Management implications of including a climate-induced recruitment shift in the stock assessment for jackass morwong (Nemadactylus macropterus) in south-eastern Australia

Abstract

Fishery responses to climate change are occurring on time-scales relevant to the provision of short-term management advice. This is especially the case in climate change ‘hotspots’ such as south-eastern Australia, where the rate of ocean-warming is 3–4 times the global average. The application of harvest strategies utilizing biological reference points that do not take into account the effect of the changing environment on fish productivity may lead to unsustainable catch recommendations. Jackass morwong (Nemadactylus macropterus) is a moderately long-lived demersal species inhabiting continental shelf waters of the southern hemisphere. The most recent Stock Synthesis assessment for the species suggests that this stock has recently declined in abundance off south-eastern Australia. The stock assessment attributes the decline to mostly below average recruitment since 1985, but the recommended catch levels are based on an assumption of average recruitment from a stationary stock–recruitment relationship. The ability of Stock Synthesis to include an environmental variable to adjust the stock–recruitment relationship is used to model a regime shift on the average level of recruitment. Management strategy evaluation (MSE) is then used to examine the consequences of using the wrong recruitment assumption in the assessment used in the harvest strategy for setting the catches. The MSE shows that the consequences of mis-specifying the assessment model are greater if the assessment continues to assume that no shift in recruitment has occurred. Thus the more precautionary approach for management wishing to meet all the aims of the Australian government Harvest Strategy Policy is to assume that a shift in recruitment has occurred. A possible mechanism for how observed oceanic changes could be impacting jackass morwong recruitment is presented.