An attempt was made by the group to reconstitute the life cycle of the round sardinella Sardinella aurita and to integrate all biological and fishery data on the basis of previous working group statistics and on recent data. For this exercise, the assumption is of one single stock centred in Ghanaian waters but extending to Côte d'Ivoire.
Spawning season
Available data indicate a main spawning season beetween June and September (with a maximum in July-August) and a secondary, less intense and more irregular spawning season in December-January (cf. Figure 14–2, page 23, CECAF/TECH/82/40 (En)).
Beach seine fishery
Catch and yield fluctuations reveal a peak production in October and January (with apparently frequent interruptions in November; Figure 16). Sardinellas caught measure between 2 and 9 cm (fork length), the average length ranging between 6 and 8 cm. Over a year, there is no significant variation in the size of fish caught. This suggests that 3–4 months after the primary spawning season there is a massive entry of new recruits in the coastal beach seine fishery (which corresponds to the month when 3 cm individuals start to be caught). The beach seine fishery has no target species.
Worth noting is the fact that the fish leave the coastal and beach seine fishery area just before (and perhaps in relation to) sexual maturity which starts when the fish reaches 10 cm.
Poli/Ali fishery
The fishery lasts throughout the year with peaks in July-August-September at a time when the target species is the round sardinella. During the other months the fishery concentrates on other species and more particularly on anchovy at the end of the cold season. The Ali net is utilized mainly for the flat sardinella and, at the end of the cold season, for the round sardinella which is then sparsely scattered instead of forming into large schools.
Purse seine fishery
The peak season covers the period July through September. The size of fish caught ranges between 9–10 cm and 25 cm, with a maximum comprised between 12 cm (beginning of effective access to the sardinella fishery) and 25 cm, and a considerable amount of fish of the average 17–20 cm length. At the time of recruitment, all sardinellas measure 16 cm. It is interesting to note that the average LC50 recruitment size is equal to the average sexual maturity size (LM50):
LC50 = LM50 = 15 cm (fork length)
This suggests that in the beach seine fishery the recruitment process is linked with the maturation and reproduction process, and the schooling phenomenon which ensures.
The above information, in Figure 20, gives an apparently coherent picture of the life cycle and of the exploitation strategies. The proposal remains, however, purely theoretical and is intended to serve as a basis for discussions, to be improved further by working groups and national laboratories.
With the succession of the A, B and C size-month blocks corresponding respectively to the spawning season, the main beach seine fishery season and the ring net fishery season, it appears that the growth curve is sharper than the one proposed by Marchal (1974).
| Present study | Marchal | |||
|---|---|---|---|---|
| Date | Age (month) | Length (cm) | Age (month) | Length (cm) |
| July-August | 0 | 0 | - | - |
| Nov-Dec | 4 | 6–8 | 6 | 11 |
| July-August | 12 | 17–20 | 12 | 14.5 |
| 18 | 17.0 | |||
| 24 | 19.0 | |||
These conclusions are provisonal and need to be confirmed using better continued series of monthly frequency distribution.
The difference could be explained by an error of interpretation either by Marchal (1974) or by this working group. Another likely assumption would be that growth as calculated by Marchal is correct for the years concerned (upwelling of lesser amplitude) and that the sardinellas reach sexual maturity when 15 cm in length which is their size at the time of massive recruitment in offshore fisheries; all the above considerations suggesting great coherence in Marchal's growth pattern.
The difference could also reflect currently observed new conditions (large amplitude upwelling) leading to a more important growth (and possibly a more important size at maturity, though this assumption is difficult to verify on the basis of data available to the meeting) and to a larger size (17 cm or more) at the time of massive entry in the fishery.
It should also be stressed that the generative period (from the date of birth of the principal cohort to the date of this cohort's main spawning time) is approximately 12 months which makes it possible to maintain the reproduction potential of the stock in phase with environment.
Another assumption could be the existence of a secondary cohort (second generation) born in December-February. The main generation, whose size at the time of second spawning season seems to reach 12 cm or so, would have very limited participation in the less important spawning season of December-February. This cohort would contribute to the sardinella catches in the 12–15 cm size class found in certain histograms as minute density observations.
The analogy with fluids flowing into a basin and the spatial dynamics of the clupeids in relation to their level of abundance, has been developed by McCall (1983). It is interesting to consider this interpretation in order to analyse the abundance of Sardinella aurita within a time series and in relation to its dynamics (Figure 21).
There is a disparity in the upwelling caused by topological effects of the continental shelf or by currents. For instance, upwelling which is centred on Ghana, extends seasonally eastwards to the Côte d'Ivoire. West of Côte d'Ivoire an intensification of upwelling occurs which is caused by currents. The Ghanaian zone is therefore the most favourable area (biotope) which can be described as a deep valley where in the liquid (biomass) retracts in periods of poor abundance because of climatic conditions. The western part of Côte d'Ivoire can be described as a depression of lesser depth (being biologically less favourable) in which the biomass concentrates when it increases throughout the area, thus generating surplus biomass which penetrates in less favourable areas.
The CPUEs (and fishery statistics) tend to corroborate this model. Figure 21 shows three situations encountered since 1966:
the years 1966–72 and 1976 which were periods of less or medium abundance when stocks occupied the Ghanaian waters and the eastern zone of Côte d'Ivoire waters (illustrated in Figure 16 where for the period 1976–80 the CPUE of Ghanaian and Côte d'Ivoire seine fleets did not vary to the same degree);
the 1973–75 period when the stock collapsed and withdrew back to the most favourable area (Ghanaian depression) and when no Sardinella aurita were caught in Côte d'Ivoire;
the recent period when considerable increases in biomass occurred which resulted in an extension of the stock to the west of Côte d'Ivoire (Figure 16 shows parallel trend for Ghanaian and Côte d'Ivoire seiners).
The biology of Sardinella maderensis is not as well known as that of Sardinella aurita. Uncertainties even exist as regards the structure of the stock. The assumption of a continuous stock all along the West African coast is plausible as well as the alternative assumption of a series of separated stocks or of stocks with limited exchanges, occupying a discontinuous series of habitats. In the latter case there would be a stock in Liberia, one in Côte d'Ivoire, one in Ghana and one in the Bay of Biafra.
Furthermore, studies associated with the “Fishery-Climate” programme suggest notable differences between the demographic strategy of S. maderensis and that of S. aurita. It appears that S. maderensis is more apt at maintaining a constant biomass because of its higher resistance to, and better integration with environmental disturbances. Populationa of S. maderensis are more resistant to unfavourable conditions (Cury and Fntana, 1988). It is understandable from all these elements that the working group was not in a position to propose a coherent biological model for S. maderensis stocks affcted by the Ghana and Cô d'Ivoire upwellings.
A study of the most attenuated CPUE fluctuations (monthly basis) was made for various gears and various zones (Figures 24 to 28), which reveals apparent abundance peaks in the catches by sardinella seinérs of Côte d'Ivoire and Ghana, which occur several times a year, without any possible identification of a biological cycle (Figure 22a, 22b, 23).
As regards the artisanal fishery, an exceptional CPUE peak was recorded in September 1986 for beach seines as well as setnets, which is attributed to S. maderensis specimens of the same size, supposedly large fish which came to the coast (for spawning) (Figures 24a and 24b). Identical studies on Benin and Togo CPUEs also revealed some peaks (Figures 25a, b, c and 26a, b, c).
Here also, it has not been possible to identify any cycle. The consensus arrived at by the group is that this could serve as a basis to build up a model but that prior to this very detailed data (in time and space) covering several years will be needed.
