Indian Ocean tropical tuna regional scaling factors that allow for seasonality and cell areas

Indian Ocean tuna assessments are spatially structured, with regions that contain separate but linked subpopulations. In such multi-region assessments we must determine the relative abundances among regions. Regional scaling, which has been used since 2005 in tuna assessments, estimates the abundance distribution from regional catch rates and areas. We describe the method and explore potential improvements to the current practice. Supported improvements included using cell ocean areas in scaling calculations; adjusting statistical weights in the standardization model based on the density of samples; including fleet effects in the standardization model; and using a region-season interaction term in the standardization model rather than a year-season term.