Net Probe

an instrument for remote monitoring of under-water conditions and the parameters of fishing gear from a fishing vessel; it is used in the immediate vicinity of the ship’s active area while fishing. Net probes are used mainly for sighting fish during trawling at various depths and along the bottom. They make it possible to monitor the operation of the trawl and to direct it toward schools of fish, as well as to determine the quantity of fish in the trawl. Because of the great accuracy of measurement of the distance between a trawl and the bottom, fishing can be conducted very close to hard, uneven bottoms without risking damage to the trawl. For purse seining, net probes are used to determine the depth and immersion time of the lower list of the seine and to ascertain the moment of its contraction when studying and testing new trawls. They also serve to monitor the entry offish into fixed fishing gear (stationary nets, traps, and so on) and are used in studying and testing new gear.

There are three types of net probes, for the measurement of one, two, or three parameters. A probe consists of the underwater apparatus, which is attached to the fishing gear, and the on-board apparatus. The underwater apparatus measures the gear parameters, studies the conditions in the vicinity of the gear, and transmits this information to the ship. The on-board apparatus provides the reception, conversion, and display of the data transmitted from the fishing gear. According to the type of communications lines used, a distinction is made among probes with a special high-strength cable-line, through which the data are transmitted from the gear to the ship; probes with a hydroacoustic communications link, in which the information is transmitted by means of ultrasonic signals; and probes with a radio link, in which the underwater information is collected by apparatus in surface buoys and transmitted to the ship as radio signals.

The most common net probes in the fishing industry have echo-sounding transducers with ultrasonic vibrators that are used by a ship not only to make observations on the fish passing into the fishing gear or nearby but also to measure the basic parameters of the gear, such as the level of its path, the distance from the bottom, and its vertical opening.