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.

The fish fauna on the shelf north of Benguela belongs to the Angola Current regime and is more diversified than that further south. A partition in an inner shelf down to 70 ml depth and an outer shelf 70 - 120 m shows assemblages of demersal fish dominated by grunts on the inner- and seabreams on the outer shelf. In addition comes the slope with the hake and the deep sea shrimps.

Benguela-Palmeirinhas

Table 1 shows the catch rates standardized to kg/hour for the main groups of resources on the two parts of the shelf and the slope. Both for the inner- and the outer shelf the highest catch rates were obtained up to about stations 70 which in a very general way indicates that the highest availability of fish was found from Benguela to Pta do Morro. The pelagic and demersal groups will be analysed further below. For Cephalopods the highest catch rates were obtained on the outer shelf and down to about 150 m on the slope. 70% of the catch was shortfin squid Illex coindetii of 12-13 cm mantle length, see ANNEX I and about 20% small sized African squid Alloteuthis africana. Catch rates up to 37 kg/hour were obtained for deepwater rose shrimp Parapenaeus longirostris and up to 17 kg/hour for the striped red shrimp Aristeus varidens. The spider shrimp Nematocarcinus africanus caused the high rate of station 85.

Table 2 shows the catch rates of the pelagic species by families from the two parts of the shelf. 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 were hardly caught by bottom trawl. 70 % of the Carangid catches from the inner shelf consisted of lookdown, Selene dorsalis, for size compositions see ANNEX I. The bumper represented 20% of the catches and the remainder was mainly horse mackerel and false scad. Nearly 100% of the Carangid catches from the outer shelf was Cunene horse mackerel dominated by small sized fish as discussed above. The hairtails, most common on the inner shelf were generally of large size, in some hauls unusually large. About 60% of the barracudas were the guachanche species Sphyraena guachancho with S. spyraena making out the rest, see size compositions ANNEX I. Again the highest catch rates were made on the inner shelf probably as for the hairtails an effect of their role as predators on the aggregations of smaller fish there.

Table 1. Benguela to Pta das Palmeirinhas. Catch rates by main groups in bottom trawl hauls, standardized to kg/hour.

INNER SHELF

ST. NO.	DEP.	Pelagic	Demersal	Sharks	Cephalopod	Other
37	39	11.8	11.3			61.0
38	50	217.1	718.9			18.1
40	44	692.7	495.2		2.2	39.8
46	53	451.2	238.8			58.4
47	41	23.7	164.6		6.0	14.0
53	70	187.8	68.6		12.6	8.5
54	20	2.0	9.6			273.0
55	63	106.5	1.0		5.7	18.5
62	28	13.0	21.5	5.2		9.0
63	29	161.8	372.4			44.8
64	30	1587.6	1080.0			486.0
65	18	6.4	60.0			4378.0
69	35	1987.8	700.6	5.0		90.0
71	25	50.3	57.5		0.0	5.9
72	42	387.1				9.4
77	28	8.2	18.7			38.0
78	48	11.1	6.4		0.3	29.2
81	65	41.3	3.9		2.0	3.5
82	18	56.2	52.0			6.8
83	36	112.8	928.2	2.4		10.3
88	34	27.4			0.1	4.4
89	16	80.1	31.2			8.0
90	45	92.2	16.2		0.0	4.0
98	16		8.6
99	61	242.0	1684.3			3.4
105	53	63.4	5.5		0.4	3.8
MEAN		254.7	259.8	0.4	1.1	216.3

