Intercalibration of nautical area scattering coefficients between research vessels
Arrhenius, Fredrik; Bethke, Eckhard; Cardinale, Massimiliano; Hakansson, N
Acoustic surveys often involve more than one research vessel and the results are combined to produce an estimate of the abundance of a stock. Intercalibration is a routine procedure carried out on surveys to check that the acoustic systems on the vessels are working properly. To fmd reasons for the differences measured in the past, an inter-vessel comparison was conducted in October 1997 in the southern Baltic Sea between the German RV Solea and the Swedish RV Argos. The results showed that a side-shifted transducer on the RV Solea again gave 1.4 to 2.5 times higher nautical area scattering coefficients (s(A) values) than a hull-mounted transducer used by RV Argos. There is no explanation for this phenomenon at this stage and it has not been possible to calculate a simple conversion factor. However, the noise from the ship was detected as the most significant factor. The fright reaction of the fish depends on the noise level and this depends on several other factors like source level, directivity function of the vessel and che water depth. Since these factors are usually different for different vessels, we can not expect the same value for each vessel. As the transducers used were optimised for different situations the variation in measurements was extremely large. However, the extent of the behavioural reaction exhibited by the fish seems to depend on two combined factors: lateral avoidance and the tilt angle effect of avoidance.
Archive Of Fishery And Marine Research
2000, Volume: 48, number: 1, pages: 31-42
Publisher: GUSTAV FISCHER VERLAG
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