Various types and sizes of midwater trawl have been designed to attempt the capture of nekton at middle depths down to 1000 m or more (Harrisson, 1967). The Isaacs-Kidd net is an elongate conical bag, usually with a mouth aperture of 8 m, and with an angled depressor plate to keep the net below the surface while towing. It can be fitted with a depth recorder, and its depth during towing can be monitored on hydrophones by attaching to the net a pressure-sensitive sound-emitter with a pulse frequency which varies with the depth. A modification is a double-ended net with the opening to the two receivers controlled by a pressure-operated valve. Above a preselected depth the captured material goes into one container, while below this level a flap deflects the catch into the other container, thereby providing a deep-level sample separate from material collected during lowering and hauling.
A midwater trawl (Baker et al., 1973; Clarke, 1969) developed for use from the RRS Discovery, and operated to depths of over 1500 m, has a mouth which can be opened and closed by remote control (Figure 3.17). With the mouth closed the net is lowered to the required depth, as indicated by the pulse frequency of a pressure-sensitive sound-emitter on the net. The mouth is then opened by a release mechanism activated by an acoustic signal from the ship. At the end of fishing a second acoustic signal from the ship causes the mouth of the net to close before hauling. An extension of this system contains two acoustically operated nets in one framework, a small upper net with a 0.32 mm mesh above a larger coarse-mesh net of 8 m2 rectangular mouth and 4.5 mm mesh. There is also a monitor on the frame which measures depth, flow and temperature, telemetering the data acoustically to the ship.
Large active bathypelagic fish and cephalopods have proved extremely difficult to catch, and little is known about their distribution. Probably very large pelagic trawls are needed for their capture. Some abyssal fish have been taken by line, either laid on the bottom or simply suspended from the surface, and some success has also been achieved with deep-water fish traps.
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