Effects of Trawling on Seamounts
2010
Malcolm R. Clark
Trawling involves the towing of nets through the water or along the seafloor to sieve out fish and marine invertebrates. It is the most widely used method to catch fish throughout the world, and there are many variations in gear design and towing methods. The main type of trawl used on seamounts is termed a bottom trawl. This equipment is towed off two wires from the stern of the fishing vessel, with the net spread in between and held apart by steel trawl doors. These doors drag along the seafloor, throwing up two sediment plumes, which helps herd fish into the net, where they are swept back into the closed cod-end of the trawl. The bottom part of the net may have large rollers (Figure 1b) to avoid snagging on rocks, which can cause serious damage to the net. Bottom trawls used on flat ground (e.g., ocean floor covered by pelagic sediment) can be large, but on seamounts they are often smaller and more heavily built because they are being towed over rocky and rough seabed. Although seamount trawls are smaller, the intensity of trawling can be extremely high, with hundreds, or even thousands, of trawls carried out on individual seamounts (Clark and Koslow, 2007). Trawling occurs from the summit down the flanks, often repeatedly along the same tow lines. The distance between the trawl doors is usually about 100 m, and between the ends of the net about 25 m. Trawl doors may weigh up to 2 t each, and the net and ground gear between 3 and 4 t.
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