An intership sonar calibration experiment in the Norwegian Sea

When migrating in the Norwegian Sea in spring and summertime, the Norwegian spring spawning herring will be surveyed by vessels from four nations. Because the herring may occur in schools close to the surface, use of horizontal guided sonar may be an advantage for mapping the geographic distribution and estimation of abundance. To be able to compare the sonar recordings of schools obtained by different sonar systems on different vessels, an intership sonar calibration is nesessary. Such an experiment, which was the first of its kind, was conducted in the Norwegian Sea in June 1995. Vessels from the Faroe Islands, Iceland, Norway and Russia lined up with an intership distance of one nautical mile, and headed in the same direction at a speed of about 4 m s^-1 over a total distance of 30 nautical miles. The number of schools recorded from 50 - 300 m to the side of the different vessels was quite similar, but the correlations between the number of schools recorded by the different vessels declined the smaller the sampling distance unit. The implication of the results for cooperative sonar surveys are discussed