimated to be 79% of unfished biomass (70–89% range across the 95% credible interval)