4. FISH DISTRIBUTION AND ABUNDANCE

In accordance with the previous surveys the acoustic registrations of fish were sorted into three categories: Pelagic 1, Pelagic 2 and demersal.

Pelagic fish

Figures 9 through 14 show the density distribution of the small pelagic fish, while Figures 15a-c show the main spatial distribution of the dominant species.

In the Baia dos Tigres area the shelf was mainly occupied by Trachurus capensis in the southern part and Trachurus trecae in the northern (Figure 15a). Schools of anchovy, Engraulis encrasicholus were observed in the near-shore areas. The pilchard Sardinops ocellata was found on one station only, close to the border to Namibia (Figure 15a), confirming that the stock had migrated south since the previous survey. Trachurus capensis was represented by juvenile and young specimens with modal lengths 12 and 18 cm TL in the samples (Appendix III). Corresponding lengths for Trachurus trecae were 12 and 15 cm. The highest densities of horse mackerel (both species), were observed at 100 to 150 m depth in the area 2 to 15 nm off the shelf edge.

From Porto Alexandre to Benguela the small pelagic registrations consisted mainly of Trachurus trecae. From abt. 15 Lat. S, Sardinella aurita was also present in the catches, but in low numbers.

From Benguela to Luanda the Pelagic 1 group consisted mainly of adult Sardinella aurita (30-35 cm TL) in the area Lobito to abt. 11 Lat. S (Figure 11) with some S. maderensis and anchovy, Engraulis encrasicholus. The pelagic 2 group was almost exclusively made up of Trachurus trecae, mostly young specimens (8 and 17 cm mod. length), but also lesser fractions of specimens in the range 20-39 cm (see Appendix III).

From Luanda to Cabinda the Pelagic 1 group was dominantly made of S. aurita and some S. maderensis at scattered locations along the coast (Figure 13) The highest abundances were found in the area Ambriz - Amriezete. The mean lengths of S. aurita were 30-32 cm. The pelagic 2 group was mostly Trachurus trecae in the region Luanda - Ambriz, with some scattered occurrences further north (Figure 14). The horsemackerel was predominantly young specimens with modal lengths 7 and 17 cm, although adults up to 37 cm were also present (Appendix III). The horsemackerel was mainly located at the outer half of the shelf. Closer to the shore Chloroscombrus chrysurus was present in Cabinda and north of Luanda.

Biomass estimates

Rounded figures of the acoustically estimated biomass from the survey are (thousand tonnes):

	Pelagic 1	Pelagic 2	Demersal	Total
Baia dos Tigres area	10	320	-	330
Porto Alex. - Benguela	-	50	10	60
Benguela - Luanda	220	120	20	360
Luanda - Cabinda	120	60	50	230
Total	350	550	80	980

30 random bottom trawl stations were carried out in the area Luanda - Cabinda, giving an average density of 25 t/nm sq. This gives an estimate of 210 thousand tonnes of demersal fish in the region, more than doubling the estimate from the previous survey (88 t. tonnes). Substantial parts of the increase is due to higher catches of Brachydeuterus auritus, and it seems like the species has moved offshore from the shallow areas not covered by the surveys. The variations in the biomass and species composition will be studied more closely in the final report.

Fishing experiments

A total of 132 hauls were made. Their distribution on areas and the range and mean value of the five highest catches in the bottom trawl were (in tonnes per hour):

	Number of hauls		Catch 5 highest demersal
Area	Pelagic	Demersal	Range	Mean
Cunene - Porto Alexandre	14	9	1.4 - 2.8	2.0
Porto Alexandre - Benguela	8	1	2.6	2.6
Benguela - Luanda	11	33	1.7 - 2.4	2.1
Luanda - Cabinda	10	46	2.1 - 6.0	4.1

Deep water trawling

From Lobito to Cabinda 29 hauls were carried out in the 200-600 m bottom range. All hauls were carried out during daylight. The distribution of the hauls by depth strata and the mean catches of the main groups were as follows:

Depth stratum	Number of hauls	MEAN CATCH (kg/hour)
		Shrimps	Shrimps, (small)	Merluccius	Mean length (cm)	Other fish
200-250	8	33	-	90	20	519
300-350	9	54	54	366	29	216
400-450	6	222	60	1050	36	102
500-550	6	9	27	60	40	102

The hake, Merluccius polli is an important by-catch in the shrimp fishery, and the estimated mean densities in the strata are provisionally:

(tonnes/nm sq.)

200-250: 3

300-350: 12

400-450: 35

500-550: 2

During the investigations the standard bottom trawl with rubber bobbins was used. It is likely that this type of gear has given lower catches of shrimp per area swept than those obtained with the gear used in the commercial shrimp fishery. The catches of Merluccius polli are probably not significantly different with or without rubber bobbins. The distance between the wings of the bottom trawl used on Fridtjof Nansen is about 20 m and by modifying the catch figures above mentioned to compensate for the greater width of the commercial trawl, the catch rate of Merluccius polli in the commercial fishery could be roughly assessed. These figures should be treated as preliminary until longer series of catch data are obtained.

Figure 1. Cruise track and stations Baia dos Tigres - Benguela.

Figure 2. Cruise track and stations Benguela - Luanda.

Figure 3. Cruse track and stations Luanda - Cabinda.

Figure 4. Temperature 4 m depth recorded by thermograph.

Figure 5. Distribution of temperature, salinity and oxygen in the hydrographical sections off Baia dos Tigres and Lobito.

Figure 6. Distribution of temperature, salinity and oxygen in the hydrographical sections off Pta. da Moita Seca and Pta. das Palmerinhas.

Figure 7. Distribution of temperature and salinity off the Congo River 3-4 Dec. 1985.

Figure 8. The hydrographical sections off the Congo River.

Figure 9. Fish distribution, Pelagic 1 Baia dos Tigres - Benguela.

Figure 10. Fish distribution, Pelagic 2, Baia dos Tigres - Benguela.

Figure 11. Fish distribution Pelagic 1, Benguela - Luanda.

Figure 12. Fish distribution Pelagic 2, Benguela - Luanda.

Figure 13. Fish distribution Pelagic 1, Luanda - Cabinda.

Figure 14. Fish distribution Pelagic 2, Luanda - Cabinda.

Figure 15. Main species distribution Nov-Dec 1985.