The biomass of krill for an area can thus be estimated if acoustic recordings are made along a systematic grid. It is essential for an acoustic estimate that the average reflected sound energy from a single krill is known. Usually, analytical models are used to estimate the average echo based on information about the recorded krill such as length, density contrast and swimming orientation. Such models are associated with quite a high uncertainty, and a focus area at the Institute of Marine Research has been to try and reduce this uncertainty.
During acoustic trawl surveys, trawl samples are normally acquired at observed targets or at regular intervals, and the samples serve many purposes. They provide essential support to the acoustic recordings both because they provide size samples of the animals and they give direct information about which species of organisms are being recorded. Trawl samples also provide valuable biological information like local species composition and demography. During their annual acoustic trawl survey around the South Orkneys, the Institute of Marine Research apply a specially designed krill trawl where the entire net is made up of very fine size – only 4 mm.