The utility of square mesh to reduce bycatch in Hawkesbury River prawn trawls

In New South Wales (NSW), Australia, small (< 10 m) prawn (penaeid) trawlers are permitted to work in areas of Sydney Harbour and the Clarence, Hunter and Hawkesbury River systems. In addition to the targeted species, (mostly School Prawns – Metapenaeus macleayi), trawlers also catch and discard large quantities of non-target organisms (termed bycatch) (Kennelly 1993). During the past 15 years, the mortality of juveniles of key species comprising bycatch has raised concerns over the sustainability of stocks. This led to the development of physical modifications, termed bycatch reduction devices (BRDs), that are installed in the bag of the funnel-shaped trawl net where the catch accumulates (i.e. the codend). These BRDs are designed to exclude bycatch organisms similar in size or larger than the targeted prawns, and so improve the size and species selectivity of the trawl net (Broadhurst 2000 for a review). In particular, two BRDs – the Nordmøre-grid (Broadhurst and Kennelly 1996) and the Hawkesbury River square-mesh panel (Broadhurst and Kennelly 1995) – were demonstrated to reduce the catches of juvenile fish by up to 70%, and have subsequently been legislated for use in relevant fisheries.