Regional scaling factors for Indian Ocean albacore tuna

Indian Ocean tuna assessments may be spatially structured, and CPUE indices developed for different regions often indicate different trends through time. In this situation it is useful to determine the relative abundances by region, so that we may either apply regional scaling in a multi-region assessment, or appropriately combine the separate indices into a single index of abundance. 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 impacts on albacore abundance scaling of changes to the approach previously applied to yellowfin and bigeye tuna. 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.