Sound travels much further underwater than light; more than 100 times further. Typically light can only penetrate up to 30 metres in clear water, whereas sound can travel many thousands of metres. This is why fishermen use active acoustic instruments, like sonar and echosounders, to find fish.
Such instruments are known as active acoustic instruments because they send out sound and receive echoes from objects in the water, like fish. These instruments operate very much like the echo-location systems of bats and dolphins.
Scientists also use these instruments to determine the abundance, distribution, and behaviour of fish, plankton and other marine organisms during acoustic surveys. The science behind this technology is known as fisheries acoustics and the main instrument is the echosounder.
Marine Scotland Science is one of the leading institutes in the world for fisheries acoustics. It has a long history of research in the field and the authoritative book on the subject was written by its staff. Most of the acoustic work focuses on herring (Clupea harengus), mackerel (Scomber scombrus) and sandeels (Ammodytes spp) and is done using acoustic instruments to:
- Detect fish in studies of gear technology
- Detect fish in studies of fish behaviour
- To determine the distribution of plankton, and
- To measure the performance of fishing gear, such as measuring the depth of trawls, their mouth openings and the distance off the botto