4. Results of the fishing experiments, catch compositions and swept area biomass estimates

Some of the results of the fishing experiments are commented on below. One should note, however, that most of the fishing forms part of a programme for “swept area” biomass estimation, and for this purpose the trawl stations are positioned in advance more or less randomly and with no intention of obtaining high catch rates. The catch rates thus do not simulate those of a commercial fishery.

All catches were sampled for composition in weight and numbers by species and size sampling was made of important species, using total length. The complete records of fishing stations are shown in ANNEX II.

Table 1. Cunene to Tombua. Catch rates by main groups in pelagic, and bottom trawl hauls, standardized to kg/hour.

INNER SHELF

ST. NO.	DEP.	Pelagic	Demersal	Sharks	Cephalopod	Other
1	35	50.5	21.1	2.2	238.8	9.0
3	43	24.2	78.0	6.6	59.4	4.5
4	46	3.7	53.6	6.3	6.6	17.3
9	57	35.1	665.4	2.4	85.2	3.0
10	18	6.6				22.0
11	42	689.8	497.4	30.0	11.2	22.9
18	69	20.0	61.4		12.8	99.0
19	18	8.1	23.0		15.2	8.8
20	46	5.6	0.0		3.7	3.7
22	36	4.8		3.3	0.7	3.7
33		590.1	24.8	16.2	1.0	51.7
MEAN		130.8	129.5	6.1	39.5	22.3

OUTER SHELF

ST. NO.	DEP.	Pelagic	Demersal	Sharks	Cephalopod	Other
2	87	6.5	2320.6			382.5
5	77	34.6	620.4		1.4	187.0
6	18	56.5	26.6		0.1	33.2
7	50	8.7	2.8			21.7
8	83	3.0	8.1		9.1	51.2
12	87	18.0	763.7		12.6	40.5
13	108	3850.0	89.6			22.0
14	96	104.0	556.6			23.0
15	43	54.0				26.1
16	136	82.4	129.2		7.20	50.4
17	108	37.0	227.8		6.00	26.0
23	85	5.0	218.0	6.6	20.00	386.5
24	145		222.6			163.4
25	28	56.2			0.6	47.3
26	118	1.2	7.2			5.4
27	134	204.0	309.4		3.6	214.8
28	112	507.6	79.2	1.0	3.6	33.6
29	134	1186.5	713.2		23.9	63.5
31	114	302.4	343.4		18.0	139.2
32	120	22.2	97.4		1.2	62.4
MEAN		327.0	336.7	0.3	5.3	99.0

Cunene to Tombua

The species diversity in this southern area is low, but there is some change of composition by depth and there is in addition a tendency for a size diversity by depth with smaller sized fish in shallow water. The trawl data are therefore analysed for the inner - and outer shelf separately using the depth ranges 0 - 70 m and 70 - 150 m respectively.

Table 2. Cunene to Tombua. Catch rates by families of pelagic fish in pelagic and bottom trawl hauls, standardized to kg/hour.

INNER SHELF

ST. NO.	DEP.	Clupeids	Anchovies	Carangids	Hairtails	Scombrids	Other
1	35			50.5			271.1
3	43			18.6		5.6	148.5
4	46			2.3		1.4	83.8
9	57			35.1			756.0
10	18			6.6			22.0
11	42			689.8			561.5
18	69			20.0			173.3
19	18	0.0		8.1			47.1
20	46			1.4	0.4	3.8	7.4
22	36			3.0	1.8		7.8
33		253.9	35.2	285.4	10.3	5.1	93.9
MEAN		23.0	3.2	101.9	1.1	1.4	197.5

OUTER SHELF

ST. NO.	DEP.	Clupeids	Anchovies	Carangids	Hairtails	Scombrids	Other
2	87			6.5			2703.1
5	77			26.6		8.0	808.8
6	18			56.0		0.5	60.0
7	50			8.7			24.5
8	83			3.0			68.4
12	87			18.0			816.8
13	108			3850.0			111.6
14	96			104.0			579.6
15	43			38.6	14.2	1.2	26.1
16	136			82.4			186.8
17	108			33.0	4.0		259.8
23	85			5.0			631.1
24	145						386.0
25	28			55.5	0.7		47.9
26	118			1.2			12.6
27	134			158.4	45.6		527.8
28	112			147.6	360.0		117.4
29	134			1185.5		1.0	800.6
31	114			271.2	31.2		500.6
32	120			22.2			161.0
MEAN				303.6	22.7	0.5	441.5

