Trawl designs and techniques used by Norwegian research vessels to sample fish in the pelagic zone

Abstract

In resource surveys, representative identification of species and sizes of fish is of vital importance. Various designs and sizes of pelagic trawls and techniques are used in the pelagic zone by Norwegian research vessels for this purpose. This paper describes trawl designs used for O-group surveys and a larger trawl used for adult fish. The largest trawl, which has a vertical opening of 30 m when towed at 3.5 - 4 knots, can be rigged for close-to-surface trawling as well as for rnid- and deep-water trawling with minor adjustment of the rigging. The performance of the various trawls is described, based on geometric measurements using Scanmar instruments and observations with a TV-camera in a towed underwater vehicle. The trawl mouth area of the O-group trawl is approximately 10 x 10 m, and it can be rigged to catch efficiently in all depths from surface downwards. The large pelagic trawl is rigged with large surface buoys and lenghtened upper bridles when used in the surface layer to sample herring and mackerel.