Habitat suitability of Indo-Pacific sailfish Istiophorus platypterus (Shaw, 1792) in the Arabian Sea

Istiophorus platypterus (Shaw, 1792) is the most common billfish species in the Arabian Sea and contributes more than 50% of the total billfish landings in India. The abundance of I. platypterus was more off Maharashtra and Gujarat in the northern Arabian Sea and in Lakshadweep islands with higher catches during March, April and December. The aim of the study was to predict the suitable habitat of occurrence and potential fishing ground of I. platypterus by correlating the spatial catch rates with environmental parameters along the Arabian Sea. Generalised Additive Modelling (GAM) showed significant relationship between the catch and the predictor variables [sea surface salinity (SSS), sea surface temperature (SST), sea surface height anomaly (SSHa), current speed (CS) and chlorophyll (Chl)]. SSS from 32 to 36 ppt had a positive effect on the expected abundance. Similarly, SST showed a positive effect from 25 to 28°C and thereafter had a negative effect on catch rate. The highest catch values associated with the SSHa was >0.4 m and partial effect of SSHa on catch was negative below 0.4 m. Current speed higher than 0.4 m s-1 showed negative effects on catch rate and lower Chl-a values which ranged between 0.04-2.14 mg m-3 in the study area showed a positive effect. All explanatory variables (SST, SSHa, CS, Chl) except SSS were found to be highly significant covariates (p<0.01, p<0.01, p<0.001 and p<0.05 respectively) and 38.2% of variance was explained by the model with these variables. The paper discusses the fishery and trend of billfishlandings along the Indian coast in relation to various remotely sensed data and defines the optimum habitat suitability for I. platypterus. Keywords: Additive models, GAMs, Large pelagics, Northern Indian Ocean