Table 1 shows the catch rates standardized to kg/hour for the main groups of resources on the two pans of the shelf. Hauls with the pelagic trawl are included. The highest catch rates for both demersal and pelagic fish were obtained on the outer shelf. Sharks and cephalopods appear, however, to be more abundant on the inner shelf.

The common cuttlefish Sepia officinalis represented 60% of the cephalopod catches on the inner shelf and 20% of those offshore while some 30% and 20% of the catches was the squid Loligo vulgaris. The size compositions of samples of these species are shown in ANNEX I. The lesser flying squid Todaropsis eblanae occurred in small amounts in the offshore catches.

Table 2 shows the catch rates of the pelagic species by families from the two parts of the shelf and for both demersal and pelagic trawl. These data are not very meaningful as indices of abundance of these types of fish, but they give an indication of the availability of the various forms. Clupeids and anchovies only occur inshore while the other groups evidently are more abundant offshore. Both species Cape- and Cunene horse mackerels occurred over the whole shelf, but while the Cape species dominated on the outer part, the Cunene species appeared to be most common inshore as can be seen from Table 3. By overall weight of catches the Cape horse mackerel represented 80% of the total. There was a distinct difference in size composition for both of these species between the outer and inner shelf with small sizes mostly juveniles dominating the inner shelf, see ANNEX I.

Table 4 shows the catch rates of demersal fish in the bottom trawl by families. The large-eye dentex Dentex macrophthalmus dominate completely the seabreams with 98% of the group in the catches of the outer shelf and 86% in those from the inner shelf. Also for this species the size compositions from the two parts differ greatly with a predominance of small sized and juvenile fish on the inner shelf. Parts of the inner shelf thus seem to form a nursery ground for this species as well as for the two horse mackerels. If these observations are confirmed by further analysis of previous data or new observations the management implications would be to consider the need for protection of these nursery grounds through fishery regulations.

Table 3. Cunene to Tombua. Catch rates of Cape horse mackerel and Cunene horse mackerel in pelagic and bottom trawl hauls, standardized to kg/hour.

INNER SHELF

ST. NO.	DEP.	Cape h.mac	Cunene h.m
1	35	2.5	48.0
3	43	1.2	17.4
4	46	0.2	2.1
9	57	31.2	3.9
10	18
11	42	257.3	432.4
18	69	12.8	7.2
19	18	4.6	0.9
20	46	0.6	0.3
22	36	3.0
33			283.9
MEAN		28.5	72.5

OUTER SHELF

ST. NO.	DEP.	Cape h.mac	Cunene h.m
2	87	1.8	4.7
5	77	25.2	1.4
6	18	51.5	4.5
7	50	8.4	0.3
8	83		3.0
12	87		18.0
13	108	3360.0	490.0
14	96	34.0	70.0
15	43	38.0	0.6
16	136	72.0	10.4
17	108	23.0	10.0
23	85	5.0
24	145
25	28	55.5
26	118	1.2
27	134	158.4
28	112	147.6
29	134	1185.5
31	114	264.0	7.2
32	120	21.6	0.6
MEAN		272.6	31.0

The striped seabream Lithognathus mormyrus represented 10% of the seabream catches on the inner shelf.

Croakers were common in many catches of the outer shelf consisting as a total of 75% of African weakfish Atractoscion aequidens of large size and 25% of Canary drum Umbrina canariensis of medium size see ANNEX I.

Hakes appeared in small amounts especially in the southern parts of the outer shelf with both the Cape hake Merluccius capensis and the Beguela hake M. polli represented.