OUTER SHELF

ST. NO.	DEP.	Pelagic	Demersal	Sharks	Cephalopod	Other
39	105		555.2			10.0
41	94	189.0	690.8			24.6
42	103	30.6	476.7			43.3
44	97	77.0	49.9		4.3	12.0
45	82	292.4	434.6		11.2	7.8
48	77	142.2	12.9		2.0	6.0
52	100	199.5	106.1		4.2	8.0
56	84	471.7	512.0		21.0	5.9
58	117	74.0	37.7		1.1	57.9
60	111	39.0	108.2		6.6	5.2
61	90	356.8	128.4			20.4
66	113	45.9	56.2		0.7	203.8
68	120	8.4	15.4		11.0	181.9
70	77	26.0	14.8		0.1	6.0
73	84	58.6	273.6			1.9
79	84	12.4	148.8		49.8	1.5
80	101	52.9	60.6	4.4	5.2	2.3
86	110	34.6	2.3		5.4	12.9
87	85	145.2	123.8		3.4	2.2
91	86	69.4	32.4		28.0	2.0
92	108	269.6	134.6	18.0	24.0	6.8
93	78	102.0	540.6	7.4	22.2	41.8
96	121	0.1	27.8		38.3	1.6
97	75	32.4	32.6		3.3	9.1
100	103	389.4	15.5		15.2	59.5
104	86	52.4	20.5		25.5	2.6
MEAN		122.0	177.3	1.1	10.8	28.3

Table 2. Benguela to Pta das Palmeirinhas. Catch rates of main pelagic families in bottom trawl hauls, standardized to kg/hour.

INNER SHELF

ST. NO.	DEP.	Clupeids	Carangids	Hairtails	Barracudas	Scombrids	Other
37	39		5.3	5.4	1.0		72.3
38	50		66.4	148.5			739.2
40	44		24.0	666.8			539.1
46	53		2.4	446.4			299.6
47	41		6.0	10.5	6.5		185.3
53	70		120.8	67.0			89.7
54	20		2.0				282.6
55	63		16.2	59.0	25.6		30.9
62	28		13.0				35.7
63	29		20.0		138.8	3.0	417.2
64	30		513.0		804.6		1836.0
65	18		6.4				4438.0
69	35	2.0	1180.0	804.0	1.8		795.6
71	25	0.3	35.7	9.3	4.9		63.6
72	42	7.0	147.7	232.4			9.4
77	28	0.0	1.2		6.0	0.9	56.7
78	48		5.9	1.4	3.8		35.9
81	65	1.7	11.5	27.2	0.9		9.5
82	18	0.5	44.2		6.9	4.6	58.8
83	36	0.6	43.2	31.2	24.6		954.1
88	34		17.6	0.0	8.2	1.4	4.7
89	16	0.1	68.5		1.5	10.0	39.2
90	45		50.7	20.3	17.8	3.4	20.2
98	16						8.6
99	61		61.8	180.2			1687.7
105	53	2.8	56.4	4.2			9.7
MEAN		0.5	96.9	104.3	40.5	0.9	489.2

OUTER SHELF

ST. NO.	DEP.	Clupeids	Carangids	Hairtails	Barracudas	Scombrids	Other
39	105						565.2
41	94		176.4	10.0		2.6	715.4
42	103		18.0	12.6			520.0
44	97		72.9			4.0	66.2
45	82		144.4	144.6		3.4	453.6
48	77			140.4	1.8		20.9
52	100		199.5				118.3
56	84		471.7				539.0
58	117		74.0				96.7
60	111		39.0				120.0
61	90		313.6	43.2			148.8
66	113		45.6	0.3			260.8
68	120		1.4	7.0			208.3
70	77		1.2	24.8			20.9
73	84	0.2	38.0	20.4			275.5
79	84		6.8			5.6	200.1
80	101	0.1	52.4			0.4	72.6
86	110		22.0			12.6	20.6
87	85	0.8	142.4			2.0	129.4
91	86	0.0	69.4				62.4
92	108		263.4			6.2	183.4
93	78		102.0				612.0
96	121		0.1				67.7
97	75			30.4	0.8	1.2	45.0
100	103		35.8	353.6			90.2
104	86	0.8	49.8	1.8			48.6
MEAN		0.0	90.0	30.3	0.1	1.4	217.7

