The working group, recognizing that knowledge of recruitment is essential for management of the fishery, re-examined the relationships between the strength of the upwelling, catch rates of beach-seines and biological factors associated with primary production. Data for these analyses, obtained from FRUB Tema, were as follows:
upwelling indices;
mean annual displacement volume of zooplankton (in ml/ 1 000 1 of seawater);
mean annual number of S. aurita eggs and larvae per 1 000 1 of seawater;
CPUE (kg/trip) of beach-seines in the Volta Region and total for Ghana.
All data, except those on fish catches, were obtained from weekly hydrographic transects off Tema. The figure showing upwelling index and larval abundance produced during the 1989 consultation was updated (Figure 11).
The following observations were made:
the concentration of zooplankton off Tema had been stable since 1980 except for 1983 and 1986;
there was some relationship between the availability of S. aurita eggs and larvae and the strength of the upwelling; the working group noted the low abundance of eggs and in 1987 and the increasing values since then.
The working group noted with concern the number of months for which there were no data on zooplankton, eggs and larvae, and recommended that effort be made to improve the data collection. This would enable the establishment of a clear relationship between egg and larval abundance and beach seine catches. The problem of patchiness of eggs and larvae was underscored: the samples could not account for the real mean concentrations.
Data on S. aurita caught by the Ivorian industrial (1984–90) and artisanal (1988–90) fisheries were used. The modal lengths were presented bimonthly for the eastern (sectors 4 and 5) and western (sectors 6 and 7) areas. The visual examination of the observed evolutions led to the presentation of some interesting results (Table 13).
It was possible to follow the cohorts exploited by the two types of fisheries: from mid-1984 to the end of 1986 and beginning of 1987, the diagram shows clearly the evolutions of lengths of the round sardinella caught by the industrial purse-seiners. The individuals of lengths 13–14 cm were certainly aged less than one year as fished in mid-1984 and must correspond to those of 16–20 cm (between 1 and 2 years) at the end of 1984-beginning of 1985, and to those of 21–24 cm (between 2 and 3 years) which appeared massively in 1986–87.
The resultant growth diagram was a compromise between what was proposed by Marchal (1974) and that of the 1987 working group (FAO, 1988). The individuals of 13–14 cm of mid-1984 were certainly less than one-year-old (born during the major upwelling season of 1983) and their growth as far as 1985 followed the diagram of Marchal which assigns a length of 19 cm to 2-year-olds. On the contrary, following the growth as far as 1987 leads to individuals of very large sizes which appeared for the first time in this fishery from 1986. This, in part, is the phenomenon which led the 1987 working group to propose a rapid growth of the round sardinella.
What caused the appearance of these large round sardinella in the Ivorian fisheries?. Was it the fast growth in the past years of these individuals or the exceptional availability of a not usually accessible age-class.
Admitting the influence of environmental conditions on the growth of clupeids, the examination of indices of upwelling of the minor cold season in 1986 and 1987 allows a certain hypothesis to be put forward: the two cold seasons are particularly intense in Côte d'Ivoire and have been able to favour the growth and availability of individuals.
These comments agree with those formulated by Roy (1990) on the increase in intensity of winds in the minor and major cold seasons and their consequences on the productivity of the environment and therefore on the growth of individual fishes.
This hypothesis can be confirmed by examination of the same type of distribution in Ghana: in 1986 and 1987, individuals of 21 cm, which were quite unusual in this region, were observed.
Finally, the appearance in Côte d'Ivoire towards the end of 1990 of fish with modal size of 23 cm, also contributed to the view regarding the intensity of the minor upwelling season of that year.
It is also noted that the length modes in Ivorian sectors 6 and 7 often appeared much higher than those of sectors 4 and 5. In more than thirty bimonthly periods for which there were simultaneous samples for the east and west of Côte d'Ivoire, the observed difference between the two zones (21 times) was in favour of the west in three quarters of the cases. This can lead to reliable hypotheses regarding the productive capacity of the environment and the growth of the round sardinella: taking into account the minor cold season, systematically very intense in the west of Côte d'Ivoire, and the equivalent of the major cold season in the two parts of the coastline, it is probable that the environment in the west could be more favourable. This links up with the fact that the lengths of the round sardinella fished in Ghana were systematically lower than those in Côte d'Ivoire: the latter country would, be very favourable from the point of view of biotic capacity as shown by two intensive cold seasons even though in Ghana there is really only one.
The difference in lengths could, however, be due to the migrations of the very old round sardinella towards the west, or the existence of two populations. This third hypothesis does not contradict the first.
An identical examination of the data of modal lengths of the flat sardinella gave no immediately interpretable results.
Gonado-somatic index data for the period 1974–89 were available for Ghana. The working group was informed that for Côte d'Ivoire a substantial amount of biological data covering the period 1968–90 was being processed. The data were not available to the working group.