Table 5 shows the swept area estimates of mean densities by species and depth strata based on the 25 successful bottom trawl hauls made. The large-eye dentex has the highest total density, but also the more “pelagic” horse mackerels can have high densities in the bottom strata. For most species the densities are highest in deeper waters. One should note that these calculations assume a value of the catchability quotient q equal to 1.

Table 4. Cunene to Tombua. Catch rates by families of demersal fish in bottom trawl hauls standardized to kg/hour.

INNER SHELF

ST. NO.	DEP.	Seabreams	Croakers	Hakes	Other
1	35	21.1			300.5
3	43	78.0			94.8
4	46	53.6			33.9
9	57	665.4			125.7
10	18				28.6
11	42	497.4			753.9
18	69	59.1	2.3		132.0
19	18	23.0			32.2
20	46	0.0			13.0
22	36				12.6
MEAN		139.7	0.2		152.7

OUTER SHELF

ST. NO.	DEP.	Seabreams	Croakers	Hakes	Other
2	87	2124.4	196.20		389.1
5	77	516.8	103.60		223.0
8	83	3.1	5.00		63.3
12	87	742.5	21.20		71.1
13	108	84.0	5.60		3872.0
14	96	424.0	132.60		127.0
16	136	124.0	2.80	2.4	140.0
17	108	198.0	22.80	7.0	69.0
23	85	188.0		30.0	418.1
24	145	138.0		84.6	163.4
27	134	285.4	9.2	14.8	422.4
28	112	79.2			545.8
29	134	621.7	82.6	8.8	1273.9
31	114	297.6	38.4	7.4	459.6
32	120	91.2	2.0	4.2	85.8
MEAN		394.5	41.4	10.6	554.9

Table 6 shows the estimates of aereal extention of the depth strata and the resulting bio-masses for all demersal species and separately for the large-eye dentex and the two croaker species caught in some quantities, African weakfish Atractoscion aequidens and Canary drum Umbrina canariensis. The mainly juvenile part of the dentex biomass found inside the 70 m depth range where the mean density is estimated at 3.8 t/nm over an area of 640 nm is assessed at 2 400 t of the total estimate of 17 000 t. Over the six coverages of the shelf between Cunene and Tombua during the 1985/86 programme the mean estimated biomass of largeye dentex was 12 500 tonnes.

Table 5. Cunene to Tombua. Swept area estimates of fish densities by species and depth ranges. Tonnes pr nm.

The estimated biomass of the hake species was only 300-400 tonnes.

Table 6. Cunene-Tombua. Biomass estimates of groups of demersal fish by depth strata. Tonnes.

	Total	0-50 m	50 - 100 m	100 - 200 m
Shelf area	1 670 nm	420 nm	620 nm	630 nm
Dentex mac.	17 000	850	11 430	4 720
Croakers	1 570		1 150	420
All demers.	26 800	2 100	16 300	8 400

Tombua to Benguela

The very few trawl hauls made in this area do not provide data for much analysis. Table 7 shows the representation in the catches of the main groups. 90% of the seabreams were large-eye dentex and Canary drum represented 70% of the croakers, the African weakfish making out the rest.

Table 7. Tombua to Benguela. Catch rates by main groups in bottom trawl standardized to kg/hour.

ST. NO.	DEP.	Seabreams	Croakers	Sharks	Carangids	Hairtails	Other
34	93	646.1	638.0	37.0	87.8	4.0	6.5
35	73	85.2	16.2		13.6	13.2	89.4
36	21	14.0	1.0		32.2	68.4	264.0
MEAN		248.4	218.4	12.3	44.5	28.5	119.9

Figure 1. Course tracks, fishing stations and hydrographical profiles, Cunene to Tombua and Tombua to Benguela.

Figure 2. Temperature at sea surface and hydrographic profile Baia dos Tigres - West.

Figure 3a. Distribution of pelagic fish type 1, Cunene to Tombua and Tombua to Benguela.

Figure 3b. Distribution of pelagic fish type 2, Cunene to Tombua and Tombua to Benguela.

Figure 3c. Distribution of demersal fish, Cunene to Tombua and Tombua to Benguela.