While shifts species distributions in south-eastern Australia associated with ocean warming, fisheries management is extending to byproduct species that are caught incidentally and have limited data. Assessment methods developed for targeted stocks may not be as reliable for byproduct species. Catch rates can reflect redirected effort as much as abundance, and stock declines can be hard to distinguish from distribution shifts. Different assessment approaches can therefore return conflicting conclusions about the same stock status.
John Dory (Zeus faber) in the Southern and Eastern Scalefish and Shark Fishery illustrates this challenge. It is a high-value byproduct species caught incidentally, in addition to catches have shifting southward over two decades. A Tier 3 catch curve assessment in 2017 indicated a healthy stock, whereas a Tier 4 CPUE-based assessment in 2020 placed it below the limit reference point and led to a reduced bycatch total allowable catch (TAC). A 2026 integrated assessment that was developed in an attempt to reconcile these results used all available data but again could not resolve the conflict between data sources.
We present a step-down approach in which the integrated model is progressively simplified to isolate the information carried by each data source. We explore how far the simpler methods can be trusted, which signal is most credible for a species that cannot be targeted, and provide guidelines for when a conflict in the data prevents conclusive assessments. We also consider the management consequences, including the cost of unnecessarily constraining a mixed-species trawl fishery against the risk of overfishing a declining stock. John Dory provides a case study for the growing number of data-limited and climate emerging stocks requiring formal management.