Population structure of the purpleback squid Sthenoteuthis oualaniensis (Lesson, 1830) along the south-west coast of India

Abstract

To understand the population structure and distribution of the purpleback squid Sthenoteuthis oualaniensis off the south-west coast of India, a total of 1,015 specimens of the species were collected from the area between lat 07o to 11oN and long 74o to 77o E at a depth of 180 – 2601m during January 2007 to December 2008. Specimens were measured for dorsal mantle length (DML), tentacle length (TL), tentacular club length (TCL), fin width (FW), total weight (TWt) and mantle weight (MWt). Morphometric analyses of the variables indicate the existence of two forms, a medium and a dwarf form with significant difference in growth between them. Study suggests that males were dominant until about 15 cm DML for medium form and 11 cm DML for the dwarf form. However, the overall sex ratio of female to male was 6:4 for the medium and 8:2 for the dwarf form. Males were invariably smaller than the females in both the forms ranging from 9 to 19 cm DML in the medium form and 9 to 13 cm DML in dwarf form. The highest frequency of the mature medium form was in the DML range of 17-19 cm for males and 19-23 cm for females. Similarly, in mature dwarf forms the DML was in the range 10-11 cm for males and 12-14 cm for females. Dwarf forms were more prevalent in 7○N latitude (60%) than the 10○N latitude (28%), whereas medium forms were more prevalent in 10○N latitude (72%) than 7○N latitude (40%). Results clearly indicate the occurrence of medium and dwarf forms of S. oualaniensis along the south-west coast of India. The shape and size of the fins were identified to be important key features to differentiate dwarf population from the medium form of S. oualaniensis in the field.