Biological Aspects and Catch Fluctuation of the Pelagic Thresher Shark, Alopias pelagicus from the Indian Ocean

Abstract

The pelagic thresher shark (Alopias pelagicus) is an oceanic shark and is generally caught as a bycatch by tuna gill net fisheries operating in the Indian Ocean. This research was conducted from April 2002 to December 2007 at Cilacap-Central Java fish landings. Biology and catch data were collected through direct observation and the use of enumerators. The results showed that the relationship between the total length and the clasper's length was sigmoid; this means that with the increase in total body length, the clasper length also increases in size until it reaches its maximum length. The relationship between the total length and the precaudal length of both the male and female were linier (R2= 0.9803 and R2= 0.9423 respectively). The length frequency of the male Alopias pelagicus was lowest at a size range of 150-170cm total length (immature), while the highest frequency was seen ranging between 251-270 cm in total length with a 260 cm mode (mature, non reproductive). The lowest frequency of female Alopias pelagicus was found at a size range of 291-310 cm in total length (mature), with the highest length frequency appearing between 231-250 cm. The sex ratio of males to females of Alopias pelagicus was 1: 1 (P>0.05). Catches of these species have decreased by 34.9% in the last six years. This indicates that the abundance of Alopias pelagicus previously existing in the Indian Ocean was significantly in decline