Spatial variation in Antarctic toothfish (Dissostichus mawsoni) catch rate, mean weight, maturity stage and sex ratio across Divisions 58.4.1, 58.4.2 and 58.4.3b

Exploratory longline fisheries targeting mainly Antarctic toothfish (Dissostichus mawsoni) have been operating in Divisions 58.4.1 and 58.4.2 since 2003, however robust stock assessments and catch limits according to CCAMLR decision rules have yet to be developed. A key step in developing management advice in these Divisions is understanding the distribution, abundance and boundaries of the target stock. WG-FSA-16/29 outlined the first multi-member toothfish exploratory fishery research plan, including research objectives, methods and milestones. Within this plan, milestones 1.5 and 2.1 involve the development of stock structure hypotheses and spatially-explicit habitat-use models. Accordingly, this paper reports on the development of generalised additive mixed models to characterise relationships between environmental variables and (1) relative density, (2) mean weight, (3) proportion mature and (4) sex ratio of D. mawsoni, including in adjacent Division 58.4.3b. Spatial heterogeneity in catch composition indicated that D. mawsoni were not randomly distributed across the area. Models were used to generate predictions of D. mawsoni density and composition across a broad spatial scale, and revise hypotheses relating to the structure and functioning of the stock.