3. Fish distribution and abundance from the acoustic system

The units of acoustic reflection is 0.1 × m²/nm² reflecting surface. An arbitrary scale has been used to illustrate different levels of concentration. The integrator values were allocated to the following groups on the basis of trawl sampling and characteristic behaviour:

Pelagic fish type 1, Clupeids and anchovies;
Pelagic fish type 2, Carangids, scombrids, barracudas etc.;
Demersal fish in mid water.

Figure 5 shows the distribution of the two categories of pelagic fish as observed with the acoustic integrations system. Very little demersal fish is found in mid water in this region. The pelagic type 1 species, mainly sardinellas with small amounts of anchovies and ilisha were found in school areas of rather limited extensions on the inner shelf of Benguela - Lobito, to the south of Pta. do Morro, between Cabo da Tres Puntas and Cabo Sao Braz and off Palmeirinhas. The sardinellas were often mixed with predators, barracudas, hairtails and some Spanish mackerel and were identified through sampling and school characteristics. Schooling often persisted during nighttime. Both the round - and flat sardinella were mostly of large size, see ANNEX I and in immediate prespawning, spawning and postspawning condition. A few large and dense solitary schools were found on the outer shelf. Identification by fishing of these was not possible, but they are shown in the distribution as pelagic type 1, although not included in the biomass assessment. From the schooling characteristic they could be sardinella, perhaps migrating along the shelf. Attempts should be made in later surveys to identify this type of school if encountered. As in most of the previous surveys a schooling area of pelagic fish was found off Pta. das Palmeirinhas. This consisted of both sardinellas and horse mackerels. The consistent location of pelagic fish here is probably related to the hydrographic conditions in this area with high currents. The pooled size compositions show predominantly large size fish, especially for the flat sardinella.

The pelagic 2 types of fish were found to be distributed over wider parts of the shelf but mostly with low densities. Inshore the aggregations consisted of various Carangids, lookdowns Selene dorsalis, bumpers Chloroscombrus chrysurus and false scad Decapterus rhonchus in addition to the larger predators mentioned above. Offshore the by far most common form was the Cunene horse mackerel Trachurus trecae. The horse mackerel was nearly everywhere of small size, below 15 cm of length. Also the dense aggregations found offshore off Cabo Ledo consisted of small sized horse mackerel. They appeared at night in a layer at 10 - 30 m depth over about 100 m bottom depth from which a half hour pelagic trawl haul yielded about 8 tonnes, but forming schools near the bottom in daytime. Off Palmeirinhas the horse mackerel was of larger size. The pooled size distribution of all horse mackerel sampled is shown in Annex I. This size distribution differs from those of the 1985/86 surveys which all had higher proportions of adult large sized horse mackerel.

Fish Distribution Luanda to Cabinda.

The fish distribution as observed by the acoustic system is shown in Figure 6. The Pelagic 1 group was very poorly represented north of Luanda, and when present, mainly by Ilisha. The sardinella was, apart from some solitary spotted observations, virtually absent from this area, but was found in groups of schools off Punta Palmeirinhas. The pelagic 2 group was more abundant, in the shallow waters with bumpers and lookdown (Chloroscombrus chrysurus, Selene dorsalis), especially between Luanda and Ambriz. The mid-shelf waters was mainly inhabited by Cunene horse mackerel (Trachurus trecae), and the species, although never forming really high densities, had an extensive distribution, and was dominated by one length group, with modal length 15 cm. See ANNEX I on pooled length distributions. The demersal species were only occasionally registrated by the acoustic system, and when, mainly as smaller concentrations of bigeye grunt (Brachydeuterus auritus).

Estimates of biomass.

An estimate of the total biomass of the pelagic type 1 fish based on the acoustic integration technique is by regions:

	Benguela -	Pta Palmeirinhas -
	Pta Palmeirinhas	Cabinda
Sardinellas	160 000	33 000
Total Pelagic 1	170 000	55 000

The aggregations of sardinella off Pta Palmeirinhas was resurveyed at the end of the survey and an estimate of 44 000 tonnes was obtained. The relative high difference between the first and the second estimate is mainly caused by the distribution pattern of the fish. When a species is concentrated in fewer and larger schools the sampling variance increases, and the precision of the estimate is lower. The aggregations of sardinella off Pta Palmeirinhas are mainly composed of large fish. This is the spawning stock, with no potential for further growth in biomass.

The corresponding Pelagic 1 estimates from the 1985-86 surveys were:

First half 1985	200 000	205 000
First half 1986	135 000	120 000

The biomass estimates of the Pelagic 2 group are by regions:

	Benguela -	Pta Palmeirinhas -
	Pta Palmeirinhas	Cabinda
Horse mackerel	75 000	28 000
Total Pelagic 2	100 000	80 000

Most of the horsemackerel figures represents juvenile fish still in fast growth for which the increase in biomass by growth will be greater than the loss by natural mortality. This indicates a strong oncoming recruitment if this small sized fish is not exposed to premature exploitation. The mean of the estimates of biomass of pelagic 2 fish from the six surveys in 1985/86 was 140 000 tonnes for the Benguela-Luanda region and 65 000 tonnes for Luanda-Cabinda.