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

October 2024

T. Mohanraj, T. Jebarani Rajathy, Guy Stevens, Daniel Fernando and Shunmugavel Chinnadurai

Keywords: Mobula • Bycatch • Monsoon • Upwelling • Tuna gillnet

Summary: The spinetail devil ray (Mobula mobular), an endangered elasmobranch species, faces extreme vulnerability to expanding fisheries due to its conservative life cycle and frequent capture in various fishing gear types. This study provides initial estimates of the length-weight relationship (LWR) for M. mobular in India’s Gulf of Mannar from 2017-2018, analyzing 355 specimens’ length, disc width, and weight by sex. Growth patterns showed negative allometry for both sexes, with high correlation (r² values between 0.82–0.88). Population trends peaked from June-August and October-December, likely due to food availability affected by monsoon cycles, offering valuable insight into this threatened species' ecological role.

Abstract

“Mobulids are amongst the highly endangered elasmobranchs which are extremely vulnerable to the expanding fisheries owing to their most conser vative 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 endan gered 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, respec tively 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.”

Author Affiliations

  • Marine Eco-biological Research Centre, Tuticorin, India

  • CSIR - National Institute of Oceanography, India

  • The Manta Trust, United Kingdom

  • Veraval Research Centre of ICAR- CIFT, India

Funding

  • The Manta Trust

  • Save Our Seas Foundation