South-east Australian waters are recognised as ocean warming hotspots and overall, Australian waters have warmed faster than the global average (Hobday and Pecl 2013, IPCC 2019). Key components of the productivity of marine fish (growth, maturity, and recruitment) are expected to be changing in response to shifts in climate and it is entirely possible that there have been changes in fundamental productivity parameters for Australian stocks.
The regularity with which the biological parameters that are used in stock assessments are evaluated and updated varies considerably among the species that are targeted in Commonwealth Fisheries. Assessment of changes in these parameters is limited largely to sensitivity analyses consisting of exploring alternate time-invariant values of natural mortality, maturity and stock recruitment steepness at values close to those used in the base-case assessment and generally agreed upon as within acceptable ranges of values. Recently an evaluation of the provenance of the biological parameters used in stock assessments found that species from the SESSF contained the largest number of parameters where provenance could not be ascertained from the literature and that SESSF species comprised over 50% of those species where biological parameters were determined to be more than 20 years old (FRDC project 2019-010, Evans et al. 2022). When plausible changes to biological parameters (such as those that might occur under environmental change) were explored quite substantial changes in biomass estimates for key target species occurs. This means that parameter mis-specification, such as due to relying on older parameter estimates that encode predator-prey and other ecosystem processes from a system state that has since changed, could be a real issue for assessments in the SESSF.
The reliance of current assessments on what is likely to be out-of-date information leads to considerable uncertainty, which cannot be easily quantified that then propagates into management decisions. Without an understanding of changes in biological parameters and how these changes might impact assessments, it is difficult (if not impossible) to evaluate whether current management measures are ensuring sustainability. Overall, the project recommended that updating parameters in stock assessments, modifying base cases, or more heavily drawing on results from sensitivity analysis in discussion of stock assessment results would be strongly advisable, especially in regions where large environmental shifts are known to be occurring, such as the SESSF.
The RV Investigator voyages to be conducted in 2023 and 2024 under the CSIRO led SEA-MES project provide a unique opportunity to access relevant biological samples that could be used to update the biological parameters identified in FRDC Project 2019-010 as a high priority (age, growth, reproduction, stock structure and although not directly used in stock assessments themselves but having significant influence on parameters that are used in assessments (such as growth and mortality), diet and food webs). These voyages have a focus on the marine ecosystem that supports the SESSF and a number of the hypotheses being posed by the study are focused on target species within the SESSF and their food webs. This will result in significant sampling of those species , with the co-benefit that there will be new samples available for the contemporary estimation of key biological parameters and evaluation of the representativeness of parameters being used in stock assessments (and with sufficient sample numbers to ensure robust updated estimates). These voyages also provide a unique opportunity to build capacity in at sea sampling, exposure to ecosystem level sampling design and post-voyage biological analyses. By linking post voyage analyses with the direct needs of both stock assessments and ecosystem models used in the SESSF, the project provides opportunities for building deeper understanding of the use of biological parameters in stock assessments, and direct application of fisheries biology.