which can consist of short lengths of chain cable shackled together or a custom made device (Fig. 12) designed to roll rather than be dragged along the bottom. Warp length/water depth ratios are similar to those used with the single demersal trawl and bridles/sweeps (d) can be steel wire, combination wire, chain or a mixture of all three. Normal towing speed used is around 2.5 knots. 13 Beam trawls are used to harvest whitefish, mainly flatfish such as sole, plaice or megrim together with angler and other species found hard down on the seabed. Each net is fished from an outrigger boom, one on each side of the vessel (Figure 14), and towed from here on a single warp (a) shackled to a three chain bridle (b) attached directly to the beam (c) which holds open the mouth of the trawl. The beam, 9-12 m in length, is constructed from heavy steel tube and supported on each side by rugged
be lethal, the most common cause of death is from drowning, starvation and loss of body heat following damage to the plumage by oil. Impact of oil on fisheries and mariculture An oil spill can directly damage the boats and gear used for catching or cultivating marine species. Floating equipment and fixed traps extending above the sea surface are more likely to become contaminated by floating oil whereas submerged nets, pots, lines and bottom trawls are usually well protected, provided they are not lifted through an oily sea surface. Experience from major spills has shown that the possibility of long-term effects on wild fish stocks is remote because the normal over-production of eggs provides a reservoir to compensate for any localised losses. Cultivated stocks are more at risk from an oil spill: natural avoidance mechanisms may be prevented in the case of captive species, and the oiling of cultivation equipment may provide a
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