Matthias Schaber (left) and Sascha Fässler (right), chairs of the Working Group on International Pelagic Surveys (WGIPS)
'Acoustic surveys are used to estimate distribution and biomass (total weight of fish in a given area) of species living in open water (pelagic) and often aggregating in large schools. In acoustic surveys, scientific echosounders are used that transmit a sound pulse into the water and receive echoes that originate from the seabed or from fishes and other marine organisms in the water. During the survey, a sound pulse is emitted by the echosounder at regular intervals, commonly every second.
Fish have a different density from the surrounding water and therefore reflect the sound pulse back to the echosounder. The strongest gradient in density and therefore the strongest echo is reflected when the sound pulse hits the gas bladder of fish. Accordingly, fish with a gas bladder like herring or blue whiting reflect strong echoes, while fishes without swimbladder (sandeel or mackerel) reflect much weaker echoes. The magnitude of the reflected echo is dependent on fish species but also fish size. The echo a specimen of a given size would reflect, (target strength), is known.
The track for the cruise vessel in acoustic surveys follows predefined transects and echosounder data is collected continuously whilst the vessel follows the track. The transects are designed to representatively cover the area under investigation and to account for e.g. patchiness in distribution of the target species. If there are notable fish registrations visible on the echosounder screen, targeted pelagic trawl samples are taken to gain information on species composition as well as length distribution within the observed fish schools.
After manual post-processing of the acoustic data, numbers of fish are estimated by dividing the total recorded echo strength of a species by the amount of echo reflected by one fish. Using length-weight relationships information, these numbers can then be converted into fish biomass.'