Since the previous survey in the area the vessel was equipped with new echosounders (Simrad EK400) and integrator (Simrad QD). This system was used to record, the distribution and abundance of small pelagic fish and of demersal fish down to 200m depth. A 38kHz system was used for abundance estimation, while a 120 kHz system was utilized only as an additional aid in identification and interpretation of the echo traces. The old QM integrators were run paralell with the new one as backup in case of failure in the new system.
The output from the integrator is expressed in millimeters on graph paper and represents indices of abundance of fish and plankton. The integrator output is separated into plankton and fish according to the characteristics of the echo traces and the trawl samples.
Figure 4 shows the distribution of pelagic fish during the present survey. Four levels of fish density are given; scattered (1-50mm) aggregated (51-100mm) dense (101-500mm) and very dense (>500mm). (Due to higher performance of the new acoustic system the output in mm was five times higher than in the system used in the previous survey.)
Figure 4. Distribution of small pelagic fish.
The biomass of small pelagic fish on the shelf is assessed to 113 thousand tonnes. Of this 60 thousand tonnes or 53% were found in dense concentrations. From the previous survey in the area 245 thousand tonnes were estimated for the total small pelagic biomass. This indicates that the shelf during the recent survey held less than 50% of the previous biomass. During a recent survey in South Yemen considerable resources of small pelagic fish were located in a scattered pattern off the shelf in the outskirts of the upwelling system. The same migration pattern could be valid for the pelagic fish community off NE- Somalia. This would explain the drastic reduction in the shelf biomass during the upwelling period. A distribution of pelagic fish off the shelf were not anticipated and the total survey time available did not allow the area surveyed.
The demersal fish group was acoustically detected only in very scattered and restricted distributions and was considered below the threshold of a reliable acoustic estimate. This indicates that the level of the demersal resources were low. Absolute estimates are obtained better from the trawl survey (next chapter).
