Length-weight Relationship of the Endangered Devil Ray Mobula mobular (Bonnaterre, 1778) off Gulf of Mannar, India

Length-weight Relationship of Mobula mobular off Gulf of Mannar, India

Abstract

Mobulids are amongst the highly endangered elasmobranchs which are extremely vulnerable to the expanding fisheries owing to their most conservative life cycle. They are susceptible to incidental capture in a wide range of gears including gillnets, purse seines, trawl nets, and occasionally long lines. The study provides a preliminary estimate of the length–weight relationship (LWR) for the endangered batoid species Mobula mobular, from the Gulf of Mannar India, between July 2017 and October 2018. A total of 355 specimens were sampled and measured for their total length (TL), disc width (DW) and total weight (TW) considering the sex ratio. The TL recorded were within size range of 43.0 – 122.0 cm and 40.0 – 124.0 cm, the DW of 96.0 – 248.0 cm and 60.0 – 232.0cm; while the TW observed was 9000 – 129000 g and 5000 – 111000 g, respectively for males and females. The growth pattern was negative allometry for both sexes ascertained from the slope (b) values (2.49 to 2.69). The Length–weight / Disc width-weight relationship showed a good fit with r² values varying from 0.82 to 0.88, indicating a high degree of positive correlation. Across the study areas, the devil ray population trend increased during June - August and October - December, attributable to the increased food availability strongly influenced by the North-East and South-West monsoon. This contemporary status  of the LWR for the species Mobula mobular is a key approach towards ecological and biogeographical evaluation of the diversity and abundance of this threatened fauna in the Indian waters.