5. FISH DISTRIBUTION AND ABUNDANCE

The acoustic data obtained from the echo sounders and integrators are processed and classified in a procedure based on experience of the types of echo recordings produced by various forms of organisms and on the information gained from the fishing experiments. Three categories are distinguished:

1. Pelagic fish type I:	Clupeids e.g. Sardinellas, sardin anchovies.
2. Pelagic fish type II:	Other schooling pelagic fish such horse mackerel, other carangids, scombrids, trichiurids, barracudas etc
3. Demersal fish:	Other than pelagic schooling fish such as hakes, sparids, sciaenids, catfishes, snappers etc.

In addition signals caused by plankton and by mesopelagic fish such as Myctophids are recorded, but no further processing is being made of these data.

Assessment of the abundance of fish resources based on acoustic observations combined with experimental fishing is a method which especially lends itself to fish found in schools or other aggregations in mid water. This is however a type of behaviour which is characteristic for most of Angolas fish species of commercial importance. There are some strictly bottom dwelling fish e.g. rays and flounders which will escape acoustic detection. Also fish in the very surface layer can not be recorded by echo sounder, but schools my be detected by horizontal ranging sonar. For navigational reasons the work with the R/V “Dr. Fridtjof Nansen” is limited to waters deeper than about 15m. Such very shallow waters form only a small part of the Angolan shelf and are mainly found in the north. The effects of any of these factors on the acoustic assessment will be towards an underestimate

In the following the survey results will be described by zonal divisions since different environmental and faunistic regimes seem to exist along the coast.

Figures 11 through 13 show the distribution of the three categories of fish mentioned above in the area off Baia dos Tigres from Porto Alexandre to the border with Namibia.

The density of fish is indicated by three arbitrary levels of the observations from the echo sounder - integrator system. Contageous distributional patterns such as those demonstrated in the figures are typical for fish species which occur in high abundance in the sea.

The trawl samples indicated that the category “Pelagic I” was a mixture of Sardinella aurita and Sardinops ocellata. Its distribution should probably be extended further north beyond the entrance to the Baia dos Tigres, but only sonar-schools were recorded when this part was covered.

The “Pelagic fish II” consisted almost exclusively of Trachurus capensis. The high concentrations were found near the shelf edge in a band a few miles wide. The schools, generally close to the bottom during daytime tended to lift and become more dispersed at night. Catch rates are shown in the list of fishing stations, Appendix I stations 2 - 16.

The “Demersal fish” consisted mostly of hake and Dentex macropthalmus and smaller amounts of a number of other bottom species. The distribution was largely similar to that of the horse mackerel.

Some size distributions of the main species are shown in Appendix III. The mean size weighted according to ratch rates were:

Trachurus capensis	26,4 cm
Sardinella aurita	27,9 cm
Sardinops ocellata	26,6 cm
Hake	40,1 cm
Dentex macropthalmus	19,0 cm

Provisional estimates of the biomass of the various fish categories can be made, but they should be treated with some reservations and may be subject to amendments later. Further analysis of the size and species composition are required and more information is needed on the properties of the various species as acoustic targets.

The rounded figures are:

“Pelagic I”	about	50 000	tonnes
“Pelagic II”	“	120 000	“
“Demersal”	“	60 000	“

The distribution of the resources in this area indicate a continuation into Namibia especially for horse mackerel and “Demersal fish”. The main species in this southern area are thus likely to be part of stocks shared between Angola and, Namibia. Surveys at other seasons are needed to elucidate further this resource aspect.

Figure 14 shows the distribution of the three categories of fish as observed between Porto Alexandre and Benguela. The fish was mostly found thinly and scattered along the narrow shelf. The survey coverage may however have been incomplete in some areas. Over deeper water off the shelf layers and schools of Myctophids was regurlarly, observed.

Provisional biomass estimates show the following:

“Pelagic I”	about	4 000	tonnes
“Pelagic II”	“	20 000	“
“Demersal”	“	8 000	“

Figure 15 shows the distribution of fish over the relatively wide shelf area from Benguela to Luanda. It is noteworthy that the main concentrations are found over the inshore and middle parts of the bank. Beyond about 100m depth very little fish could be located. This may be related to the low oxygen content below the thermocline over the offshore shelf.

The “Pelagic I” category consisted of the two sardinellas, aurita and maderensis, anchovy and some Ilisha. There was a predominance of juvenile sardinella in the inshore waters, usually found together with anchovy. Bigger sardinella was found further offshore, but in some of the areas of high concentration there is some uncertainty about the identification of large fast swimming schools which proved difficult to catch. The alternative identification would be carangids or scombrids.

In addition to the horse mackerel Trachurus trecae which dominated the “Pelagic II” group a number of other fish was identified by the fishing: Chloroscombrus was quite common, also barracudas and some Caranx, Decapterus, scombrids and smaller tunas. The small areas of high concentration south of Luanda contained horse mackerel.

The “Demersal” group is more evenly distributed. The survey may have missed concentrations close inshore. The caches consisted predominantly of sparids, sciaenids, Trichiuridae and Pomadasidae

The rounded figures of a provisional biomass estimate are:

“Pelagic I”	about	200 000	tonnes
“Pelagic II”	about	400 000	“
“Demersal”	“	70 000	“

Because of the difficulties of identification the combined estimate of the two pelagic groups is more reliable than the split figures.

Figure 16 shows the fish distribution from Luanda and northwards. The relatively low densities and mixed occurrences of small pelagic fish made it impossible to split the Pelagic I and Pelagic II into separate biomass estimates. In Figure 18 the fish species have been roughly grouped in main communities according to their occurrence in the trawl catches. Off Luanda there is an area dominated by Trichiurus lepturus. In shallow waters from Luanda to Bambrizete the fish fauna consists mainly of Chloroscombrus chrysurus and Brachydeuterus auritus. In the region Bamrizete-Congo River the shelf is dominated by seabreams (Dentex spp. and Pagellus bellottii). Off Congo river some few trawl samples confirmed a brackish water fish community with croakers (Sciaenidae), threadfins (Polynemidae) and Ilisha africana. In the shallow waters in the Cabinda region Brachydeuterus and Chloroscombrus are again dominant, while Sardinella maderensis and S. aurita dominates the shelf more offshore.

Provisional biomass estimates for the region Luanda-Cabinda are:

“Pelagic I & II”	about	220 000	tonnes
“Demersal”	about	95 000	tonnes

On basis of the randomized bottom trawl stations and the so called “swept area method” the demersal fish biomass is assessed to 150 000 tonnes.