Biology of the widenose guitarfish Glaucostegus obtusus (Muller and Henle, 1841) (Rhinopristiformes: Glaucostegidae) from the eastern Arabian Sea

Purushottama, G B and Thomas, Sujitha and Kizhakudan, Shoba Joe (2025) Biology of the widenose guitarfish Glaucostegus obtusus (Muller and Henle, 1841) (Rhinopristiformes: Glaucostegidae) from the eastern Arabian Sea. Indian Journal of Fisheries, 72 (3). pp. 45-57. ISSN 0970-6011

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Abstract

The study delves into the reproductive biology of Glaucostegus obtusus, a benthic, inshore batoid species currently listed as Critically Endangered, for which life history information remains scarce. A total of 331 specimens, collected as bycatch from shrimp trawl and gillnet fisheries operating at depths of 2-65 m between 2016 and 2022 off Karnataka, the south-west coast of India, were examined and analysed. Specimens ranging from 22 to 109.2 cm total length (TL) and 0.023 to 5.7 kg total weight (TW) were observed in the fishery. The length–weight relationship (LWR) derived was W = 0.000392 TL 3.506478 (r2=0.991), with no significant difference observed between sexes. Length-at-maturity (Lm50%) estimates for females and males are 60.4 and 50.4 cm TL, respectively. G. obtusus has two functional ovaries, with the ovarian cycle and gestation occurring simultaneously. Pregnant females were observed year-round, suggesting a non-seasonal reproductive cycle. Uterine fecundity ranged from 2 to 14 and estimated birth sizes between 22 and 25 cm TL. Breeding peaks were identified in October–December and May-July, with the highest abundance recorded in the post-monsoon. Stomach content analysis revealed G. obtusus feeds on diverse prey, comprising mainly crustaceans (82% IRI) and teleosts (18% IRI). The present study provides preliminary insights into G. obtusus distribution and critical habitats based on fishery interaction and can contribute to marine conservation spatial planning. This study provides baseline data for fisheries management, conservation and population assessment of the widenose guitarfish in the eastern Arabian Sea.

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