Selectivity of conventional diamond- and novel square-mesh codends in an Australian estuarine penaeid-trawl fishery

Published source details Broadhurst M.K., Millar R.B., Kennelly S.J., Macbeth W.G., Young D.J. & Gray C.A. (2004) Selectivity of conventional diamond- and novel square-mesh codends in an Australian estuarine penaeid-trawl fishery. Fisheries Research, 67, 183-194.
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This study is summarised as evidence for the following.

	Action	Category
	Decrease the circumference or diameter of the codend of a trawl net Action Link
	Use a square mesh instead of a diamond mesh codend in a trawl net Action Link

Decrease the circumference or diameter of the codend of a trawl net

A replicated, randomized, paired, controlled study in 2002 in an area of seabed in an estuary leading to the Tasman Sea off New South Wales, Australia (Broadhurst et al. 2004) found that prawn trawl codends of smaller circumference (two types) reduced the unwanted catch of six fish species in only three of 12 cases compared to a larger circumference. In diamond mesh codends, average number of unwanted fish catch was lower with a smaller circumference than a larger for three of six species (small: 18–25 fish/haul, large: 45–75 fish/haul) and similar for three (small: 2–6 fish/haul, large: 2–8 fish/haul). In square mesh codends, there were no differences in average catch numbers of the six species between a smaller circumference of tapered design and a larger, non-tapered, codend (small, tapered: 0–9 fish/haul, large, non-tapered: 0–8 fish/haul). See original paper for data by individual species. In addition, the square mesh codends caught lower average numbers of unwanted fish for four of the six species. Data were collected on a commercial prawn Penaeidae trawler using a twin-net configuration on commercial grounds in Lake Woolooweyah in the Clarence Estuary in March 2002. Combinations of four experimental trawl nets codends (see original paper for gear specifications), with size-sorting escape grids (Nørdmore type, 20 mm bar spacing) were tested in pairs, one either side of the vessel: two 40 mm diamond mesh codends of either 100 or 200 meshes (standard) circumference, and two square mesh codends (one tapered from 82 to 54 meshes, and one 110 mesh non-tapered). All catch in each codend was sorted and counted by species.

(Summarised by: Leo Clarke)
Use a square mesh instead of a diamond mesh codend in a trawl net

A replicated, randomized, paired, controlled study in 2002 in a seabed area in the Clarence Estuary (Tasman Sea), New South Wales, Australia (Broadhurst et al. 2004) found that square mesh codends fitted to prawn trawls reduced the catch numbers of unwanted young fish of three of six commercially important species compared to diamond mesh codends. Average catch numbers of non-target southern herring Herklotsichthys castelnaui, Tasmanian whitebait Lovettia sealii, and pink-breasted siphonfish Siphamia roseigaster were lower in square mesh than diamond mesh codends (square: 0–9 fish, diamond: 17–75 fish) and were similar for catfish Siluriformes, Ramsey’s perchlet Ambassis marianus, and silver biddies Gerreidae (square: 2–4 fish, diamond: 2–9 fish). In March 2002, experimental fishing was done on commercial prawn-trawl fishing grounds in Lake Woolooweyah using a commercial trawler. One of four designs of trawl codend were deployed on one side of a twin trawl, paired on the other side with small mesh control codends, all with Nordmøre escape grids (20 mm bar spacing): two square mesh codends (20 mm mesh, one tapered and one non-tapered), and two diamond mesh codends (40 mm, 100 or 200 meshes circumference). Twenty replicate hauls of each test codend/control were done. All catches were sorted and counted separately.

(Summarised by: Leo Clarke)