Size selectivity of diamond and square mesh cod ends for four by-catch species in the crustacean fishery off the Portuguese south coast

Published source details Campos A., Fonseca P. & Erzini K. (2003) Size selectivity of diamond and square mesh cod ends for four by-catch species in the crustacean fishery off the Portuguese south coast. Fisheries Research, 60, 79-97.
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This study is summarised as evidence for the following.

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	Use a square mesh instead of a diamond mesh codend in a trawl net Action Link

Use a square mesh instead of a diamond mesh codend in a trawl net

A replicated, controlled study in 1993 in a fished area of seabed in the Atlantic Ocean off the south coast of Portugal (Campos et al. 2003) found that a square mesh codend improved the size-selectivity of a crustacean trawl net for three of three non-target fish, compared to diamond mesh codends of similar mesh size. The estimated length at which fish had a 50% chance of escape was greater with the square mesh than diamond meshes of increasing sizes for blue whiting Micromesistius poutassou (square: 30 cm, diamond: 23–27 cm). For horse mackerel Trachurus trachurus, the 50% escape length was greater with square mesh than the two smallest diamond mesh codends (square: 22 cm, diamond: 18–20 cm), but similar to the larger diamond mesh codend of 70 mm (22cm). In addition, for European hake Merluccius merluccius, the proportions of escapees below the minimum landing size relative to those retained were improved in the square mesh compared to the two smallest diamond meshes (see paper for data). However, since only very small proportions were retained overall for both the square and the largest diamond mesh, their size-selectivity was not calculable. Data were collected on two surveys in March/April and May 1993 from 133 deployments of a crustacean trawl (1 h) by a research vessel in depths of 152–706 m. A square mesh cod end of 55 mm mesh size (24 hauls), and three diamond mesh cod ends of 55 mm (41 hauls), 60 mm (33 hauls) and 70 mm (35 hauls) were tested. Covers fitted over each codend collected fish escaping through the meshes. Fish in both the codend and cover catches were separately identified, weighed, and total lengths measured.

(Summarised by: Natasha Taylor)