The giant cuttlefish Sepia apama, which annually migrate to northern Spencer Gulf, South Australia, has recently undergone substantial declines in abundance and attracted significant media and public attention. Whilst it is accepted that the Marine Stewardship Council (MSC)-accredited SGPF, which has been operating and reducing its effort for almost 50 years, has not been the cause of the decline, several consecutive years of diminishing numbers and a particularly low population estimate in 2013 requires all efforts to be made to minimise the incidental catch of this species. The SGPF has been pro-active in taking steps to minimise interactions during this species’ annual migration. One such measure is the commitment to investigate the development of a bycatch reduction device (BRD). During its development, it would be prudent to also ensure that the BRD also reduces the bycatch of blue swimmer crabs Portunus armatus, because these require additional handling and, owing to their exoskeletons, physically damage the soft-bodied prawns and cuttlefish.
A recent pilot study (2013/052) identified the parameters within which a successful BRD should exist. This work involved testing two Nordmøre-grids over a few deployments, yet yielded promising results (i.e. both designs reduced the numbers and weights of cuttlefish, crabs and total bycatch, and one of the designs maintained prawn catches). Through rigorous testing of refined versions of these Nordmøre-grids across larger spatial and temporal scales, the proposed study aims to produce an optimal design for potential implementation in the fishery.
Relative to other fishing methods, prawn trawling is considered to be poorly selective, and can result in large quantities of bycatch being discarded, which sometimes includes charismatic species. Attempts at reducing bycatch or mitigating trawl impacts to the discarded bycatch have involved three broad techniques: (1) avoidance by spatial and/or temporal closures; (2) on-board handling procedures that minimise the mortality of discarded bycatch; and (3) retrospectively fitting BRDs into trawls. The latter approach can be particularly effective, with some BRDs reducing bycatches by up to 90%.
The aim of this study was to test incremental technical refinements to a generic Nordmøre-grid to identify an optimal design for the SGPF with respect to criteria of: (i) reducing total bycatch, with particular focus on maximising the escape of Blue Swimmer Crabs and Giant Cuttlefish; (ii) maintaining and improving the quality of Western King Prawn catches; and (iii) minimising technical handling issues in relation to the grid.
A double‐rigged trawler from the SGPF fleet was chartered for three experiments in northern Spencer Gulf over 13 nights (four in each of April 2015 and 2016, and five in November 2015). Each experiment involved paired comparisons between two or three grid configurations and a conventional codend (the control). Primary data collected from each codend were catch weights of Blue Swimmer Crabs, Giant Cuttlefish and broad categories for remaining bycatch (i.e. elasmobranchs, porifera, seagrasses/algae and teleosts), and Western King Prawns (including a breakdown by industry size grades and condition). In total, six grid configurations were tested over the course of the study; these were differentiated by the grid bar spaces, location of the horizontal support bar, area of the escape exit, and length of the guiding panel.
The effects of varying grid bar spaces, escape-exit areas and guiding-panel lengths were investigated. Compared to a control, the greatest reductions (by weight) in total bycatch (~80%), Blue Swimmer Crabs and Giant Cuttlefish (both ~90%), and elasmobranchs and porifera (almost 100%), were achieved with a large, low-angled Nordmøre-grid with 38-mm bar spaces, a support bar two thirds up the length, a 2.7-m guiding panel terminating ~0.6 m anterior to the grid base, and a large escape exit (≥0.8 m2). Importantly, this configuration did not negatively impact catches of prawns, but rather improved their quality and value (presumably owing to fewer crabs causing less damage).
While the bycatch reductions achieved in this study are impressive by world standards, an area of concern for industry relates to the dimensions (~2 × 1 m) and weight (~24 kg) of the grid and the operational difficulties and safety concerns they may pose to the crew, particularly under fishing conditions worse than those experienced during the study (e.g. winds >35 km h-1, swells >1.5 m). Acknowledging these concerns, an appropriate next step would be to test the preferred grid across broader spatio-temporal scales on a number of vessels in the fishery under various conditions. By including operational data with catch assessments, it should be possible to objectively assess any concerns fishers have with using the grid and perhaps modify deployment and on-board handling procedures so they are more acceptable/suitable to industry operations.