Table 3 shows the catch rates of demersal fish in the bottom trawl by families. The grunts dominate the catches from the inner shelf and the seabreams those from the outer. 82% of the total grant catches consisted of the bigeye grant Brachydeuterus auritus, a small sized species of doubtful commercial value. The larger sized and more interesting sompat grant Pomadasys jubelini gave some occasional good catch rates in inshore waters and counted in total for 13% of the grant catch. Among the seabreams the large eye dentex Dentex macrophthalmus was common on the outer shelf and gave good catch rates between Benguela and Cabeca da Baleia, but was then almost absent from the catches up to Palmeirinhas where it reappeared in some of the catches in the slope at 140 and 270 m of depth. This species accounted for about 60% of the sparid catches. Also the other sparids of commercial interest were found on the outer shelf. The most common, the red pandora Pagellus bellottii contributed 20% and the Angola dentex Dentex angolensis, the pink dentex Dentex gibbosus and the southern common seabream Sparus pagrus represented together some 14%. The pooled size compositions of these sparid species are shown in ANNEX I.

Croakers and groupers made more occasional contributions to the catches. The cassava croaker Pseudotolithus senegalensis was only caught close inshore and represented 40% of the croaker catches while the canary dram Umbrina canariensis caught on the outer shelf provided 50%. The smaller grouper catches came both from the inner- and the outer shelf and consisted almost only of white grouper Epinephelus aenus.

Hake Merluccius polli was caught in the slope between 150 and 450 m with the highest catch rate 440 kg/hour at 300 m. The pooled size composition of the samples is shown in ANNEX I.

Table 4 shows the swept area estimates of mean densities by species and depth strata, based on 60 successful random bottom trawl hauls made. The bigeye grants dominates both the 0-50 and 50-100 m bottom depth zone followed by Pomadasys jubelini and Pomadasys incisus in the shallow waters and by Dentex marcrophthalmus in the 50-100 m zone. The deeper waters are dominated by cardinal fish Synagrops microlepis. The catchability coefficient applied in the estimation is 1.0.

Table 3. Benguela to Pta das Palmeirinhas. Catch rates of main demersal families in bottom trawl hauls, standardized to kg/hour.

INNER SHELF

ST. NO.	DEP.	Seabreams	Grunts	Croakers	Groupers	Hake	Other
37	39	9.9	1.3				72.8
38	50	34.9	670.4	13.5			235.2
40	44	70.4	419.2	3.0	2.6		734.7
46	53	20.0	218.8				509.6
47	41	28.6	100.0		36.0		43.7
53	70	0.8	67.8				208.9
54	20		9.6				275.0
55	63	1.0					130.8
62	28	3.8	17.0		0.7		27.2
63	29		354.4	18.0			206.6
64	30	27.0	853.2	199.8			2073.6
65	18	60.0					4384.4
69	35		668.8	31.8			2082.8
71	25		57.5				56.4
72	42						396.5
77	28		18.7				46.2
78	48	2.8	3.6				40.6
81	65	3.7	0.2				46.9
82	18		52.0				63.0
83	36	14.0	906.0	8.2			125.5
88	34						32.0
89	16		31.2				88.1
90	45	4.8	11.4				96.2
98	16				8.6
99	61		1670.4		13.9		245.4
105	53	5.5					67.6
MEAN		11.0	235.8	10.5	2.3		472.7

OUTER SHELF

ST. NO.	DEP.	Seabreams	Grunts	Croakers	Groupers	Hake	Other
39	105	435.2		120.0			10.0
41	94	662.8	1.6	26.4			213.6
42	103	436.2		37.5	3.0		73.9
44	97	49.9					93.3
45	82	371.6	59.2	2.4	1.4		311.4
48	77	5.0	4.1	3.8			150.2
52	100	105.6		0.5			211.7
56	84	494.3	11.3	6.4			498.6
58	117	33.1		3.4	1.2		133.0
60	111	103.6			4.6		50.8
61	90	16.4	112.0				377.2
66	113	16.8		38.2		1.2	250.5
68	120	15.4					201.3
70	77	0.4	14.4				32.1
73	84	5.0	267.6	1.0			60.5
79	84	10.2	138.0	0.6			63.7
80	101	0.6	60.0				64.9
86	110	2.3					52.9
87	85	31.4	53.4	2.4	36.6		150.8
91	86	28.4	4.0				99.4
92	108	79.6		0.9	54.0		318.4
93	78	441.0	99.6				173.4
96	121	8.6	19.2				40.0
97	75	20.8		1.2	10.6		44.8
100	103	3.5	12.0				464.2
104	86	20.5					80.6
MEAN		130.7	32.9	9.4	4.2	0.0	162.3

