Mitigating the bycatch of giant cuttlefish Sepia apama and blue swimmer crabs Portunus armatus in an Australian penaeid-trawl fishery

Published source details Kennelly S. & Broadhurst M. (2014) Mitigating the bycatch of giant cuttlefish Sepia apama and blue swimmer crabs Portunus armatus in an Australian penaeid-trawl fishery. Endangered Species Research, 26, 161-166.
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This study is summarised as evidence for the following.

	Action	Category
	Increase the mesh size of pots and traps Action Link
	Fit one or more soft, semi-rigid, or rigid grids or frames to trawl nets Action Link

Increase the mesh size of pots and traps

A replicated, controlled study (date unspecified but appears to be 2012) in a muddy and sandy area in the Corindi River system, eastern Australia (Broadhurst et al. 2014) found that traps used to catch giant mud crabs Scylla serrata appeared to catch fewer unwanted undersized mud crabs when designed with larger mesh size, compared to conventional traps. The proportion of undersized crabs caught in traps with 101 mm mesh appeared lower (22%) compared to conventional traps with 51 mm mesh (29%; results not tested for statistical significance). In addition, the number of wounded mud crabs (undersized and commercial size) was statistically lower in traps with larger mesh size (0.03 crabs/trap) compared to conventional traps (0.13 crabs/trap). Conventional traps are designed with 51 mm mesh. Conventional traps were modified by increasing the mesh size to 101 mm. Seven modified traps and seven conventional traps were tested during 20 deployments. All traps were baited with sea mullet Mugil cephalus. Traps were recovered after 24 hours, and all catch identified, counted, and any wounds assessed.

(Summarised by: Anaëlle Lemasson)
Fit one or more soft, semi-rigid, or rigid grids or frames to trawl nets

A replicated, paired, controlled study in 2014 in a sandy area in Spencer Gulf, Southern Australia (Kennelly & Broadhurst 2014) found that, when fitted to trawl nets, two grids reduced the number and biomass of unwanted giant cuttlefish Sepia apama and blue swimmer crabs Portunus armatus caught, compared to conventional nets without grids. Compared to conventional nets, nets fitted with a small grid resulted in a 50% decrease in the number and a 60% decrease in the biomass of giant cuttlefish caught, as well as a 40% decrease in the number and a 48% decrease in the biomass of blue swimmer crab caught. Nets fitted with a large grid resulted in 34% decrease in the number and a 37% decrease in the biomass of giant cuttlefish caught, as well as a 34% decrease in the number and a 50% decrease in the biomass of blue swimmer crab caught. There were no differences in cuttlefish abundance and biomass between the grid sizes. Catch of commercially targeted western king prawns Melicertus latisulcatus was reduced by 8% when using a small grid compared to a large grid and the conventional net (which had identical catches). Two grids were tested: a small grid (1.4 m long, 45° angle) and a large grid (1.98 m long, 30° angle) (see paper for full details). For 30 min at night, a trawler towed two identical nets (one on each side) fitted with a 41 mm mesh codend during simultaneous, paired deployments: one net fitted with a grid and one unmodified conventional net (eight deployments using small grids, seven using large grids). For each deployment, the weight and numbers of cuttlefish and crabs were recorded, as well as the weight of other unwanted catch. Weight and size of prawns were also recorded.

(Summarised by: Anaëlle Lemasson & Laura Pettit)