Stock assessment of Queensland east coast burrowing blackfish (Actinopyga spinea), with data to June 2023

Abstract

Burrowing blackfish are species of sea cucumber from the family Holothuriidae that is found in northeastern Australia, New Caledonia, and possibly other Melanesian countries. In Australia, burrowing blackfish distributions extend along the entire Great Barrier Reef (GBR). They often occur in shallow to deeper depths from 1–25 m.

This is the first stock assessment conducted on Queensland east coast burrowing blackfish by Fisheries Queensland. This stock assessment considered burrowing blackfish as three distinct populations, one associated with Gould Reef (Gould), one associated with the Capricorn Bunker Group (Bunker), and one associated with Lizard Island (Lizard). No stock assessment result is provided for Lizard.

All assessment inputs and outputs are referenced on a financial year basis (that is, ‘2023’ means July 2022–June 2023).

This assessment used a one-sex age-structured population model and a delay-difference model which led to similar results. The outputs of the age-structured model are presented as the main results for all three species in this assessment.

The assessment incorporated commercial catch and effort data spanning 1995 to 2023 as well as length composition data and estimates of absolute abundance from recent surveys undertaken in 2023. No recreational or Indigenous catch data were available and catches from these sectors are considered negligible. There are no discards due to the highly selective nature of the fishery.

Several Stock Synthesis scenarios were run to examine the implications of different fixed model parameters such as steepness (h) and natural mortality (M) on model outcomes. All scenarios were optimised using Markov chain Monte Carlo (MCMC) to better explore the robustness of the models. From these exploratory scenarios a final base case was chosen for each species. The base case Stock Synthesis results indicated that the biomass ratio of Gould at the beginning of 2024 financial year was between 51% and 101% of unfished levels. The base case Stock Synthesis results indicated that the biomass ratio of Bunker at the beginning of 2024 financial year was between 78% and 109% of unfished levels. The Lizard stock assessment modelling process failed to reconcile fishing pressure with the biomass decline observed through survey estimates of absolute biomass.