SLOPE (200-800 m)

ST. NO.	DEP.	Hake	Seabreams	Shrimp	Sharks	Cephalopod	Other
49	294	201.4	6.6	9.5	5.7	2.8	410.4
50	292	443.0	1.3	2.3	5.1	4.1	454.9
51	441	68.5		21.0	0.7		46.8
57	274	13.2		37.6	6.6		574.8
59	556			9.9		0.2	41.6
67	447	4.8		21.6	0.8		69.6
85	453	13.0		140.2			30.4
95	153	25.2	4.6	26.4		142.2	212.0
101	143		17.7	3.2		19.5	1980.9
103	269	9.3	219.9	6.2		3.1	965.6
MEAN		77.8	25.0	27.8	1.9	17.2	478.7

Table 4. Benguela to Pta Palmeirinhas. Swept area estimates of demersal fish densities by species and depth ranges.

Table 5 shows the estimated area of the shelf and the resulting biomasses for all demersal species and separately for the dominating species in each zone.

Table 5. Benguela-Palmeirinhas. Biomass estimates of groups of demersal fish by depth strata. Tonnes.

	Total	0-50m	50-100m	100-200m
Shelf area nm	4700	1750	1680	1280
Brachydeuterus	18000	8000	10000
D. macrophthalmus	6500		3500	3000
Pom. Jubelini	2900	2400	500
Pom. Incisus	6000	6000
Pagellus bellottii			2300
All demersal	61000	22000	19000	20000

From the Dr. Fridtjof Nansen surveys in 1985-86 the estimated demersal biomass varied between 50 and 100 thousand tonnes with a mean value from six surveys of 70 thousand tonnes.

In the 200-600 m bottom depth zone only 8 hauls were carried out. Dominating species were cardinal fish (Synagrops microlepis), greeneyes (Chloropthalmus atlanticus) and hake. The areal extension of this zone has been estimated to 1300 nm, and with the limitations of the few number of hauls in mind (n=8) the following biomass figures are indicated:

Cardinal fish	4300	tonnes
Greeneyes	7500	tonnes
Hake	4400	tonnes

Palmeirinhas-Cabinda

Table 6 shows the catch rates by main groups for the successful hauls in the region, and divided by inner shelf, outer shelf and the slope, as above. The mean catch rate in the shallow waters of the pelagic species are reduced to less than one third of the level of the Benguela - Palmeirinhas region, indicating the considerable lower density level of the group. Likewise the catch rate of the demersal group is reduced to about half of the level further south. On the outer shelf, however, the mean catch rates are in close agreement with the results from the southern region. The pelagic species catch broken down to families, Table 7, show that the main pelagic family in the shallow waters is the clupeids (43%), followed by equal shares of carangids and hairtails (27 %). The clupeids are mainly Ilisha africana and some Sardinella maderensis, while the carangids are lookdown, bumpers and some juvenile horsemackerel. On the outer shelf the main pelagic family is the horsemackerel, almost exclusively one cohort of about 15 cm modal length. See ANNEX I on length frequency distributions. The catch rates of the clupeids on the outer shelf are negligible.

The catch rates of the main demersal families are shown in Table 8. The dominating group in the shallow waters is the grunts, mainly the bigeye grunt, Brachydeuterus auritus. Seabreams and croakers hold about equal shares in the total catch of demersal fish (25 and 27%). On the outer shelf the seabreams is the dominating group (56%), followed by grunts and croakers (23 and 16%).

