The recordings on the acoustic instruments were grouped in four categories: small pelagic fish, demersal fish, mesopelagic fish and 0-group fish and plankton. The survey area were divided into 10 sub-areas (Fig. 1). Within each sub-area the area of recording of each group and the average integrator deflection were calculated, and indices of abundance were worked out by multiplying the areas of recording by the average integrator deflection (Table 1).
In order to convert the indices of abundance into volumes or weights of fish a conversion factor, C, have to be established. This conversion factor which depends strongly on the target strength of the fish can be calculated from direct observations of target strength, or directly measured by putting a known amount of alive fish into the sound beam. Such experiments will be carried out during 1976. However, in order to get an idea of the absolute quantities of fish which can be derived from the abundance indices in Table 1, results from the Pelagic Fishery Project in Cochin are used. The conversion factor, C, for the acoustic system onboard the project vessel "Rastrelliger", corresponding to a target strength of -29 dB/kg, was estimated to be 6.5 tonnes/mm per nautical mile x square nautical mile. An intercalibration run between "Dr. Fridtjof Nansen" and "Rastrelliger" in June 1975 gave the following equation for the readings of the two acoustic systems:
Rast = Dr. Fr. Nansen x 1.1
Consequently a reasonable estimate of C for "Dr. Fridtjof Nansen" will be 7 tonnes/mm per nautical mile x square nautical mile. This value of C was applied to the abundance indices of small pelagic fish and demersal fish in Table 1 resulting in the weights in Table 2.
The maximum catch rates obtained for small pelagic and demersal fish are given in Table 3.
