Environmental and spatial effects on the size distribution of sailfish in the Atlantic Ocean

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 cast of 25 degrees W in areas with a shallow mixed layer (<30 m), between 3000 and 6000 m depth, and SSTs lower than 24 degrees C. The proportion of adults was highest at chlorophyll concentrations between 0.3 and 0.8 mg m(-3). High adult densities were also observed to the west of 40 degrees W, both in the northern and southern parts of the study area. An opposite trend was observed in relation to juveniles, since they appear to be associated with waters with SSTs higher than 28 degrees C and a deep mixed layer (>50 m) on the western side, particularly between 10-20 degrees S and 25-35 degrees W.