Length-weight relationship, condition factor, and diet analysis of thresher sharks (Family: Alopiidae) along the southern coast of India

Abstract

Sharks are fascinating groups of elasmobranchs that are consistently threatened by commercial tuna fishing operations, as they share a common habitat with most scombrids. This raises ecological concerns to explore the vulnerability of the species in its prevailing ecosystem. The length-weight relationship of three known species of thresher sharks viz, the common thresher (Alopias vulpinus), bigeye thresher (A. superciliosus), and pelagic thresher (A. pelagicus) were estimated based on 771 specimens landed as bycatches from the commercial tuna gillnetter and multi-gear utilizing boats (MGUB) off Bay of Bengal between June 2022 and November 2023. The fork length (FL) ranged from a minimum of 51 cm to a maximum of 288 cm. The dominant fork length class for A. vulpinus, A. superciliosus, and A. pelagicus were 101–120, 101–140 cm, and 61–80 cm respectively. The estimated slope (b) values in the range between 2.48 and 2.66 reveal negative allometry for all three species. The coefficient of determination was highly significant with r2 values of 0.98–0.99. The male-to-female ratio for A. vulpinus (1.03:1) and A. superciliosus (1.07:1), were male-biased except for A. pelagicus, (1:1). We also present data on the foraging components of the bigeye thresher shark that typically comprised teleosts and cephalopods, with occasional crustacean prey. Biological data collection and constant surveillance would help better understand the population structure of this iconic marine species and preserve them from overexploitation.