Research theme: Hydroacoustics

Hydroacoustics or echo-sounding is a non-destructive way of sampling fish in lakes and works using the application of sound in water. Hydroacoustics can be passive, listening out for sounds within the surrounding water or active, where sound is transmitted, with the returning echo analysed using an “echogram”. This echogram is a computer-generated image, much like a graph, and allows us to see various underwater features.

When we survey lakes using this technology for Inland Fisheries Ireland, we transmit a beam of sound into the water column and listen for the returning echo. If fish are encountered and return an echo, we see little distortions or dots in the echogram image. The time it takes for an echo to reach us tells us how far away the fish are and the loudness of the echo tells us how big the fish are. Lot of “noise” or dots may indicate a large shoal of fish.

Hydroacoustics enable us to gather information on fish presence and absence, their numbers, the depth they are at and their sizes. Unfortunately, we cannot identify the species of fish but by cross-referencing these surveys with historical survey data and proofing it with small amounts of real “ground-truthed” data we can fill in the gaps. This in effect, allows us to survey the lake without intensive conventional netting surveys. At present our hydroacoustic surveys are conducted at night because fish disperse more in the dark, making echos easier to interpret. This technology, while not giving us perfect information, is always improving.

Pollan and Arctic char are rare in Ireland and have high conservation importance; therefore, it is desirable to monitor them using minimum impact techniques such as hydroacoustics and eDNA.