Management strategy evaluation of static and dynamic harvest control rules under long-term changes in stock productivity: A case study from the SESSF

Abstract

The harvest control rules (HCRs) for many fished marine populations rely on biological reference points. These reference points are often related to unfished biomass conditions (B0) and are calculated assuming that biological parameters and their associated functional forms (e.g., unfished recruitment (R0), natural mortality (M), growth) are stationary (‘static B0’). However, there is increasing evidence that the assumption of stationarity is untenable in the face of environmental change. In principle, non-stationarity can be addressed by defining stock status (i.e., spawning biomass relative to unfished spawning biomass) using ‘dynamic B0’ (the spawning biomass that would be expected in the absence of fishing). Previous work has examined the implications of adopting HCRs under the assumption that spawning biomass and hence target fishing mortalities are known exactly. However, this is an unrealistic assumption so management strategy evaluation (MSE) is used here to contrast the performance of a static B0 HCR and three variants of a dynamic B0 HCR for four stocks in Australia’s Southern and Eastern Scalefish and Shark Fishery. The MSE explores scenarios in which non-stationary is due to time-trends in M and R0, with performance measures based on static and dynamic B0, as well as total catches and catch variability. In declining productivity scenarios the use of a dynamic B0 HCR will on average maintain slightly higher catches than a static B0 HCR, but the stock biomass (in magnitude) will be less than if a static B0 HCR was adopted. Stock status then depends on the frame of reference – relative to static B0 the status will be lower than relative to an estimate of annual unfished biomass. In addition, the annual variability in catches is reduced with the dynamic B0 HCR compared to the static B0 HCR and there is a lower probability of zero Recommended Biological Catches. The preference for dynamic B0 or static B0 HCRs therefore relates to the aims of the harvest strategy to be implemented. Some harvest strategies may be focused on preserving the relative stock status through time and the management objectives do not pertain to the absolute numbers of individuals in the population, in which case a dynamic B0 HCR may meet the requirements of the harvest strategy. Other strategies may require that a population does not drop below a pre-specified absolute size, in which case an HCR that uses static B0 as the limit reference point would be preferable.