Catch composition, reproductive biology and diet of the bowmouth guitarfish Rhina ancylostomus Bloch and Shneider, 1801 (Batoidea: Rhinidae) in the eastern Arabian Sea, India

Abstract

Scarce information exists on the life history of the bowmouth guitarfish Rhina ancylostomus (=R. ancylostoma) landed by trawlers and gillnetters that operate in the eastern Arabian Sea off Karnataka, on the south-west coast of India. This study was carried out to update information on this species by analysing the size distribution, sex ratio, length-at-maturity (Lm50%), length–weight relationship and diet using data collected during 2016-2019. Three hundred and sixty-nine individuals were collected with a total length (TL) range of 44.0 to 295 cm and total weight (TW) range of 0.2 to 127 kg. The length-weight relationship (LWR) of females and males did not differ significantly and therefore, a common equation was derived as TW = 0.006604TL 3.027504(r2=0.979). The length-at-maturity (Lm50%) for females and males were estimated to be 183.0 and 164.0 cm TL, respectively. R. ancylostoma has two functional ovaries and the ovarian cycle and gestation run concurrently. The number of embryos ranged from 2 to 8 and size at-birth was estimated to range between 44.0 and 50.0 cm TL. Overall sex ratio indicated dominance of females. Dietary analysis of stomach contents (%IRI) revealed that R. ancylostomus fed primarily on teleosts (73.3%), crustaceans (20.5%) and molluscs (6.2%). Gravid females enter coastal waters possibly for parturition and feeding in some seasons, where they become vulnerable to trawl and gillnets used along Karnataka coast. The species is categorised in IUCN Red List as “Critically Endangered” and the present study revealed the urgent need to construct an effective management plan to conserve this species in the region.
Keywords: Bycatch, Elasmobranchs, Life history, Shark ray