FACTORS AFFECTING DISTRIBUTION OF ADULT YELLOWFIN TUNA (THUNNUS ALBACARES) AND ITS REPRODUCTIVE ECOLOGY IN THE INDIAN OCEAN BASED ON JAPANESE TUNA LONGLINE FISHERIES AND SURVEY INFORMATION

Environmental influence on yellowfin tuna (Thunnus albacares) distribution and reproductive ecology was analyzed using long line fisheries and oceanographic data in the Indian Ocean for the period of 1953 – 1997. Data were obtained from National Research Institute of Far Seas Fisheries and the National Oceanographic Data Center, respectively. Hooking rates, sexual maturity and larval density were compared with thermocline depth and depth-specific data on temperature, salinity, oxygen and nutrients using geographic information system (GIS) application and simple numerical analyses in order to describe their relationships. Results predicted optimum range for every environmental parameter at each respective life stages of yellowfin tuna. It was found out that water temperature, dissolved oxygen concentration and thermocline depth influenced the distribution of adult yellowfin tuna while, temperature and dissolve oxygen concentration influenced spawning activities. For the larvae, temperature, dissolved oxygen and thermocline depth were also considered important factors. Other environmental parameters studied had more influence on intermediary factors that affects the distribution as well.