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

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.

Cunene to Tombua

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, as in previouse cruise, analysed for the inner- and outer shelf separately using the depth ranges 0-70 m and 70-150 m respectively.

Table 1 shows the catch rates standardized to kg/hour for the main groups of resources on the two parts of the shelf. Hauls with the pelagic trawl are not included. The highest catch rates for both demersal and pelagic fish including sharks were obtained on the outer shelf.

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

INNER SHELF

ST. NO.	PEP.	Pelagic	Demersal	Sharks	Cephalopod	Other
196	55	58.29	188.58			72.00
200	70	835.00	875.00	60.00	5.00	27.40
207	70	112.20	951.90	16.80	42.00	151.20
211	37	66.40	102.00	10.00	400.00	30.00
215	37	5.00	33.00	11.00	45.00	43.54
216	51	71.40	239.40		8.54	8.40
MEAN		191.38	398.31	16.30	83.42	55.42

OUTER SHELF

ST. NO.	DEP.	Pelagic	Demersal	Sharks	Cephalopod	Other
192	96	8.00	56.40	5.60	196.80	37.60
193	114	194.20	560.80	6.80	52.00	121.40
195	84		88.00	24.00		258.00
197	108	263.40	1734.80	2685.00	101.32	212.60
198	125	1616.91	1601.74	33.00	13.20	196.83
201	101	1233.60	2212.40	86.00	41.40	62.00
202	136	50.00	309.00	4.00	9.00	134.00
203	100	3.00	622.50	22.50	40.50	76.50
204	76	43.20	1800.00	288.00	54.00	234.00
205	120	64.80	2511.00	75.60	75.60	405.00
206	137	8.40	618.80		30.80	204.40
212	74	31.60	2377.20	4.80		39.60
213	82		105.80	5.20	15.80	99.80
214	71	11.40	69.60	3.00	53.20	34.50
217	97	82.97	2254.97		15.43	217.38
MEAN		240.77	1128.20	216.23	46.94	155.57

The african cuttlefish Sepia bertheloti represented some 60 % of the cephalopod catches, while some 30% of the catches was the squid Loligo vulgaris. The size compositions of samples of these species are shown in ANNEX I. The pink cuttlefish Sepia orbignyana occurred in small amounts in the offshore catches. Some 90 % of the shark catches consisted of the bigeyed spurdog Squalus megalops.

Table 2 shows the catch rates of the pelagic species by families from the two parts of the shelf by bottom 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. Pelagic 1 type fish does not occur in the bottom hauls, while the horse mackerel are occasionally very abundant. Both species Cape- and Cunene horse mackerels occurred over the whole shelf, but while the Cape species dominated on the outer part and to the south, the Cunene species appeared to be most common inshore and in the northern part (Table 3). As mention previously there was a distinct difference in size composition for both of these species between the outer and inner shelf with small sizes juveniles dominating on the inner shelf, see ANNEX I.

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
191	62			201.94			6.86
194	36						47.13
196	55			58.29			260.58
200	70			830.00	5.00		967.40
207	70			8.40	100.80	3.00	1161.90
208	19			10.15			83.08
209	15	103.15	0.23	86.77			40.50
210	17	3288.60		610.68			300.72
211	37			64.00		2.40	542.00
215	37			5.00			132.54
216	51			15.40	56.00		256.34
218	26			13.85			434.77
MEAN		282.65	0.02	158.71	13.48	0.45	352.82

OUTER SHELF

ST. NO.	DEP.	Clupeids	Anchovies	Carangids	Hairtails	Scombrids	Other
192	96			8.00			296.40
193	114			192.00	2.20		741.00
195	84						370.00
197	108			263.40			4733.72
198	125			1616.91			1844.77
199	170			17.76			12.11
201	101			1233.60			2401.80
202	136			35.00	15.00		456.00
203	100			3.00			762.00
204	76			43.20			2376.00
205	120			64.80			3067.20
206	137				8.40		854.00
212	74			31.60			2421.60
213	82						226.60
214	71			8.40		3.00	165.30
217	97			82.97			2487.78
MEAN				225.04	1.60	0.19	1451.02

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.

ST. NO.	DEP.	Cape h. mac	Cunene h. m	Other
191	62	201.94		6.86
192	96	8.00		296.40
193	114	176.00	10.00	743.20
194	36			47.13
195	84			370.00
196	55	58.29		260.58
197	108	263.40		4733.72
198	125	1550.91	66.00	1844.77
199	170	17.76		12.11
200	70	830.00		972.40
201	101	1233.60		2401.80
202	136	35.00		471.00
203	100	3.00		762.00
204	76	43.20		2376.00
205	120	64.80		3067.20
206	137			862.40
207	70	8.40		1265.70
208	19	10.15		83.08
209	15		86.77	143.88
210	17		610.68	3589.32
211	37		64.00	544.40
212	74		31.60	2421.60
213	82			226.60
214	71		8.40	168.30
215	37		5.00	132.54
216	51		15.40	312.34
217	97		82.97	2487.78
218	26		13.85	434.77
MEAN		160.87	35.74	1108.50

Table 4 shows the catch rates of demersal fish in the successful bottom hauls by families. The large-eye dentex Dentex macropthalmus dominate completely the catches of seabreams in both areas. Also for this species the size compositions from the two parts differ with a predominance of small sized and juvenile fish on the inner shelf.

