Development of an industry-funded fishery-independent survey and associated indices for managing a deep sea crab resource in Western Australia

Abstract

The inclusion of fishery-independent data in stock assessments has been shown to have positive stock and financial outcomes for fisheries. A co-management approach between the government and commercial fishers in this limited-entry fishery led to the successful implementation of industry-funded, fishery-independent surveys of crystal crab, a deep water Geryonid crab, in two areas off Western Australia.While the instigation of these surveys was driven by a need for improved abundance indices for stock assessment, they have also provided an opportunity to better understand stock dynamics and composition. Structuring the surveys across the depth distribution (400–700 m) of crystal crabs demonstrated an apparent lack of depth-related size stratification in crystal crabs with large crabs captured throughout the species depth distribution. Female crabs, which mature below the legal minimum size for harvest, were a minor component of the catch and were more prevalent at the shallower end of their depth distribution. There also appeared to be a movement of pre-molt crabs to the periphery of their distribution (shallow and deep), in preparation to molt.There was a slight decline in the fishery-independent abundance (FIA) index from 2017 to 2019 mirroring the pattern displayed by the current standardized commercial catch rate, with an increase during the most recent survey (2020). The FIA index will be incorporated as an additional data source for the weight of evidence and size-structured integrated model for the assessment of the crystal crab resource off the west coast of Western Australia.The surveys, which were funded by industry and supported by government research staff onboard, have proved cost-effective, robust and sustainable, and a preferable means by which fishery-independent data could be collected for this commercially important Geryonid crab. The cost of the survey was partially offset by the retention of the legal catch. Such a collaborative model could be applied in fisheries world-wide to address the inherent biases of commercially-derived indices and the ever-increasing financial constraints on regulators to conduct stock assessments.