This report presents the findings of bycatch reduction device (BRD) trials undertaken for the Spencer Gulf Prawn Fishery (SGPF) in South Australia using a ‘Nordmøre-grid’—a type of BRD that mechanically separates organisms based on size and/or morphological differences. Combined with previous work by the Co-Investigators, who identified the parameters required for the successful implementation of a Nordmøre-grid in this fishery, this report highlights an incremental approach to refining the grid over a series of experiments (in April and November 2015 and April 2016) to maximise the reductions in total bycatch and selected bycatch species of interest without affecting the targeted catch. This work represents a collaborative effort between the Spencer Gulf and West Coast Prawn Fishermen’s Association, the South Australian Research and Development Institute (SARDI), the New South Wales Department of Primary Industries and IC Independent Consulting. While the project was undertaken to address bycatch issues specific to the SGPF, the incremental approach used to develop an optimal grid design has potential application among other prawn-trawl fisheries.
Background
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%.
One of the few remaining Australia prawn trawl fisheries that currently does not use any type of BRD is the SGPF. This fishery has been accredited by the Marine Stewardship Council in recognition of its effective management through a suite of controls within the first (fishing closures) and second (on-board handling) techniques above. Historically, these efforts have been sufficient to mitigate bycatch issues that have mostly involved Blue Swimmer Crabs (Portunus armatus), a key species targeted by other commercial and recreational trap fisheries in Spencer Gulf. Despite the potentially low impact of trawling on discarded Blue Swimmer Crabs, their exoskeleton and claws are known to cause considerable damage to Western King Prawns.
Another species interaction with Spencer Gulf trawlers is that of the Giant Cuttlefish (Sepia apama). Giant Cuttlefish are incidentally caught in relatively small quantities, but in recent years this species has attracted considerable attention when, in 2013, its annual spawning aggregation in northern Spencer Gulf (between May and July)—the largest known Sepia aggregation in the world—declined to record low levels. Several studies were undertaken on potential causes of the decline, but none provided any evidence that the SGPF had a detrimental impact. Nevertheless, due to the iconic status of Giant Cuttlefish and extent of the decline, all sources of potential mortality, including trawl bycatch, should be minimised.
Aims/objectives
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.
Methodology
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.
Results/key findings
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).
Recommendations
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.
