Estimating the catching efficiency of a 2-m beam trawl for sampling epifauna by removal experiments

The catching efficiency of a standard 2-m beam trawl for sampling epifauna was estimated by removal experiments at two sites in the southern North Sea. In order to allow repeated sampling of the same area, a rig of three beam trawls one behind the other was constructed, the three being tied together by steel ropes. Catching efficiency was estimated on the basis of the sum of the catches of all three trawls relative to the numbers caught in the first trawl. The catching efficiency of the beam trawl for epifauna ranged from 36% to 44% of total abundance between sites. Efficiency was least for partly buried species (Liocarcinus holsatus, 9% and 18%; Buglossidum luteum, 27%; Arnoglossus laterna, 35%), and slightly better for species living on the surface of the sediment (Asterias rubens, 42% and 46%; Pomatoschistus minutus, 58% and 46%; Pagurus bernhardus, 51%). The abundance of epifauna will be underestimated by a factor of 1.4-11 relative to the abundance in the three trawls. Also, sediment type seems to influence the catching efficiency of the beam trawl, efficiency being less at the sandy study site than at the muddy site for most species.