Refining a Nordmøre-grid to minimise the incidental catch of cuttlefish and crabs in the Spencer Gulf prawn fishery
Noell, C., Broadhurst, M. and Kennelly, S., 2017. Refining a Nordmøre-grid to minimise the incidental catch of cuttlefish and crabs in the Spencer Gulf prawn fishery. Final Report to the FRDC (Project No. 2015/019), 35p.
The most common fishing method for catching prawns is ‘otter trawling’; a method used throughout more than 25 countries and one of the world’s least selective. Prawn trawlers also catch large quantities of unwanted organisms (bycatch), including charismatic species. Attempts at reducing ‘bycatch’ have involved restricting fishing across different areas or times to avoid problematic catches; using on-board handling techniques to minimise the mortality of released bycatches; and most commonly, retroactively fitting ‘bycatch reduction devices’ (BRDs) into trawls. The latter approach is particularly effective, with some BRDs reducing bycatches by up to 90% without affecting the targeted catches of prawns.
Bycatch reduction devices currently are mandated in nearly all of Australia’s prawn-trawl fisheries - one of the few exceptions being in Spencer Gulf, South Australia. This fishery involves 39 vessels and has ‘Marine Stewardship Certification’ in recognition of its effective management through a suite of regulations within the first (restricted fishing) and second (on-board handling) techniques above. Historically, these efforts have been sufficient to mitigate problematic bycatches that mostly just involved blue swimmer crabs (a key species targeted in other fisheries). Blue swimmer crabs often are caught in very large quantities by Spencer Gulf trawlers, but are quickly separated from the catch onboard and discarded with an assumed low associated mortality (<~16%). More recently, concerns have been raised about the incidental catches of the giant cuttlefish; a species listed as ‘near threatened’ by the International Union for Conservation of Nature, and one that occurs on trawl grounds during their annual spawning migration between March and May. Strategies are required to limit interactions between giant cuttlefish and trawlers.
The aim of this project was to mitigate catches of both species via progressive refinements to a BRD (termed the Nordmøre-grid) originally tested and adopted in another Australian fishery (the Clarence River, NSW). The generic Nordmøre-grid comprised a rigid, inclined grid (~2 × 1 m) located in the end of the trawl designed to mechanically separate catches by allowing prawns to pass through and be retained, but larger animals to be directed up and out of an escape exit. The effects of varying grid bar spaces, escape-exit areas and guiding-panel lengths were investigated. Compared to a control (no BRD), maximum reductions (of up to 90%) in catches of giant cuttlefish and blue swimmer crabs were achieved by a Nordmøre-grid with 38-mm bar spaces, a very large escape exit (>1 m2), and a short (2.7 m) guiding panel. This configuration did not negatively affect catches of the targeted western king prawns, but rather improved their quality.
Assuming most giant cuttlefish and blue swimmer crabs survive escaping from Nordmøre-grids, we postulate the recommended configuration will alleviate concerns over impacts to cuttlefish stocks and, by saving millions of crabs, greatly contribute to the regional biomass of this species (available for capture by other fisheries). Work is now required to extend the results among fishers, and to encourage participation in ongoing technical adjustments.