Scaling factors for multi-region stock assessments, with an application to Indian Ocean tropical tunas

Abstract Stock assessments may be spatially structured, with regions that contain separate but linked subpopulations. In such multi-region models we must determine the relative abundances among regions. Regional scaling, which has been used since 2005 in tuna assessments, estimates the relative 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. We also recommend the use of operational data to estimate regional scaling factors.