Netsounder

The netsounder is merely an echosounder with a transducer mounted on the headline of the net rather than on the bottom of the vessel. Nevertheless, to accommodate the distance from the transducer to the display unit, which is much greater than in a normal echosounder, several refinements have to be made. Two main types are available. The first is the cable type in which the signals are sent along a cable. In this case there has to be the provision of a cable drum on which to haul, shoot and stow the cable during the different phases of the operation. The second type is the cable less netsounder in which the signals are sent acoustically between the net and hull mounted receiver/hydrophone or a small receiver towed a short distance behind the vessel. In this case no cable drum is required but sophisticated electronics are needed at the transducer and receiver.

The display on a netsounder shows the distance of the net from the bottom (or the surface), rather than the depth of water as with the echosounder's hull-mounted transducer. Fixed to the headline of the net, the footrope can usually be seen which gives an indication of the net performance. Any fish passing into the net can also be seen, allowing fine adjustments to be made to catch the most fish possible. Some versions have transducers mounted on the otter boards, transmitting horizontally across the face of the net and receiving an echo from the other otter board. This is particularly important in twin trawl gear which permits very fine control over the nets. Again, this arrangement gives a good indication of the performance of the net. In other fisheries, where the amount of fish in the net is important, tension transducers are mounted at various positions on the codend of the net. As the codend fills up these tension transducers are triggered one by one and this information is transmitted acoustically or by cable to monitors on the bridge of the vessel. The skipper can then decide when to haul the net.

Modern versions of the netsounder, using multiple element transducers, function more like a sonar than an echosounder and show slices of the area in front of the net and not merely the vertical view that the initial netsounders used.