The programme is based on an agreement between the Government of People's Republic of Angola the Food and Agriculture Organization of the United Nations and the Norwegian Agency for Development Cooperation. The Fisheries Research Centre, (CIP) of the Fisheries Ministry of Angola and the Institute of Marine Research, Bergen jointly defined the objectives of the programme and developed the main plans for the surveys. Representatives of CIP participated in all surveys.
The objectives were to investigate the marine fish resources of Angola with emphasis on surveys using acoustic integration technique to study the distribution, composition and abundance of the stocks of small pelagic schooling fish. Further objectives were to investigate the stocks of demersal fish on the northern shelf by bottom trawling using the swept area method. A similar programme for the deep sea shrimp - and hake resource on the slope was included after the second survey. A programme for the monitoring of the main hydrographical features of the sea off the Angolan coast was defined with 5–6 main profiles and a more detailed coverage around the Congo estuary.
The programme was originally to include four complete coverages of the shelf and the slope from the Cunene River in the south to Cabinda in the north distributed during the year 1985 to cover the different seasons. An extension was later made to allow two further coverages during the first half of 1986. The objectives for this extension were to study possible inter-annual variations of the distribution and abundance of the stocks of small pelagics and to intensify the deep water trawling programme in order to obtain a better data base for the assessments of the hake and shrimp resources. An additional objective if time permitted was to explore offshore parts of the Bengula Current system for the occurence of horse mackerel, a possibility suggested by the distributional characteristics of a closely related species off the Peruvian coast.
Cruise reports with a description of the work conducted and outlining some main findings were issued after each of the 1985 srveys and distributed in accordance with an agreed plan. A brief interime progress report was submitted to the Angolan authorities upon request in January 1986.
The present report contains an analysis of all main findings and is prepared for the seminar on the Fish Resources of Angola, to take place in Luanda April 1987. It is, however, recognized that the great amount of observations and data collected during the six extensive surveys will allow and indeed call for more detailed analyses of the biology and distribution of the commercial fish species of Angola than has been possible in this report. To facilitate such further work a special data file report has been compiled and issued in a limited number of copies for the CIP. (For contents see Annex 4.)
The survey programme covered a period of 1 1/2 year from January 1985 until June 1986. The details of the six coverages are set out in Table 1. During 1985 the surveys were extended northwards to include also the shelves of Congo and Gabon. The programme during January-June 1986 was restricted to Angolan waters and the work could thus be more detailed and the programme of experimental fishing could be expanded, particularly that for the deep water hake and shrimp resources.
All surveys covered the shelf area from a depth of about 10 fathoms along the shore out to the shelf edge at about 200m depth. Also the slope down to 500m depth was covered extensively and the hydrographic sections including acoustic recording ranged a further 20–30nm into the ocean. During survey 5 a few exploratory tracks were made, extending up to 240 nm offshore in search for possible resources of horse mackerel in these oceanic parts of the Benguela Current, but only squid was observed. Except for some concentrations during Survey 4 of medium sized horse mackerel just outside the shelf off Bahia dos Tigres, very little pelagic fish was observed over depths of more than about 200m, and the main concentrations were located over the shelf inside this depth. One can not, however, altogether exclude the possibility that possible offshore resources may have been missed by the programme.
Table 1. Details of investigational efforts in the six survey.
| SURVEY | REGION | DATES | DAYS | DISTANCE TRAVELLED IN SURVEY AREA | SURVEY INTENSITY nm /100nm | NUMBER OF FISHING STATIONS | NUMBER OF HYDROGRAPHIC STATIONS |
|---|---|---|---|---|---|---|---|
| I | 28.1-26.2/85 | ||||||
| Cunene-Porto Alexandre | 1.2–4.2 | 4 | 620 | 46 | 16 | 5 | |
| Porto Alexandre-Benguela | 5.2 | 1 | 320 | 29 | 6 | 0 | |
| Benguela-Luanda | 6.2–14.2+17.2 | 10 | 1631 | 25 | 50 | 16 | |
| Luanda-Cabinda | 16.2–25.2 | 8 | 1339(3910) | 16 | 45 (117) | 6 (27) | |
| II | 23.4–28.5/85 | ||||||
| Cunene-Porto Alexandre | 25.4–29.4 | 5 | 583 | 43 | 12 | 5 | |
| Porto Alexandre-Benguela | 30.4-1.5 | 2 | 344 | 31 | 9 | 0 | |
| Benguela-Luanda | 2.5–12.5 | 11 | 1458 | 22 | 57 | 6 | |
| Luanda-Cabinda | 18.5–27.5 | 10 | 1993 (4378) | 23 | 53 (131) | 19 (30) | |
| III | 8.8–10.9/85 | ||||||
| Cunene-Porto Alexandre | 10.8–15.8 | 6 | 841 | 62 | 29 | 5 | |
| Porto Alexandre-Benguela | 16.8 | 1 | 308 | 27 | 4 | 0 | |
| Benguela-Luanda | 17.8–24.8 | 8 | 1201 | 18 | 40 | 10 | |
| Luanda-Cabinda | 25.8–28.8 | ||||||
| +1.9 +9.9 | 13 | 2494(4844) | 29 | 81 (154) | 16 (31) | ||
| IV | 5.11–5.12/85 | ||||||
| Cunene-Porto Alexandre | 10.8–15.8 | 7 | 1017 | 75 | 23 | 5 | |
| Porto Alexandre-Benguela | 14.11–15.11 | 2 | 537 | 48 | 9 | 0 | |
