Elasmobranch bycatch by prawn trawls in the Gulf of California: First comprehensive analysis and the effect of fish escape devices

Abstract

We report results from the first detailed investigation of elasmobranch bycatch that contains data on species, sex, and length-frequency distributions of animals collected in the coastal south-eastern and entrance region of the Gulf of California. Using data from fishery-independent prawn trawl surveys between 2011–17, we found differences between years and zones in the number of species per tow in summer when more samples were taken, but we did not find differences in autumn and winter. We present size-frequency distributions with size at first maturity for Urobatis halleri, Urotrygon chilensis, Rhinoptera steindachneri, Hypanus dipterurus, Gymnura marmorata, and Pseudobatos glaucostigmus, which were the species most frequently present in the prawn trawls during the surveys. These distributions are presented by zone, depth stratum, and season (mainly summer, when commercial prawn trawling is prohibited, and thus information from commercial catches is not available). We found significant differences in the mean size between mature females and mature males for five of these six species. We also found that fish escape devices installed in the prawn nets early in 2016 improved the escape of mid-sized rays, demonstrating size selectivity of the fishery and suggesting the potential to improve further the escape of large-sized rays by modifying fish escape devices. Furthermore, the large number of rays caught (21 species) compared with the number of sharks caught (four species) suggests much lower catchabilities for sharks than for rays in demersal prawn trawl gear.