Developing an integrated habitat index for bigeye tuna (Thunnus obesus) in the Indian Ocean based on longline fisheries data

Two surveys were conducted in 2005 and 2006 aboard Chinese longliners in the Indian Ocean to evaluate how environmental variables might influence the spatial distributions of bigeye tuna, Thunnus obesus. Models were developed to estimate integrated habitat indices (IHI) for bigeye tuna at eight depth strata between 80 m and 400 m and to predict the spatial distributions of bigeye tuna in these depths. Catch rates and environmental variables (water temperature, salinity, chlorophyll-a, and dissolved oxygen) at these depth strata were obtained at 30 sampling stations in the survey, and the quantile regression method was used to develop the IHI models. This study suggests that the weighted average temperature, chlorophyll-a, and dissolved oxygen (when the dissolved oxygen was less than 0.85 mg L−1) had significant impacts on the spatial distribution of bigeye tuna. The IHI models developed in this study performed well in predicting catch rates with selected environmental variables. The key environmental parameters in the IHI models differed among the depth strata, suggesting impacts of the environmental variables on the tuna distribution differed among different depth strata. This study provides an effective approach to identify optimal habitats for the bigeye tuna.