| Benguela-Luanda | 16.11–23.11 | 8 | 1292 | 20 | 43 | 10 | |
| Luanda-Cabinda | 24.11–26.11 | ||||||
| +28.11-5.12 | 11 | 1654(4500) | 19 | 56 (131) | 12 (27) | ||
| V | 22.1-10.3/86 | ||||||
| Cunene-Porto Alexandre | 23.1–29.1 | 7 | 1480 | 83 | 52 | 6 | |
| Porto Alexandre-Benguela | 1.2 | 1 | 393 | 24 | 6 | 0 | |
| Benguela-Luanda | 2.2–14.2 | 12 | 1580 | 24 | 60 | 29 | |
| Luanda-Cabinda | 15.2-4.3 | ||||||
| +5.3–11.3 | 24 | 3419(6872) | 40 | 126 (244) | 11 (46) | ||
| VI | 22.4-5.6/86 | ||||||
| Cunene-Porto Alexandre | 24.4–29.4 | 5 | 905 | 51 | 22 | 6 | |
| Porto Alexandre-Benguela | 30.4-1.5 | 2 | 443 | 28 | 8 | 18 | |
| Benguela-Luanda | 2.5–11.5 | 9 | 1334 | 21 | 55 | 11 | |
| Luanda-Cabinda | 11.5–14.5 | ||||||
| 19.5-5.6 | 19 | 3637(6319) | 42 | 130 (215) | 19 (54) | ||
| Total | 30823 | 992 | 215 | ||||
The density of the course tracks are indicated in Table 1 as the ratio between nautical miles steamed and 100nm2 covered. A ratio of 30 corresponds approximately to a spacing of course tracks 10 nm apart. The adequacy of the survey density is difficult to access since it will depend on the characteristics of the fish distribution. The densities used in the Angola survey are, however, generally high compared with other surveys of similar resources.
Surveys 1 through 4 were intended to provide data for a description of seasonal changes in the systems. Surveys 5 and 6 are in the seasonal sense repetitions of surveys 1 and 2 and may provide information on changes between years. All surveys started in the south and the time interval between the six surveys of each of the four regions is thus about the same.
As it became evident during the work that there would be time available for a more extensive programme of experimental fishing, this was expanded both for the demersal fish on the shelf and for the deep water resources of hake and shrimp along the slope.
The total survey effort of the programme is demonstrated by the following statistics:
| Days at sea: | 186 |
| Distance steamed: | 30.800 n.m. |
| Number of fishing st.: | 992 |
| Number of hydrographic st.: | 215 |
The layout of the course tracks and the positions of fishing-and hydrographic stations are shown in maps in the special data file report.
The main survey effort was spent in investigating the pelagic schooling fish using acoustic integration technique combined with fishing with bottom and mid water trawls for identification and sampling. Fish near the surface and close to the bottom are not properly observed by this system.
Horizontal ranging sonars were used to observe surface schooling fish, but such observations are not easily quantified in terms of measures of biomass. To include bottom dwelling fish and shrimp a programme of prelocated trawl stations were worked to provide data for swept are measurements.
The reliability of the acoustic technique in providing estimates of biomass is under continuous review by the scientific community. In Angolan waters its limitations would particularly relate to the behaviour of the sardinella species which at times were found to form surface schools out of reach of the echo sounders and which tended to avoid the vessel when occurring in shallow waters. A further problem is the limited information available on the acoustic target strength of the species surveyed. Some efforts were made to acquire direct observations of this during the survey, but the results are not considered reliable, and for the biomass estimates data on target strength of herring in European waters have been made use of throughout. Another limitation is found in the incomplete coverage of inshore waters by the survey, since the parts of the shelf with depths less than about 10 fathoms could not be navigated by the vessel.
The overall effect of these various limitations are assessed to lead to an underestimation of the biomasses.
When allocating the integrator readings on different types of resources use is made of the following categories:
| Pelagic fish type 1, clupeids and engraulids; | |
| Pelagic fish type 2, carangids, smaller scombroids, | |
| barracudas, trichurids; | |
| Demersal fish, sparids, sciaenids etc. |
The allocation is based on the catch compositions and on general knowledge of the schooling behaviour of the various species. One should, however, note that this allocation is approximate and that the total biomass estimates are thus more reliable than those of the subgroups.
As much time is required for a full swept area trawl survey, the method was at first limited to some selected parts of the shelf. The swept area method involves a special problem of estimation viz. what value to assign to the catchability quotient, q. We have in this case used 1, which implies that all organisms in the path of the trawl are caugth and contained in the catch. This is likely to be true for certain types of shrimp, but for fish the q no doubt will take on values that are often lower than 1, with fish escaping through meshes or under or over the trawl opening. The assumption q=1 thus probably gives underestimates of biomass.
All catches were sampled for species and sizes so that a total composition by weight and usually also number could be estimated.
The sea surface temperature was observed continuously and the hydrographic profiles were worked with Nansen bottles casts, with salinity and oxygen analysed onboard.
A record of the acoustic instruments used and their calibration is presented in Appendix 1 together with a description of the fishing gears.
