The acoustic instruments consist of a 38 kHz and a 120 kHz echo sounder, both connected to echo integrators.
The integrated echo values are averaged over five mile intervals and plotted in the chart along the sailing track to provide a fish distribution chart. Figure 11 presents the relative distribution of pelagic fish biomass by integrated echo in millimetres per nautical mile.
Before plotting the fish values, the echogrammes are scrutinized and contributions from plankton and fish are separated. The 38 kHz sounder was chosen as the main instrument, and the 120 kHz sounder was used as an aid for this separation. Integrator values may, if possible, be divided further into demersal fish, various types of pelagic fish and mesopelagic fish.
For identification of the various types of echo traces both midwater and demersal trawl was used frequently.
A searchlight sonar was used during parts of the cruise for counting of pelagic fish schools. The sonar was set 90 to starboard, horizontally. No schools were recorded. In the shallow areas with coral reefs and knolls the sonar was used for navigational purposes.
Some small pelagic fish schools were recorded in shallow waters by the echo sounder. In some places, pelagic species were also recorded as “knots” and “needles” on the bottom. At night these concentrations would rise and scatter in the whole water column. Generally the fish recordings were poor.
In the shallow areas, mainly the Zanzibar Channel, there were patches of relatively good concentrations of small organisms. Sampling by midwater trawl revealed fish larvae and juveniles and small crustaceans (“krill”).
The vessel is too large to investigate the very near-shore shallows which has many coral reef surprises. In the south, where the shelf is very narrow, relatively good recordings were made only in the bays where the vessel could survey a fairly even shallow plateau.
Off the shelf, near the slope a “cloudy” recording was made in midwater and near the bottom. This consisted of plankton, fish larvae and juveniles, and small mesopelagic species. Large scad (Decapterus kurroides) would feed on these organisms and could be seen as distinct darker spots in the “cloud”. Some groups of distinct spots in the echogrammes may also have been caused by flocks of sharks.
Further offshore, the echogramme was practically blank for long distances.
