Decision rules for quota setting to support spatial management in a lobster (Jasus edwardsii) fishery

Abstract

The Northern Zone rock lobster fishery in South Australia is extensive, covering ca. 207 000 km2. Managed under transferable quotas, the majority of the annual total allowable commercial catch (TACC) is taken on eastern inshore grounds, targeting the smaller redder lobsters favoured by Chinese buyers. The confined nature of fishing under the TACC system has led to concerns of localized spatial depletion and suggested a need for spatial management. A regional partition of the TACC was proposed. Formal prescriptive decision-rule tables for setting regional TACCs annually were constructed based on target exploitation rates. A target exploitation rate of 20% historically produced zero average yearly change in biomass and, in previous bioeconomic projection modelling, achieved near-optimal net economic return. Using an age-based fishery assessment model that fits to catch totals by both weight and number landed, conditioning on fishing effort and mean weights-at-age, we estimated harvestable biomass for three regions. The management response was to establish inner and outer subzones where separate TACC decision rules are now adopted. Basing the harvest strategy on a target exploitation rate underlies a simple direct method for constructing decision-rule tables for setting TACC using the indicator of previous year’s catch per unit effort (cpue). In the normal cpue range, a constant target exploitation rate is applied. At lower levels of stock abundance, below a designated upper limit reference point of cpue, the exploitation rates used to assign TACCs are set to decline linearly from the normal target level, reaching zero at a designated lower-limit reference cpue point below which the fishery is closed. At higher levels of cpue, stakeholders agreed to a TACC cap, under which exploitation rate decreases if biomass rises. This case study applies where formal harvest strategy decision rules for quota-based lobster fisheries are considered at finer spatial scales.