Reducing discards in a temperate prawn trawl fishery: a collaborative approach to bycatch research in South Australia

Published source details Gorman D. & Dixon C. (2015) Reducing discards in a temperate prawn trawl fishery: a collaborative approach to bycatch research in South Australia. ICES Journal of Marine Science, 72, 2609-2617.
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This study is summarised as evidence for the following.

	Action	Category
	Fit one or more soft, semi-rigid, or rigid grids or frames to trawl nets and use square mesh instead of a diamond mesh at the codend Action Link
	Fit a size-sorting escape grid (rigid or flexible) to a prawn/shrimp trawl net Action Link

Fit one or more soft, semi-rigid, or rigid grids or frames to trawl nets and use square mesh instead of a diamond mesh at the codend

A replicated, paired, controlled study in 2012 in the Gulf of St Vincent, off the coast of South Australia (Gorman & Dixon 2015) found that trawl nets fitted with a rigid U-shaped grid (“bycatch reduction device”) and a square-oriented mesh codend resulted in lower catch rates of three dominant groups of unwanted invertebrate catch species, compared to unmodified nets. Compared to unmodified nets, the modified nets led to a 92% decrease in catch rate (kg/h) of sponges, 78–82% decrease in catch rate of crabs and other crustaceans, and a 61% decrease in catch rate of molluscs (excluding commercially valuable species of octopus, squid and cuttlefish; raw data not provided). A 15% decrease in catch rates of the commercially targeted western king prawn Penaeus latisulcatus was recorded due to reduced catch of less valuable smaller-sized prawns. In May 2012, unwanted catch of invertebrates in modified and unmodified nets were compared (see paper for details). Nets were deployed by four vessels during 29 paired hauls for 30 min (one modified; one unmodified; side-by-side simultaneously). All invertebrates were identified, sorted as commercial prawn catch or unwanted catch, and weighed.

(Summarised by: Anaëlle Lemasson)
Fit a size-sorting escape grid (rigid or flexible) to a prawn/shrimp trawl net

A replicated, paired, controlled study in 2012 in the Gulf St Vincent, Australia (Gorman & Dixon 2015) found that prawn trawls modified with a rigid size-sorting excluder grid, and a diamond mesh codend with the mesh orientation turned by 90°, caught fewer unwanted fish compared to conventional diamond mesh nets with no grid. Modified trawls caught fewer unwanted individuals than conventional trawls of rays Batoidea (2.6 vs 0.7/h), sharks Selachii (2.0 vs 0.3/h), porcupinefish Diodontidae (0.8 vs 0.0/h), bream Sparidae (0.2 vs 0.0/h), armourheads Pentacerotidae (0.1 vs 0.0/h), croaker Sciaenidae (0.1 vs 0.0/h), filefish Monocanthidae (22.4 vs 10.8/h), jacks and pompanos Carangidae (2.8 vs 2.2/h), dragonets Callionymidae (2.3 vs 1.0/h) and gurnard Triglidae (2.2 vs 1.3/h). Numbers of flatheads Platycephalidae caught in each trawl design were similar (data reported as statistical model results). Target western King prawn Penaeus latisulcatus catches were 15% lower in modified trawls (results not tested for statistical significance), although this was largely due to losses of small, low-value individuals. Twenty-nine, 30-min, replicate paired trawl deployments were undertaken at randomly chosen sites in May 2012 using modified and conventional trawl nets. Modified nets were fitted with a U-shaped plastic grid with 50 mm bar spacing and top escape opening and had a codend of 58 mm diamond mesh rotated 90° in orientation. Conventional trawls used 58 mm diamond mesh codends. Full details of trawl design are given in the original study.

(Summarised by: Leo Clarke)