A meta-analysis of haddock size-selection data

Published source details Fryer R.J., O'Neill F.G. & Edridge A. (2016) A meta-analysis of haddock size-selection data. Fish and Fisheries, 17, 358-374
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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 larger mesh size Action Link
	Fit mesh escape panels/windows to a trawl net Action Link

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

A review in 2016 of 40 experimental fishing trials in the northeast Atlantic Ocean (Fryer et al. 2016) found that overall, decreasing the circumference of trawl codends resulted in an increase in the size-selectivity of haddock Melanogrammus aeglefinus. The length at which haddock had a 50% chance of escape from the codend was greater by 1.3 cm for every reduction of 10 meshes in circumference around the codend. In addition, the 50% escape length increased by 3.4 cm for every 10 mm increase in codend mesh size and by 1.4 cm for every 1 mm decrease in twine thickness. The study was a meta-analysis of the effects of various changes to codend characteristics, including the number of meshes around the circumference, on the selectivity of haddock in the northeast Atlantic. Data were from 40 trials, covering the years 1991–2009, taken from published studies and other data collected by a Scottish fisheries research organisation (Marine Scotland Science).

(Summarised by: Leo Clarke)
Use a larger mesh size

A review in 2016 of 40 experimental fishing trials in the northeast Atlantic (Fryer et al. 2016) found that trawl nets with larger codend mesh sizes, smaller codend circumferences and thicker netting twine had better size-selectivity for haddock Melanogrammus aeglefinus, and that the amount of smaller haddock caught in the gear was affected by the position of square mesh escape panels, when present. The length at which haddock had a 50% chance of escape increased by 3.4 cm for every 10 mm increase in codend mesh size, 1.3 cm for every decrease in codend circumference by 10 meshes and by 1.4 cm for every 1 mm decrease in twine thickness (data reported as statistical model results). In addition, escape of smaller haddock was higher with square mesh escape panels located closer to the codend. The study was a meta-analysis of data from 40 trials on the effects of changes to codend characteristics on the selectivity of haddock in the northeast Atlantic.

(Summarised by: Leo Clarke)
Fit mesh escape panels/windows to a trawl net

A review in 2016 of bottom trawl size selection data from fishing trials in the northeast Atlantic Ocean (Fryer et al. 2016) found that the effect of fitting a square mesh panel in a diamond mesh trawl on improving size selectivity of the gear for haddock Melanogrammus aeglefinus depended on panel mesh size and position, and time of year, and in diamond mesh trawl nets without square mesh panels haddock selectivity increased with larger mesh size codends, smaller codend circumference and thinner twine thickness. The length at which 50% of haddock are likely to escape from the gear increased by 3.8 cm for each 10 mm increase in panel mesh size, but the effect of square mesh panels on the overall (panel plus codend) size selectivity of the gear for haddock varied with panel mesh size, position (greater closer to the codend), and time of year (data reported as statistical results). The length at which 50% of haddock are likely to escape from the gear increased by 3.4 cm for every 10 mm increase in codend mesh size, 1.3 cm for every decrease in codend circumference by 10 meshes and by 1.4 cm for every 1 mm decrease in twine thickness. This study presents a meta‐analysis of haddock size‐selection data collected on 24 vessels between 1991–2009, by Marine Scotland Science (formerly Fisheries Research Services). The final dataset was based on 614 fishing deployments from 18 trials (one excluded) of the combined size selection of a diamond mesh codend and a square mesh escape panel in the upper part of the codend or extension, and 20 trials (one excluded) of diamond mesh codend selection.

(Summarised by: Leo Clarke)