Fishing and Gear Technology

Unit 16 - Electronic equipments for fishing

16.2.Principles of acoustic fish detection
All acoustic fish detection devices are based on echo-sounding or echo ranging techniques in which a pulse of sound energy is transmitted through water and received back from a reflecting object. Sound is transmitted through sea water at a nominal speed of 1500 m per second at a temperature of 13ºC and 35.5 ppt salinity. The speed ranges from 1450 – 1570 m/sec. depending on temperature, salinity and pressure. The speed increases with temperature at the rate of 4.5 m/sec/^oc, with salinity at the rate of 1.3 m/sec. per ppt and with pressure at the rate of 1.7 m/sec. per 100 m depth. Speed in sea water is about 3% higher than in fresh water.