Square-mesh codend circumference and selectivity

Published source details Broadhurst M.K. & Millar R.B. (2009) Square-mesh codend circumference and selectivity. ICES Journal of Marine Science, 66, 566-572.
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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

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

A replicated, paired, controlled study in 2007 of an area of seabed in an estuary leading to the Tasman Sea off New South Wales, Australia (Broadhurst & Millar 2009) found that using square mesh codends of smaller circumferences in prawn trawls resulted in lower catches of one of four non-target fish compared to larger circumferences. Average catch numbers of unwanted pink-breasted siphonfish Siphamia roseigaster decreased with decreasing codend circumference (small: 8 fish/haul, intermediate: 20 fish/haul, large: 25 fish/haul). However, for three other unwanted fish species (silver biddy Gerres subfasciatus, southern herring Herklotsichthys castelnaui and Ramsey’s perchlet Ambassis marianus – see original paper for species individual data) there were no statistical differences in average catch rates between codend circumferences (small: 7–25 fish/haul, intermediate: 4–18 fish/haul, large: 5–9 fish/haul). Average catch weights of all commercial target prawns Penaeidae were similar across codend designs (all 5 kg/haul). Experimental fishing was done in October and November 2007 on commercial prawn trawl grounds in Lake Wooloweyah on the Clarence River estuary using a local twin-rigged trawler. Three square mesh codends (27 mm nominal mesh) of varying circumference (standard 90 meshes, 150 meshes and 200 meshes) were deployed simultaneously with a small mesh (9 mm) codend, 18 deployments for each paired comparison. After each tow, codend catches were separated by species and numbers or weights recorded.

(Summarised by: Chris Barrett)