Generalized regression analysis and spatial prediction (GRASP) was applied to size data for sailfish (Istiophorus platypterus) to better describe its preferential habitats in relation to juveniles and adults, based on environmental and spatial factors in the equatorial and southwestern Atlantic. We analyzed a total of 9954 lower jaw fork length size data (7541 fish caught by longline and 2413 by gillnet) from the International Commission for the Conservation of Atlantic Tunas database for 1998 to 2007. Four main environmental variables were considered: sea surface temperature (SST), mixed layer depth, chlorophyll concentration, and bathymetry. Results indicated that adults were more frequent to the east of 25ºW in areas with a shallow mixed layer (50 m) on the western side, particularly between 10–20ºS and 25–35ºW.
