Observations of the bowmouth guitarfish (Rhina ancylostoma) by-catches by the tropical tuna purse seine fleet in the Indian Ocean's open waters

Abstract

Bowmouth guitarfish (Rhina ancylostoma) is typically described in the literature as a coastal ray species that inhabits nearshore waters up to 70 m deep on the continental shelf, dwelling on or near the seafloor, primarily over sandy or muddy substrates and around coral reefs. However, the scientific monitoring programme aboard the Spanish and Seychellois tropical tuna purse seine fleets has documented 37 incidental captures of this species in open waters of the Indian Ocean, through onboard observers and electronic monitoring systems. All these captures occurred within the first 200 m of the water column, at locations where the maximum depth exceeded 2000 m. The study was conducted based on at-sea observations onboard the Spanish and Seychellois tropical tuna purse seine fleets operating in the Indian Ocean from 2017 to 2023. The observation coverage varied throughout this time-series, with more than 85\% of fishing activity monitored since 2017. The interactions recorded were predominantly between January and May, and more frequent in the years 2018 and 2021. The captured individuals ranged from 150 to 250 cm in total length, with females recorded more frequently than males. These observations provide valuable insights into the large-scale movements of this neritic species beyond its typical coastal and inshore habitats in the Indian Ocean.