We present an optimised spatially explicit age-structured population dynamics operating model for Antarctic toothfish over the entire Ross Sea region, for a medium scale spatial resolution (189 spatial cells) covering the Ross Sea region.
The model was developed as a generalised Bayesian population dynamics model implemented using the Spatial Population Model software (SPM), and a single sex age-structured model that categorised fish as immature, mature, pre-spawning, spawning, or post-spawning. Observations include spatially explicit commercial catch proportions-at-age, proportions mature and proportions spawning (based on GSI data), CPUE, and tag-release and tag-recapture observations. Estimates of parameters appeared to broadly reflect the hypothesised spatial distribution of Antarctic toothfish, suggesting that younger fish were found predominantly in southern areas of the Ross Sea shelf, mature fish on the slope and spawning fish in the northern areas of the Ross Sea region. Fits to the observations were generally good.
Further investigations are recommended in order to further validate and refine this model. These include testing alternative model assumptions around fishing selectivity, maturity and migrations; as well as collecting further data to better inform the maturity ogive and spawning migration. We also recommend a survey of the assumed spawning grounds during winter, and the controlled collection of data in areas not fished to date. In addition, further external validation diagnostics need to be developed for this model, and also for any other such models being developed. Due to the complexity of the problems encountered in validating spatial models, a spatial modelling workshop would be of great benefit.
Although still undergoing development, this model is useful for investigating the direction of potential biases in the current single-area stock assessment model, and for carrying out management strategy evaluations under various alternative hypotheses of fish movement.