Changes in the underlying productivity for a stock can result in changes in the carrying capacity, and hence effect biomass-based reference points. The CCAMLR toothfish Decision Rules use reference points based on B0 and assume that the underlying productivity of a stock does not change over time. Approaches to consider such effects are referred to as dynamic B0 approaches.
In this study, we evaluate the effect on the CCAMLR integrated toothfish assessments when changes in the underlying productivity (dynamic B0) are assumed. The consideration of potential changes in productivity has implications for management targets, and we show that assuming productivity changes can result in significant changes in the status of stocks.
As a first step, before adopting a dynamic B0 approach, CCAMLR should consider further developing methods for identifying productivity changes that could be considered as regime shifts resulting from environmental changes rather than fishing. Further, we recommend that:
- WG-SAM should develop a program to conduct Management Strategy Evaluations that allow for comparison of the static and dynamic B0 approaches while using different harvest control rules.
- Future toothfish stock assessments summarise the evidence for changes in stock assessment parameters or processes that could be due to the effects of environmental variability or climate change.
- WG-SAM identify methods for including time-variant biological parameters within integrated assessments, such as growth (Linf, K) or natural mortality or fitting time-varying stock recruitment models.