Comparing top- and bottom-opening Nordmøre-grids in south-eastern Australian estuarine squid trawls

Abstract

In response to prolonged, regional jellyfish (mostly Catostylus mosaicus) blooms in the Hawkesbury River, south-eastern Australia, trawlers targeting broad squid, Uroteuthis sp. 3 and Luminous Bay squid, U. noctiluca have modified their trawls to include mechanical-separating devices, and mostly bottom-opening Nordmøre-grids. Three experiments were done here to incrementally assess the relative performances of Nordmøre-grids for maintaining squid and by-product (>35 mostly fish species) catches, while reducing jellyfish and other bycatch. The Nordmøre-grids had identical bar spaces (∼42 mm), sizes (1200 × 725 mm), installation angles (30°) and escape-exit designs (with a mesh cover) but were compared as (1) top- or bottom-opening orientations with long guiding panels in the extension, and then bottom-opening designs with either (2) long or short guiding panels or (3) short or no guiding panels. There were few significant differences in catches of key groups or species between any of the grids, reiterating the generic utility of the design for consistently separating out large unwanted organisms (i.e. jellyfish). Nevertheless, trawls with the top-opening design retained significantly fewer small- and medium-sized silver biddy, Gerres subfasciatus (a by-product species) and slightly less total by-product (attributed to a tendency for some fish to rise in the extension). Among the bottom-opening designs, using a short guiding panel and a modification involving opening meshes opposite the leading edge of the panel allowed one strong-swimming bycatch species (silver trevally, Pseudocaranx georgianus) to escape, although there were few other perceived benefits. While Nordmøre-grids permit squid trawling during moderate jellyfish blooms in the Hawkesbury River, alternative catching methods (e.g. jigging) might also warrant investigation for more holistically improving environmental efficiencies.