Improving catch prediction for tiger prawns in the Australian northern prawn fishery

Population models form the basis for the assessments of species in the tiger prawn component of Australia's northern prawn fishery. Penaeus semisulcatus and P. esculentus are assessed using a size-structured population model. These assessments form the basis for a control rule which predicts future total allowable catches (TACs) for P. semisulcatus and P. esculentus so that the discounted profit from the fishery is maximized. However, there are concerns with this approach: (i) the TAC predictions have consistently overpredicted actual catches and (ii) the assessment for one of the species exhibits a retrospective pattern. A series of analyses was conducted to explore the causes of these observations. Results indicate that catch, effort, and recruitment prediction can be improved substantially by changing the assumed selectivity pattern for one of the surveys, changing how the length frequency data are assembled from the raw data collected, changing the constraints on the minimum amount of effort by target fleet, modifying how the distribution of effort by week is forecasted, and dropping the length frequency data from the most recent recruitment survey. More generally, the analyses illustrate how retrospective analysis can be used to improve how assessments and projections are undertaken when the quantities of interest are known retrospectively.

From input to output controls in a short-lived species: the case of Australia's Northern Prawn Fishery

A management strategy evaluation (MSE) framework is developed to evaluate strategies that provide total allowable catches (TACs) when the target biomass corresponds to maximum economic yield (MEY). The framework is applied to Australia’s Northern Prawn Fishery (NPF), which has been actively managed using a tradable input-control system, but is to move to output controls based on individual transferable quotas (ITQs), with a consequence that the current management strategy needs to be replaced. Because the fleet is small, it is possible to set a TAC that cannot be taken entirely. Whereas input controls tend to self-adjust if recruitment is not accurately predicted, and consequently catch variability tends to be low, this is not the case for the reasonably variable species caught in the NPF when they are managed using TACs. The management strategy recovered the simulated stock to the target reference point when it was initially depleted, and avoided dropping the stock below the limit reference point (LRP) for five scenarios based on the current best understanding of resource status; however, in some cases, the stock was left above the target biomass because of biases in the assessment. A scenario in which the stock was forced to be initially depleted to below the limit reference point showed that the management strategy allows recovery.