Confounding effects of knot orientation in penaeid trawls

Published source details Broadhurst M.K., Sterling D.J. & Millar R.B. (2016) Confounding effects of knot orientation in penaeid trawls. Fisheries Research, 179, 124-130.
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This study is summarised as evidence for the following.

	Action	Category
	Modify the design or configuration of trawl gear (mixed measures) Action Link

Modify the design or configuration of trawl gear (mixed measures)

A replicated, paired, controlled study (year not provided) of sand and mud bottom in an estuary off the Tasman Sea, New South Wales, Australia (Broadhurst et al. 2016) found that modifying a prawn trawl net (knot orientation) reduced the catches of unwanted fish species compared to a conventional trawl. Overall catch rate of unwanted fish species was lower in a modified trawl with a negative knot angle of attack than the conventional positive attack angle (modified: 90 fish/ha, conventional: 110 fish/ha). For the five most abundant non-target fish species caught, individual catch rates were lower with negative angles of attack for three: yellowfin bream Acanthopagrus australis (modified: 4, conventional: 8 fish/ha), southern herring Herklotsichthys castelnaui (modified: 2, conventional: 8 fish/ha) and silver biddy Gerres subfasciatus (modified: 0.2, conventional: 1.3 fish/ha); but similar for forktail catfish Arius graeffei (modified: 80, conventional: 88 fish/ha) and Ramsey’s perchlet Ambassis marianus (modified: 0.4, conventional: 0.7 fish/ha). Target school prawn Metepenaeus macleayi catches were also lower with the modified knot orientation (modified: 675, conventional: 775 ind/ha). Fishing trials were done in the austral summer in the Clarence River estuary (2–20 m depth) using a local trawler. Knot-force direction in the top and bottom net panels was compared by turning a conventional trawl with a positive angle of attack inside out to create a negative angle of attack (see original paper for full gear specifications). A total of 24 paired deployments (45 min) of both trawl types were completed. The authors reported that altering the knot orientation may have affected the overall geometry of the trawl during fishing (e.g. a lower position in the water column and lower headline height).

(Summarised by: Chris Barrett)