Table 6. Pta das Palmeirinhas to Cabinda. Catch rates by main groups in bottom trawl hauls, standardized to kg/hour.

INNER SHELF

ST. NO.	DEP.	Pelagic	Demersal	Sharks	Squid	Shrimp	Other
114	64	214.2	260.9		45.0		63.8
115	34	22.0	98.1		11.0		15.6
120	17	228.3	114.0			3.0	60.7
121	34	144.0	106.5			1.8	37.8
125	56	12.8	54.8		0.2		7.0
127	65	8.9	16.7		9.0		8.0
128	64	18.1	51.9	9.8	3.8		12.2
129	14	11.4	1.2		2.0		7.8
136	13	141.8	70.1	1.6	1.0		34.0
137	25		39.3		1.8		15.0
138	41	1.0	1.0		3.2		3.2
142	64	48.0	199.8			0.0	1.4
143	44	33.6	441.8	5.0		7.4	19.2
148	9	294.9	316.2	2.1		84.0	94.8
149	44	2.6	137.2		0.6	0.5	3.8
156	11	247.4	140.4	7.6		1.3	79.4
157	16	93.7	73.0	6.0		1.4	23.6
158	34	2.9	42.8				2.7
159	30	5.6	172.2	6.2			6.8
160	45	12.2	173.6	5.6	6.6		5.2
171	55	1.0	41.6	8.7	3.2		0.2
MEAN		73.5	121.5	2.5	4.1	4.7	23.9

OUTER SHELF

ST. NO.	DEP.	Pelagic	Demersal	Sharks	Squid	Shrimp	Other
119	75	166.3	873.2			1.8	1.4
122	104	54.0	19.0		5.8	0.4	1007.4
130	75	17.6	706.0	2.2	0.5		23.5
131	103	1.5	44.8		7.2		5.0
132	125	0.4	39.5		3.0		7.6
134	73	16.2	543.4				15.4
135	87	1400.0	92.4				47.6
139	87	224.0	185.6				20.8
140	110	10.8	82.8		0.2		12.6
141	132	8.4	30.2	5.2	17.5		167.6
144	80	42.0	68.7		0.3		12.3
146	181	21.8	49.5	6.0	5.2	3.1	132.3
147	80	183.7	118.6	6.2	2.5	1.8	31.1
150	130	103.8	13.2		2.4	0.2	9.0
152	97	30.6	9.0	3.0	4.0	0.1	36.1
161	94	6.0	24.0	3.2	0.2		28.8
162	91	32.2	20.3	3.8	2.4		15.1
163	110	17.4	83.6	19.6	1.2		15.0
164	79	6.0	321.4		1.2		6.0
165	99		28.3		0.3		4.5
166	70	10.5	43.2		11.4		3.5
167	90	667.2	101.1				108.0
168	79	30.3	112.1	1.6	1.5		9.5
169	100	183.6	35.9	3.0	2.6		4.4
170	72	3.5	274.0				16.0
MEAN		129.5	156.8	2.1	2.7	0.3	69.6

Table 7. Pta das Palmeirinhas to Cabinda. Catch rates by main pelgic families in bottom trawl hauls, standardized to kg/hour.

INNER SHELF

ST. NO.	DEP.	Clupeids	Anchovies	Carangids	Hairtails	Scombrids	Other
114	64			5.4	208.8	1.5	368.2
115	34		1.6	7.0		3.6	134.6
120	17	135.3		58.2	22.8	1.3	188.4
121	34	85.2		24.4	28.0		152.5
125	56				1.7	5.5	67.6
127	65	0.1		0.5	8.0		34.1
128	64			7.7	9.0		79.2
129	14					6.0	16.4
136	13	0.3		106.5	3.6	13.0	125.1
137	25						56.1
138	41				1.0		7.4
142	64	0.2		46.8	1.0		201.2
143	44	3.2		14.4	16.0		473.4
148	9	182.4		40.8	69.6		499.2
149	44			0.2	2.4		142.1
156	11	169.2		42.8	4.8		259.3
157	16	23.1		9.6	2.4	2.8	159.8
158	34	0.2		1.9			46.3
159	30			5.6			185.2
160	45			6.8			196.4
171	55			0.2			54.5
MEAN		28.5	0.08	18.0	18.0	1.6	164.1

