Fig. 7 shows the distribution of echo abundance at Sofala Bank during the cruise. The recordings were nearly exclusively made up by pelagic species. North of 17º50’S the Sardinella spp. and Dussumieria acuta dominated the recordings. The isolated patches along the shore were mainly Pellona ditchela and Thryssa vitrirostris while Decapterus spp. and Rastrelliger sp. were observed near the shelf edge. The largest contribution to the echo abundance came from the Stolephorus spp. In the southernmost area it was not possible to carry out neither bottom nor pelagic trawling due to the bad bottom conditions. It is therefore not confirmed if these recordings really consisted of Stolephorus sp.
Fig. 3. Surface salinity distribution - 29 May-8 June 1983.
The acoustic abundance estimate was calculated by the equation
where B is the fish biomass, M is the average integrator reading, A the corresponding area and C is a conversion coefficient. The numerical value for C applied in these calculations was
C = 0.8 · L tonnes/mm · (n.mil)2when L is the average length in cm of the fish.
Fig. 4. Water colour according to the Forel scale - 29 May-8 June 1983.
If all the recordings in the C areas of Fig. 7 consist of the small anchovy (Stolephorus sp.), this stock size will be about 170,000 tonnes while the sardines of area C amounts to about 20,000 tonnes. For the mackerel, area B, it is not possible to give any abundance estimate due to insufficient coverage of the area. In addition to the patches called areas D in Fig. 7, the Pellona sp. and Thryssa sp. are distributed scattered along the whole coast at depths shallower than 30 m. Calculations based on Fig. 7 will therefore give an underestimate for these species Most likely, this stock will be of the order of 50.000 tonnes.
Fig. 5. Secchi depth distribution - 29 May-8 June 1983.
Fig. 6. Vertical temperature distribution in three sections indicated in Fig. 1.
Fig. 7. Echo abundance distribution of pelagic fish - 29 May-8 June 1983.
