DEVELOPMENT OF A SPATIALLY EXPLICIT AGE-STRUCTURED STATISTICAL CATCH-AT-AGE POPULATION DYNAMICS MODEL FOR MODELLING MOVEMENT OF ANTARCTIC TOOTHFISH IN THE ROSS SEA

We present a generalised spatially explicit Bayesian statistical catch-at-age population dynamics model (SPM) for developing and investigating plausible spatial movement models, and apply a preliminary development version of this model to Antarctic toothfish in the Ross Sea as an age and maturity state spatial movement model. SPM is an aggregate movement model suitable for use with large numbers of areas, and is implemented as a discrete time-step state-space model that represents a cohort-based population age structure in a spatially explicit manner. The model is 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. SPM was designed to be flexible, allow for the estimation of both population and movement parameters based on local or aggregated spatially explicit observations, and optimised for speed. Model validation consisted of three types: implementation checking; development-driven unit tests; and comparative software evaluation. Comparisons with expected output from CASAL and movement processes coded in S+/R were essentially identical, and estimates of example parameters for models implemented in both CASAL and SPM gave essentially identical results. We have also developed a preliminary model for Antarctic toothfish in the Ross Sea and describe the spatial and population structure and processes, data, observations, and likelihoods used to estimate movement parameters. The model was a single sex model that categorised fish as immature, mature, or spawning. Observations included within the model were spatially explicit commercial catch proportions-at-age and CPUE indices. While we caution that model results are preliminary, we note that they appeared reasonable, and suggested immature fish were located in the southern Ross Sea on the continental shelf, mature fish were located on the continental slope, and spawning fish were located on the northern banks of the Ross Sea. The results also suggested that parameterising of movement based on latitude, depth and distance provided a significantly better fit to the observations than a model where depth was ignored. However, further development to the SPM model is required, including processes and observation classes to incorporate year class variability, stock recruitment relationships, tag-release and tag-recapture observations, and maturation state observations. Further, the current implementation of the MCMC algorithm in SPM is only partially complete, and there is some further work on parallelisation algorithms for MCMC that could be investigated. And, in order to address the questions of the adequacy of the Antarctic toothfish Ross Sea assessment model, SPM needs to be modified to allow simulation of observations from underlying movement parameters. Finally, once adequate models for Antarctic toothfish in the Ross Sea have been developed using SPM, the current assessment model (Dunn & Hanchet 2007) would need to be evaluated within a simulation-experiment in order to address current assessment model uncertainties.