6.1 ACOUSTIC SURVEYS
6.2 OTHER STUDIES
Three research vessels have carried out acoustic surveys in the area, the R/V GOA (Angola), the R/V ANDRE NIZERY (Congo), and the R/V FIOLENT (CECAF).
R/V GOA carried out five coverages of the Angolan area in 1972 and 1973. The results of only one coverage have been published so far, giving maps of the relative density of all pelagic species together.
R/V FIOLENT covered the whole area of interest for the southern stock of sardinella, using an echo integrator (FAO, 1977). This survey was carried out in 1976.
It appeared that the species mix in the area, and the difficulty to catch small pelagic species such as sardinella, with the gear used during the survey, made it impossible to estimate the distribution and biomass by species. Also, a zone of 25 miles wide along the Angola coast could not be covered. The results for the remaining area have been given for all pelagic species together.
Neither the results from R/V GOA nor those from R/V FIOLENT can therefore be used in the present studies.
The results of the R/V ANDRE NIZERY have been reported in internal reports of the institute in Pointe Noire. Ghéno and Campos Rosado (1972) have mentioned these results from which M'Fina gave the following summary to the Ad hoc working group.
The most important concentrations of sardinella are found:
(a) between Cap Lopez and 3°S; this is the area where bait is fished for the tuna fishery;During the short warm season the fish is dispersed in very small shoals which show as small dots on the echosounder and which can be caught with the purse seine even though they cannot be seen at the surface.(b) south of the Congo River, from Cabeça da Cobra as far as Luanda and sometimes further south, the whole year round;
(c) between the mouth of the Congo River and Mayunba, especially during the cold season.
During the long cold season, the region around Pointe Noire is invaded by large concentrations of S. aurita of large size, which give only faint echo traces bearing no relation to the excellent catch rates obtained by the local sardinella fishermen.
6.2.1 Angola
6.2.2 Congo
6.2.3 Comparison with West Africa
The available information does not allow a precise description of the distribution and migration of the two sardinella species in the Congo-Angola area. Some hypotheses on the migration have been mentioned in the literature. These hypotheses and the evidence available to the Ad hoc working group in this respect, can be summarized as follows:
According to scientists who worked in the area, sardinella catches in Angola seem to take place mainly in the north during the cold season and in the south during the warm season. Therefore, there seems to be a movement of sardinellas along the Angola coast towards the north at the beginning of the cold season, and southward when the warm season begins (Baptista, 1977; also Troadec, 1964; Ghéno and Poinsard, 1969).
The catch data available to the Ad hoc working group for both species, by month and division (Table 2), do not seem to support this hypothesis, because most of the catch, both in Division 1.1 (northern Angola) and Division 1.2 (central Angola), is made from October till May and this is the warm season. Unfortunately, the catch data are not separated by species and only very few length frequency data are available from which to determine variation in the length compositions in each area.
Data on catch, catch per unit of effort, and length frequency of catches for each species in the Congo area have led the scientists working in that area to the following conclusions;
(a) S. auritaThe catch per unit of effort of adult fish of this species (more than three years old) increases spectacularly during the “long cold season”, and also during the “short cold season”, and decreases drastically during the “long” and “short warm season”. These facts have led to the hypothesis that adults arrive in the Point Noire area at the beginning of the cold seasons and disappear at the beginning of the warm seasons (Ghéno and Campos Rosado, 1972; Bouchereau, 1976). A review of the length composition of the catches (Annex 3) seems to confirm this pattern, in particular the data for the years since 1970.
Unfortunately, the Ad hoc working group did not have enough time at its disposal to further analyse the catch rates by size class or age class, and the data in the available literature are not sufficient to check the conclusions. It should however be borne in mind that changes in catch rates may be due to migration, but also to changes in vulnerability, e.g., by the schooling behaviour. Because the cold season is also the spawning season, changes in the vulnerability of the fish are likely.
Baptista (1976) stated that the migration of sardinella starts offshore from north to south at the end of the cold season or beginning of the hot season and appears off central and southern Angola during September or October. After February, they start moving north but this time in inshore waters. This could explain the seasonal variation in catches in the area.
Young specimens of this species do not seem to show a clear pattern in the seasonal catches, which may mean that they remain in the nursery areas for some time. Drastic changes in catch rates have been observed from season to season, but the period of larger catches varies from year to year and from area to area, and is not so clearly bound to a special season (Ghéno and Campos Rosado, 1972; see also Appendix 3 of this report).
The few available data on length compositions in Angola show that adult concentrations exist in that area but are not sufficient to determine the seasonal and annual fluctuations.
(b) S. maderensis
There is no evidence of large migratory movements in either the young or the old fish of this species. Angolan scientists mention that both S. aurita and S. maderensis show a north-south migration, but this may have been based mainly on the observations on S. aurita, in particular when it is taken into account that the Angolan catch statistics do not distinguish between the two species.
Comparison with the occurrences in West Africa, of which the Congo-Angola stocks could well present a mirror image, would support the hypothesis of substantial migrations along the coast of the adult S. aurita, and less extended migration of S. maderensis.
In view of the statements made in the literature by various authors on the seasonal variation in the catches of small and large fish of each of the two species, at various places along the Congolese and Angolan coasts, and the conclusions drawn therefrom on seasonal north-south migrations, the Ad hoc working group believes that more evidence exists on this matter than the group has been able to review in the time available during the meeting. It would be most useful if a complete review of all the relevant evidence could be prepared by a scientist who has access to all available data.