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
200	70	10.00	10.00		1782.40
207	70	794.10	36.00		444.00
211	37	102.00			506.40
215	37	25.50			112.04
216	51	236.60			91.14
MEAN		233.64	9.20		587.20

OUTER SHELF

ST. NO.	DEP.	Seabreams	Croakers	Hakes	Other
192	96	8.00	8.00		288.40
193	114	427.20	76.80	4.00	427.20
197	108	1175.20	81.00		3740.92
198	125	969.00	73.54	4.46	2414.68
201	101	1240.60	792.60	17.20	1585.00
202	136	137.00	3.00	5.00	361.00
203	100	469.50	7.50	7.50	280.50
204	76	3.60	32.40	3.60	2379.60
205	120	1350.00		324.00	1458.00
206	137	168.00	126.00	18.20	550.20
212	74	2189.00	37.80		226.40
213	82	47.80			178.80
214	71	29.40			147.30
217	97	2177.31		20.74	372.70
MEAN		742.26	88.47	28.91	1029.34

Croakers were common in many catches of the outer shelf and consist of 95 % African weakfish Atractoscion aequidens of large size and 5 % of Canary drum Umbrina canariensis.

Hakes appeared in small amounts in the southern parts of the outer shelf. The Cape hake Merluccius capensis contributed with 20 % and the Beguela hake M. polli with 80 % of the catches.

Table 5 shows the swept area estimates of mean densities by species and depth strata based on the 19 successful bottom trawl hauls made. The large-eye dentex has the highest density at all depths above 50 m. For most species the densities are highest in deeper waters. One should note that these calculations assume a value of the catchability quotient equal to 1.

Table 5. Cunene to Tombua. Swept area estimates of fish densities by species and depth ranges. Tonnes/nm².

Table 6 shows the estimates of aereal extention of the depth strata and the resulting biomasses for all demersal species (excluding sharks), 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. All demersals are estimated at 66 400 tonnes compared to 26 800 tonnes for the same area in February. The croakers are also more abundant this time, 4 370 t against 1 570 t last time.

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

Shelf area	Total 1 670 nm2	0 - 50 m 420 nm2	50 - 100 m 620 nm2	100 - 200 m 630 nm2
Dentex mac.	35 400	400	15 000	20 000
Croakers	4 370	70	300	4 000
All demers.	66 400	1 400	27 000	38 000

Tombua - Benguela

Table 7 shows the catches of the main groups from Tombua to Namibe (St. no. 188-221) and from Namibe to Benguela. The demersals dominate in the catches. Some 60 % of the demersals were seabreams and 10 % croakers, the croakers being most aboundent in the Tombua-Namibe area. One extraordinary large catch(St. No. 221) consisted of Dentex macropthalmus mainly. Canary drum dominated the croakers. The thinlip split-fin Synagrops microlepis is dominating in the deeper water of the northern part. Most of the pelagic catch was horse mackerel and of the Cunene type only. The size composition of the horse mackerel has two modes, one at 14 cm another at 32 cm.

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

ST. NO.	DEP.	Pelagic	Demersal	Sharks	Cephalopod	Other
188	33		299.10			9.78
189	81		602.40			94.20
190	107		504.40		2.60	76.60
221	97	388.74	5555.40			56.52
222	102	3.00	204.40	20.00	3.00	135.00
223	115		210.96			104.40
224	114	7.20	248.40			94.80
225	96	455.00	1461.60		14.40	124.40
226	48	655.00	462.50			60.00
227	73	93.60	419.40		3.60	100.98
228	101		2672.00			84.00
229	95	40.00	1376.00			155.00
MEAN		136.88	1168.05	1.67	1.97	91.31

ST. NO.	DEP.	Seabreams	Crokers	Grunts	Hakes	Splitfin	Other
180	33	207.20	77.00	7.70			16.98
189	81	446.40	132.00	24.00			94.20
190	107	265.20	239.20				79.20
221	97	5194.95	360.45				445.26
222	102	141.40	45.00			18.00	161.00
223	115	174.96	36.00				104.40
224	114	174.00	74.40				102.00
225	96	133.00	26.60			1281.00	614.80
226	48	27.50	15.00	420.00			715.00
227	73	21.60	25.20	12.60		360.00	198.18
228	101	352.00				2320.00	84.00
229	95	110.00	8.00		4.00	1204.00	245.00
MEAN		604.02	86.57	38.69	0.33	431.92	238.34

The shelf between Tombua and Namibe is partly inaccessible for bottom trawling and few successful bottom hauls were made. Based on three hauls the stock biomass of demersal species was estimated at an order of magnitude of 30 000 t, mostly seabreams. In the Namibe-Benguela part 7 successful hauls were made at depth 50-200 m. Based on these the biomass of demersal fish was estimated at 6 000 tonnes. The thinlip splitfin contributed to the stock by some 60 %, the seabreams by 20 %.

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.

BAIA DOS TIGRES 25.4 1989

Figure 3. Distribution of pelagic type 1, pelagic fish type 2 and demersal fish, Cunene to Tombua and Tombua to Benguela