The echosounder EKS-38 with the 30 × 30 cm ceramic hull mounted transducer and the QM-Mk II analog integrator were used for the fish density evaluation. The calibration performed with the steel ball of TST = -29.4 dB as a standard target gave SL + VR = 116.7 dB for standard settings used during the survey. Integrated echo intensity from the standard target at a depth of 20.25 m for the unit distance of 1 nautical mile gave the integrator output of 260 mm.
More detailed performance data and standard settings are listed in Annex III.
Calculation of C-value
The relationship between target strength of 1 kg of small pelagic fish with swim bladder and the length of the fish can be written:
Analytical relation between C value, the acoustic system performance constants and the target strength of 1 kg of fish can be written:
It can be seen from those two formulas that the C value expressed in tons/n.mile2 mm is straightly related to the fish length in cm
where
For the actual performance data of the acoustic system of R/V "Dr. Fridtjof Nansen":
And this is exactly the same value as obtained from the intercalibration with R/V "Johan Hjort" performed in March, 1979. So, for the standard fish length of 17 cm
Using the "Aberdeen" method (Simmonds and Stewart, 1980) and the steel ball as a reference target we can calculate the C value from the formula:
The C value calculated by both methods is exactly the same and this confirms that both methods are equivalent for the same target strength of fish.