OUTER SHELF

ST. NO.	DEP.	Clupeids	Anchovies	Carangids	Hairtails	Scombrids	Other
119	75			58.3	90.0		894.4
122	104			43.2	10.8		1032.6
130	75			17.6			732.2
131	103	0.1		1.4			57.0
132	125			0.4	0.0		50.1
134	73	4.2		5.1			565.7
135	87			1400.0			140.0
139	87			224.0		12.8	193.6
140	110			10.8			95.6
141	132				8.4		220.5
144	80			23.0	18.0		82.3
146	181				21.8		196.1
147	80			183.7			160.2
150	130			103.0	0.8		24.8
152	97			1.8	28.8		52.2
161	94			5.0	0.2		57.1
162	91			2.6	29.6		41.6
163	110			16.0	1.4		119.4
164	79	1.2		4.8			328.6
165	99						33.1
166	70	5.4		3.9	1.2		58.1
167	90			667.3			209.1
168	79	2.6		27.5			124.9
169	100	0.6		183.0			45.9
170	72			3.5			290.0
MEAN		0.5		119.4	8.4	0.5	232.2

Table 8. Pta das Palmeirinhas to Cabinda. Catch rates by main demersal families in bottom trawl hauls, standardized to kg/hour.

INNER SHELF

ST. NO.	DEP.	Seabreams	Grunts	Croakers	Groupers	Hake	Other
114	64	19.8	193.1	46.2	1.8		323.0
115	34	59.4	20.2	1.9	16.6		48.6
120	17		87.6	26.4			292.1
121	34	0.3	58.0	48.2			183.6
125	56	38.0	7.3	1.7	7.7		20.1
127	65	16.1	0.6				26.0
128	64	50.5	1.4				44.0
129	14	1.2					21.2
136	13	1.2	10.5	58.4			178.5
137	25	39.3					16.8
138	41	1.0					7.4
142	64	19.8	128.8	42.8	8.4		49.4
143	44		338.4	103.4			65.2
148	9		24.0	292.2			475.8
149	44	130.0	0.6		6.6		7.5
156	11		84.6	55.8			335.7
157	16		66.8	6.2			124.7
158	34	15.6	24.0		3.2		5.6
159	30	39.6	108.6		24.0		18.6
160	45	158.0	3.6		12.0		29.6
171	55	41.6					13.1
MEAN		30.0	55.1	32.5	3.8		108.9

OUTER SHELF

ST. NO.	DEP.	Seabreams	Grunts	Croakers	Groupers	Hake	Other
119	75	6.0	864.0	3.2			169.5
122	104	8.8		7.2		3.0	1067.6
130	75	351.6	2.6	342.8	9.0		43.8
131	103	24.6		20.2			13.7
132	125	39.5					11.0
134	73	398.5		27.4	117.4		31.7
135	87	92.4					1447.6
139	87	163.2	22.4				244.8
140	110	82.0	0.8				23.6
141	132	30.2					198.7
144	80	17.0	1.8	36.5	13.4		54.6
146	181	4.0		45.5			168.4
147	80	20.2		98.4			225.3
150	130	13.2					115.4
152	97	4.8		4.2			73.8
161	94	4.2		19.8			38.2
162	91	11.3		9.0			53.5
163	110	65.2	18.4				53.2
164	79	311.8	9.6				13.2
165	99	8.8			19.5		4.8
166	70	43.2					25.4
167	90	101.1					775.4
168	79	106.1			6.0		42.9
169	100	25.3			10.6		193.6
170	72	266.0			8.0		19.5
MEAN		87.9	36.7	24.5	7.3	0.1	204.3

LOPE (200-800 m)