Further development of coarse- and medium-scale spatially explicit population dynamics operating models for Antarctic toothfish in the Ross Sea region

We present developments towards spatially explicit age-structured population dynamics operating models for Antarctic toothfish in the Ross Sea. The operating models consider both a coarse-scale and medium-scale spatial resolution and consider scenarios where abundance can be present over the entire Ross Sea region or restricted to areas where the fishery has operated.



The models are implemented in the generalised Bayesian population dynamics model, the Spatial Population Model (SPM). The SPM program allows implementation of an aggregate movement model for use with large numbers of areas as a discrete time-step state-space model that represents a cohort-based population age structure in a spatially explicit manner. Models can be parameterised by both population processes (i.e., ageing, recruitment, and mortality), as well as movement processes defined as the product of a set of preference functions that are based on known attributes of spatial location.



The operating models considered were single sex age-structured models 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 when the operating models were used as estimation models with observations from the Ross Sea Antarctic toothfish fishery appeared to broadly reflect the hypothesised spatial distribution of Antarctic toothfish, suggesting that younger fish were found predominantly in the southern shelf areas, mature fish on the slope and spawning fish in the northern areas of the Ross Sea region. Fits to the commercial catch proportions-at-age observations were generally good in most models, although fits to the plus group of the proportions-at-age catch data were less than ideal. Model estimates of proportions-mature appeared to be sensible, with a clear pattern that the proportions mature were a function of location and age. Tag release and recapture data were less well fitted by the models due, in part, to the conflict with assumptions of known abundance in the model and the abundance information inherent in the tag-recapture observations. Whilst these models are an improvement on earlier versions, further work is required to improve these residual patterns and to better match the observations.