Navigation and Seamanship

6.1.5. Netsonde

NETSONDE

Net sonde is a special acoustic instrument which collects the operational information of trawl net. It has the same working principle of echosounder ,but for the position of the transducer.The transducer of the net sonde is fixed in the head rope of the trawl net. It monitors the operation of trawl net and its catch and transmits it for display in the wheel house during trawling.Net sonde is also called as Trawl eye or Net sounder or Trawl Monitoring System.

Net sounds was initially developed for midwater trawling. Later it was found that it can also be most useful in bottom trawling for measuring the opening height of the trawl mouth and for ensuring continuous bottom contact. In addition to this continuous control of gear performance the netsonde used in bottom trawling also provides a much better estimate of catching efficiency and amount of catch by showing the fish in and above the net opening. Net sonde equipment requires a direct connection between fishing gear and wheel house a vessel which can be either by cable or by a wireless link. For observing the trawl opening and fish entering, only one transducer is mounted on the headline of the trawl, usually sounding straight downwards. When midwater trawling over very deep water with the bottom outside the sounding range the transducer may be attached to the foot rope sounding straight up to the surface.

In the operation of aimed pelagic or mid water trawl it is essential to identify the position of the trawl net relative to the fish shoal. Net sonde also obtain information about the fish distribution and their reaction in front of and around the trawl, as well as providing more complete data on the gear itself. Using the display the skipper can observe whether a fish school being approached is higher or lower than the trawl opening or to the right or left, and an adjustment of trawl depth and course may be made accordingly.