A spatially explicit population dynamics operating model for Antarctic toothfish in the habitable depths of the Ross Sea region

We present a 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. In this model run we spatially restrict the stock to cells where at least 5% of the depth is deemed suitable as habitat for toothfish (120 cells – semi-restricted model), and compare with previous models where the stock was either restricted to the fished cells only (65 cells – restricted model) or allowed to occupy the entire Ross Sea region including depths outside of those normally considered suitable habitat for toothfish (unrestricted model). The semi-restricted model provides a plausible distribution hypothesis in-between the two extreme bounding hypotheses represented by the previously presented restricted and unrestricted models (Mormede et al. 2013).



The most plausible model is the one that restricts toothfish distribution to depths which are known to provide suitable toothfish habitat. This is unsurprising because the model utilises fishery-dependent data and has no information about the distribution of toothfish in areas where no fishing has occurred, such that the unrestricted model estimates fish movements into cells outside of the fished area including those with implausible depths for toothfish.

 

Further data collection would be useful to improve the parameterisation of the model, in particular making collection of gonad weight measurements routine on all fishing vessels, surveying likely spawning grounds during winter, and obtaining fishery or survey data from 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 it can also be used to examine the likely effects on the toothfish stock assessment of alternate spatial management options affecting the distribution of fishing effort, data collection, and/or tagged fish releases.