5.1 Ivory Coast
5.2 Ghana
5.3 Togo
At the first working group (ORSTOM, 1976) a satisfactory estimation of abundance variations was obtained by weighting the annual cpue with an index of upwelling. Owing to lack of comparable data it has not been possible to recalculate the cpue for the last few years. Moreover, the tendency towards increased power of the sardinella boats has not been taken into account (Chaali, 1977). Thus, two abundance indices have been calculated for the recovery period alone without weighting with an upwelling index since this has remained relatively stable from 1975 to 1978 as seen earlier.
- The first index was obtained by dividing the total catch of the Ivorian seiners by the total effort, expressed in hours fishing.It is apparent that the first index indicates an increase in abundance starting in 1976 and the second, which is more representative, shows the increase from the following year only as indicated by the cpue of the Ghanaian seiners (Figure 4).- The second corresponds to the average cpue per fortnight and per sector on the assumption that a significant effort existed in all the various strata (Table 8).
Data available by sector and month from 1967 to 1978 have been analysed so as to study details of the evolution in space and time of fishing effort in yield. A sufficiently wide range of cpue has been used in the graphic representation in order to reduce to a minimum the biases introduced by increasing fishing power and a change in the method used to calculate fishing effort during the period under consideration (Figure 5). The results indicate that:
(i) the fishing effort has shifted westward in recent years and the Ivorian zone has been more intensively fished during the upwelling season as mentioned above;(ii) in Ivorian waters, the localization of strata with high catches does not seem to have significantly changed with the exception of the two years proceeding the collapse of S. aurita.
(iii) in the last two years high catches have been recorded of Abidjan during the first six months of the year, an unusual occurrence. This was particularly apparent in 1972 when Ivorian seiners were able to fish in Ivory Coast without having to fish in Ghanaian waters as usual;
It is thus very likely that the reduction in the zone fished in 1972 corresponds to an increase in the area of availability of the stock and not just to a westward shift of this area. This is confirmed by the high catches obtained during the same period by the Ghanaian fleets.
(iv) it has been noted that the stock collapsed suddenly at the end of 1972 and only began to recover at the end of 1976, after which time it increased progressively. Thus, the observed increase in cpue cannot be explained by a simple increase in availability since the intensity of the upwelling has only changed slightly.
Annual catch per unit of effort available for purse seiners was obtained by dividing total catches by the total number of days fishing (Table 9). This index is biased since, rather than indicating abundance variations in space and time, it expressed concentration related to fishing effort and availability as a function of the intensity and duration of upwelling. It would, for instance, have been preferable to calculate the average monthly cpue for each sector during the period of maximum abundance, but no data were available at the time of the meeting.
In future, it will thus be advisable to process data according to boat category, grouping it by sector and month. In this way, although the cpue calculated will not indicate precisely the relative values of annual abundance it can be taken to represent the overall trend of the stock which, following its collapse after 1972, has been markedly restored since the end of 1976 (Figure 4).
Data on the fishing effort of the artisanal fleet are collected by gear and sector. However, these were not available to the working group. The working group therefore recommends that these be presented at the earliest possible date. The data are of the greatest importance since the Ghanaian artisanal fishery is preponderant in the region and important recruitment information may be obtained from this analysis.
Annual catches per day fishing are available from 1976 to 1978. These show a sharp increase in yields in 1978 (Appendix 3). However, the results are difficult to interpret since they do not take into account the likely increase in number and in variety of gear used. Recommendations similar to those proposed for Ivory Coast and Ghana were adopted for Togo.
