Biological observations on the Bengal guitarfish Rhinobatos annandalei Norman, 1926 from the Eastern Arabian Sea, India

Abstract

Biological data for the little known Bengal guitarfish Rhinobatos annandalei Norman, 1926 (Rhinopristiformes: Rhinobatidae) are presented based on specimens collected from bycatch of commercial shrimp trawlers, gill netters and bag or dol netters operating in the Arabian Sea at depths of 2-70 m off the north-west coast of India. Five hundred ninety-three specimens measuring 30.0 to 95.0 cm total length (TL) and weighing between 64.0 to 3300 g total weight (TW) were collected for the study. The length-weight relationships (LWR) were significantly different between the sexes (p<0.001) and for the combined sexes was derived as TW= 0.000604 TL3.408256 (r2= 0.997). The co-efficients ‘a’, ‘b’ of the LWR were estimated as 0.000621, 3.410115 (r2= 0.999) for females and 0.000766, 3.333872 (r2= 0.999) for males. The length  at maturity (TL50) for females and males was estimated to be 61.0 and 63.3 cm TL, respectively. In a single female, the number of embryos ranged from 2 to 11 and the size at birth was estimated between 25.0 to 30.0 cm TL. The overall sex ratio favoured females at the rate of 1.6:1. An analysis of the stomach contents (%IRI) revealed that R. annandalei mainly fed on Solenocera spp. (18.7% IRI), along with P. sculptilis (0.5% IRI), P. stylifera (0.4% IRI), Loligo spp. (0.4% IRI) and sciaenids (0.3% IRI). Since, the species is poorly studied and assessed as ‘Data Deficient’ in the IUCN red list criterion, it is anticipated that the biological results from the present study, will update information on the species thereby enabling more effective management decisions.

Keywords: Arabian Sea, Diet, Length at maturity, Length-weight relationship, Reproductive biology, Rhinobatos annandalei