|Physical impact of beam trawls on seabed sediments|
Fonteyne, R. (2000). Physical impact of beam trawls on seabed sediments, in: Kaiser, M.J. et al. The effects of fishing on non-target species and habitats: biological, conservation and socio-economic issues. Fishing News Books, : pp. 15-36
In: Kaiser, M.J.; de Groot, S.J. (2000). The effects of fishing on non-target species and habitats: biological, conservation and socio-economic issues. Fishing News Books. Blackwell Science: Oxford. ISBN 0-632-05355-0. XVI, 399 pp., more
In: Fishing News Books. Blackwell Science, more
Bottom trawls; Collisions; Ocean floor; Penetration depth; Pressure; Resuspended sediments; Sediments; Tracking; Marine
1. The first data on the physical impact of beam trawling on the seabed were obtained during the 1970s, and consequently relate to rather light gears compared with those currently used, This paper deals with the impact on the sea bed of modern, heavy beam trawls. It concentrates on the pressure exerted by the gears and on the changes to the seabed topography and sediment characteristics. A 4-m beam trawl equipped with a chain matrix was used in all experimental work. This gear is typical for 'Eurocutters' operating in coastal areas.2. An instrumented trawl head was developed to measure directly the pressure of the trawl heads on the seabed. This device also allowed a description of the mechanical behaviour of the gear in contact with the seabed. The effect of gear and vessel size on gear pressure was modelled. The changes to the seabed topography were observed by side-scan sonar, and changes in sediment characteristics were measured using the RoxAnn seabed classification system.3. The pressure exerted on the seabed by beam trawls is strongly related to the towing speed. As the speed increases, the lift of the gear increases and the resultant pressure force decreases. At higher speeds, the weight of the gear is fully compensated, and the trawl lifts off the bottom.4. For the 4-m beam trawl studied, the pressure exerted by the trawl heads varied from 17 to 32 hPa at towing speeds of 4-6 kn. Bottom contact was lost at a towing speed of 7 kn.5. Although larger vessels use heavier gears, this is compensated for by larger sole-plate dimensions and higher towing speeds, hence the pressure exerted is roughly equal to the 4-m beam trawl.6. Beam trawls leave detectable marks on the seabed. The length of time that the beam trawl marks remain visible depends on the upper sediment layer. On a seabed consisting of mainly coarse sand, the tracks remained visible for up to 52 h, whereas on sediments with mainly finer particles, the tracks had completely faded after 37 h. The penetration depth could not be deduced from the side-scan sonar recordings, since the traces were too weak.7. The movement of the gear causes the resuspension of the lighter sediment fraction. The changes are most pronounced in areas with finer sand. The suspended particles, however, settle down within a few hours.