Data-poor Stock Assessment Approach: Catch-Maximum Sustainable Yield (MSY)
In addition to routine analyses of spatio-temporal trends in catch and effort, we used several catch-only methods to estimate stock depletion and catch relative to the estimated maximum sustainable yield (MSY). The results shown here are based on the “Catch-MSY” method (Martell and Froese 2013), which refers to a model-assisted stock assessment approach developed for data-poor conditions. The approach relies on the Schaefer production model, which defines the relationship between biomass and catch based on the intrinsic population growth rate (r), and which assumes that the biomass delivering MSY is equal to 50% of the unfished biomass. According to a time series of catch records and the assumed resilience of the target species (“very low” ,“low”, “medium”, or “high”, and associated ranges of plausible r values), Catch-MSY can be used for a stock reduction analysis based upon which credible Schaefer model predictions are inferred to estimate management reference points for MSY and biomass depletion (Haddon 2018, Haddon et al. 2019) .
Biomass depletion fluctuating around 50% of unfished levels is a commonly defined target (Btarget = BMSY = 0.5 B/B0, where B = biomass and B0 = unfished biomass), but has also been used as a threshold in precautionary Australian harvest strategies to initiate reductions in catch of data-poor fish populations so that biomass remains above or recovers back to target levels (B > 0.5 B/B0) (see e.g., DPIRP (2020)). Biomass depletion below 20% is an internationally applied limit reference point (Blimit), beyond which directed fisheries under Australian harvest strategies are closed (Rayns 2007, Smith et al. 2009, Punt et al. 2014).
The Catch-MSY analyses conducted here were based on the commercial component of total fishery catch, generally excluding estimates of recreational catch. The Catch-MSY method appears to be robust to the exclusion of recreational catch data unless trends in recreational vs commercial catch over time are divergent (Haddon 2018).
Scalefish species selected for Catch-MSY analyses were those for which we assumed that changes to management over the duration of recorded fishery catch did not severely undermine the use of catch data to infer trends in abundance. The same implicit assumption was made with respect to changes in the spatial distribution of fishing effort and catch.
