The units of acoustic reflection is 0.1 x 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;Distribution Benguela to Luanda
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 rather limited areas and with low density on the inner shelf. In the pelagic trawl catches worked at nighttime the sardinellas were often mixed with barracudas and hairtails. The pelagic 1 type schools were however identified through school characteristics recorded on the echointegrator. Both the round- and flat sardinella were of mixed sizes, see ANNEX I. In most of the previous surveys a schooling area of pelagic fish has been found off Pta. das Palmeirinhas which has consisted of both sardinellas and horse mackerels. No such aggregation of pelagic fish were recorded during this cruice. A fleet of large purse seiners was operating in the area.
The pelagic 2 types of fish were found to be distributed over wider parts of the shelf but also with low densities except for a small location south of Pta. do Morro. On the inner shelf the pelagic 2 types consisted of various carangids, lookdowns Selene dorsalis, bumpers Chloroscombrus chrysurus and a small amount of false scad Decapterus rhoncus in addition to barracudas and hairtails. Offshore the by far most common form was the Cunene horse mackerel Trachurus trecae. The horse mackerel was mostly of small size, with mode at 13 cm of length.
Fish Distribution Luanda to Cabinda
The distribution of the two categories of pelagic species detected by the acoustic system is given in Figure 6. In previous surveys the Pelagic 1 group have been very poorly represented in the area north of Luanda and when present mainly represented by Ilisha. The observation of sardinella have been very scare and only in small patches mainly in the area of Luanda and Punta Palmeirinhas. The pelagic registration have been dominated by Pelagic 2 species as bumpers and lockdown (Chloroscomberus chrysurus, Selene dorsalis) in shallow waters and mainly Cunene horse mackrel (Trachurus trecae) in mid-shelf. The demersal species normally contributes very little to the acoustic registrations in this area.
The distribution of pelagic 2 species in this survey shows large similarities with previous findings and the species composition is the same with bumpers and lookdown in shallow waters and Cunene horse mackerel extensively distributed in mid-shelf from Luanda to the boarder of Congo. The mean length of the horse mackerel is about 15 cm or the same as obtained in previous survey. See APPENDIX I on pooled length frequency distributions. The contribution of demersal species are very limited and when present as scattered concentrations of bigeye grunt (Brachydeuterus auritus).
In contrast to survey I sardinella occured in two areas: from Luanda to Ambrizete and Pta. da Moita Seca to the boarder of Congo. In the sourthern area the sardinella were distributed mostly near to the coast where as in the northern area the distribution extended over the shelf to about 120 m depth. At Cabinda sardinella shoals were observed at surface in very shallow waters less than 20 m. This shallow waters have not been adequately surveyed due to navigational reasons. In the area off Luanda round sardinella (Sardinella aurita) was most common where as in the northern area madeiran sardinella (Sardinella maderensis) dominated. The sardinella found in both areas were mature fish. Mean length of round and madeiran sardinella were 24 cm and 26 cm respectively.
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 |
|
Total pelagic 1 |
39 000 |
131 000 |
|
First half 1985 |
200 000 |
205 000 |
|
First half 1986 |
135 000 |
120 000 |
|
February 1989 |
170 000 |
55 000 |
|
|
Benguela - Pta. Palmeirinhas |
Pta. Palmeirinhas-Cabinda |
|
Total pelagic 2 |
43 000 |
202 000 |
|
Februar 1989 |
100 000 |
80 000 |
The present acoustic estimates indicate a significant decline in the pelagic fish stock abundance of the Benguela-Luanda region, compared to the pelagic fish recordings in 1985/86 and in the February cruise this year as well. The new estimate of the stock of sardinella north of Pta Palmeirinhas is well above the February estimate and the result indicate northward displacement of the stock. The total estimate of sardinella 170 000 tonnes is lower than the February estimate and significantly lower than the 1985/86 level. The result strongly indicate that the stock are heavely exploited and the fishing mortality could be well above a level that will secure sustainable catches.
The estimate of pelagic 2 species north of Pta. Palmeirinhas are much higher than in February and at the same level as the 1985/86 estimates. The present estimate is, however, largely dependent of a small and very dense patch of false scad which gave 130 000 tonnes or about 75 % of the total estimate.
