Application of a weekly delay-difference model to commercial catch and effort data for tiger prawns in Australia’s Northern Prawn Fishery

Abstract

The two species of tiger prawn ( Penaeus semisulcatus and P. esculentus) harvested in Australia’s Northern Prawn Fishery are assessed by fitting a Deriso–Schnute delay-difference model to catch and effort data. The population dynamics model has a weekly time-step and allows for week-specificity in recruitment, spawning, availability and fishing mortality. The stock–recruitment relationship is fitted assuming temporally correlated environmental variability and by downweighting recruitments that are poorly determined by the catch and effort data. Uncertainty is quantified through sensitivity tests, variance estimation and future projections. The projections account for the technical interaction between the two species in that effort directed at one species leads to some mortality on the other species. Recruitment and spawning stock size are robustly estimated to have declined substantially but the status of the resource relative to MSY-based reference points is uncertain. The three factors to which the results are most sensitive are the value assumed for the catchability coefficient, the rate of change over time in fishing efficiency, and the future within-year effort distribution. Seasonal closures are shown to lead to increased yields at similar levels of risk to the resource, particularly for P. semisulcatus.