Skates (Rajidae) represent the greatest biomass of incidental bycatch caught in the Patagonian toothfish (Dissostichus eleginoides) longline fishery operating within Australian EEZ waters around HIMI in the Southern Ocean. Due to their life history characteristics of slow growth and late maturation, their populations are particularly vulnerable to fishing mortality which can lead to overall and localised population declines. As bycatch managed species, sustainable bycatch limits are informed by a stock assessment (using the Generalised Yield Model) which models long-term population viability. Such models can be sensitive to the input parameters of assumed total fishing mortality. Currently, in the HIMI fisheries, only retained skates are counted towards fishery removals, but the overall fisheries-induced mortality may be substantially higher due to the high number of skates released after capture and the unknown, but potentially low, post-release survival. Post-release survival becomes increasingly important as the scale of discarding increases; with ~90% of all skate bycatch in the HIMI longline fishery being released it is imperative that post-release survival is quantified to get an accurate estimate of total fishing mortality.
This project aims to quantify post-release survival of Bathyraja spp. skates caught in the deep waters of the Kerguelen Plateau using the latest in pop-up satellite tag technology. In doing so, this project will improve the estimation of total fishing mortality to inform the skate bycatch assessment model and the estimation sustainable yield. Thus, it supports the Commonwealth Research Advisory Committee 2018-2021 Research Development and Extension (RD&E) Plan by addressing the following priority area actions:
Program 1. Environment
• Better Assessment Approaches, and Harvest and Management Strategies
o By quantifying post-release survival we provide a demonstrable improvement in fisheries removals data quality and decision-making by providing evidence-based recommendations for setting accurate bycatch limits.
• Better Managed Fishing Impacts and Interactions
o By underpinning accurate bycatch limits with accurate long-term population viability estimates we enable fisheries managers to demonstrate environmentally sustainable bycatch management practices.
In addition to the COMRAC RD&E Plan, this project addresses the following priority area outcomes within the Southern Ocean Industry Partner Agreement RD&E Plan:
Program 1. Environment
• TEP and bycatch management:
o The need to monitor skate/ray bycatch by providing accurate estimates of total predicted fishing mortality based on past and current catches.
o Develop stock assessment approaches for skate bycatch species (which are generally not retained for commercial use) by integrating quantified fishing mortality estimates into the stock assessment model.
o Provide post-release survival estimates and insight into behavioural ecology of deep-water skates for development of a risk assessment for multiple incidental bycatch species.
Program 2. Industry
• Management efficiency
o By resolving a key component of uncertainty in the stock assessment this project reduces risk in fisheries management decision-making processes by improving accuracy in bycatch limit recommendations.
Furthermore, this project meets the primary objectives of the AFMA Strategic Research Plan (2017-2022), namely to collect appropriate information to support stock assessments, support the management of Commonwealth fisheries, and inform policy development [Research Strategy 1a]. The results of this study will also inform fisheries managers on appropriate specifications for the release and retention of caught skates.