Calculating optimal effort and catch trajectories for multiple species modelled using a mix of size-structured, delay-difference and biomass dynamics models

A framework is described whereby effort levels and their associated catches consistent with maximizing the net present value (NPV) of fishery profits over time can be calculated when each harvested species is modelled using a different population dynamics model. Results are presented based on three species ( Penaeus semisulcatus , P. esculentus , and Metapenaeus endeavouri ) in Australia's Northern Prawn Fishery and three population dynamics models (size-structured, delay-difference, and biomass dynamics). The results indicate that there is a considerable between-model variation in key model outputs such as the catch predicted for 2010 and the estimated future long-term catches corresponding to maximum economic yield. This variation is comparable with that due to uncertainty about economic parameters when all species are modelled using a size-structured population dynamics model, highlighting the importance of both good population dynamics models and accurate economic parameter inputs. The results also highlight some of the implications (in terms of estimating effort and catch levels which maximize NPV) of not having sufficient data when using population dynamics models to explicitly represent some of the species caught in a multi-species fishery.