We present an updated spatially explicit age-structured population dynamics operating model for Antarctic toothfish in the Ross Sea region, for a medium scale spatial resolution (189 spatial cells) covering the Ross Sea region. We build on the previous working medium-scale model in which the population was restricted to those cells in which at least 5% of cell area was of a depth deemed suitable as habitat for toothfish (120 cells – semi-restricted model).
Model fits to the reproductive data were unsatisfactory in previous models, with a much flatter maturity ogive than that derived through histology. In the previous model reproductive parameters (proportion maturing and proportion spawning) were derived from two gonadosomatic index thresholds. In this model we keep only proportion spawning data, and retain the assumption that a proportion of the mature fish spawn each year. The model estimation of the maturity ogive is now similar to that estimated by histological analysis. This model represents our current best model of the spatial distribution and movement of Antarctic toothfish in the Ross Sea region given the movement hypothesis proposed.
Further data collection would be useful to improve the parameterisation of the model, in particular obtaining gonad weight measurements from all fished areas, biological characterisation of fish on the spawning grounds during winter, and characterising the fish population in areas not fished to date.
We propose that the modelling platform presented here can be used to test the likelihood and examine the consequences of alternative fish movement hypotheses given the data available. For a given movement scenario the model can also be used to examine the likely effects on the toothfish stock assessment of alternate spatial management options or alternate tagging programme designs affecting the distribution of fishing effort, data collection, and/or tagged fish releases.