Fishery biology of purpleback squid, Sthenoteuthis oualaniensis, in the northwest Indian Ocean

Abstract

Three surveys of purpleback squid ( Sthenoteuthis oualaniensis) resource were carried out by Chinese squid jigging boats in the waters of 4°N–21°N, 58°E–65°E in the northwestern Indian Ocean during September and November 2003, and from September 2004 to April 2005. The daily catch ranged from 0.1 t/days to 36.0 t/days, with majority of the catch from areas defined by 15°N–18°N and 60°E–62°E, and by 18°30′N–20°N and 62°30′E–64°E. The sex ratio (M:F) for the total catch was 1:30.5. The mean mantle length (ML) was 299 mm for males (range 106–462 mm) and 352 mm for females (range 106–612 mm). Three separate groups with different sizes were identified: those larger than 400 mm ML located in the north of 18°N; squid ranging 300–400 mm ML in the area from 12°N to 18°N, and those shorter than 300 mm ML in the south of 12°N. There was no systematic difference in squid size along the longitude direction. The relationship between body weight (BW, g) and mantle length (mm) was quantified by BW = 40.64348 ML 2.9115 and BW = 45.78507 ML 2.5842 for females and males, respectively. In the waters south of 16°N, more than 70% of females were immature, while in the area north of 18°N, more than 48% were mature. Squid in the same maturity stage varied greatly in size, suggesting that S. oualaniensis may spawn all year around. The oldest squid from September 2004 to April 2005 were slightly younger than 1 year old, suggesting that all the examined individuals were hatched during January and November 2004. The peak spawning time is from March to May. Most of the stomachs analyzed had food remains and only 8% were empty. The stomachs contained three major groups: fish, cephalopods and crustaceans, mainly Cypselurus spp and S. oualaniensis. More than 60% of the stomachs had evidence of cannibalism for the squid larger than 400 mm ML.