9. STATE OF S. AURITA STOCKS

9.1 Present State of Stocks
9.2 Management of the Stocks and Work to be undertaken to manage the Fisheries

The aims of the working party were:

(a) to determine the present state of stocks;
(b) to consider management measures if so required;
(c) to propose a coherent programme for analysis of data collected and of future research.

9.1 Present State of Stocks

The following information was available to the working group: catches by gear, fishing effort, length measurements, biomass estimation by echo-integration, physical oceanographic data, statistics on other coastal pelagic species concurrently exploited, egg and larval abundance, etc. A certain amount of important information, for example, gonadosomatic indices, monthly catches and fishing effort, surface temperatures and currents on the Ghana/Ivory Coast continental shelf, were collected but could not be processed before the meeting. From all the available information it can be said that the stock of S. aurita appears to have now reached a level of abundance comparable with that of the period preceding its collapse. In fact, the 51 000 t caught in 1978, when upwelling conditions were average, ranks second in annual catch figures for the 1963-1978 period. It would therefore appear that the stock demonstrated strong resilience and has been able very rapidly (between 1976 and 1978) to reach a level of abundace comparable with that of previous years. This feature is probably inherent in the biological characteristics of the reproduction of the stock. It is further favoured by many other phenomena:

Fisheries

- Significant reduction in Ghanaian fishing effort following transfer to other species and reduction in the global fishing effort due to certain economic problems, for example, lack of spare parts for vessels;

- Reduction in the fishing effort of Ivorian seiners which have not fished the main concentration of S. aurita in Ghana;

- Several years in succession of cold upwelling waters (1974-1978) and high salinity (1973-1978); both conditions appear favourable to S. aurita (Figure 8);

- Return to apparent normal level of zooplankton abundance (Table 21, Figure 8);

- Reduction in the abundance of B. capriscus. Although the interaction between B. capriscus and S. aurita remains to be ascertained, it is probable that excessive abundance of B. capriscus helps to reduce the S. aurita biomass and vice versa.

- The stock decreased to a very low level between 1973 and 1975. Indeed, the upwelling during this period was average or strong, thus favouring good availability of the stock;

- The same stock rapidly recovered to a certain biomass level. However, the level actually reached by the stock remains unresolved: the large 1978 catch is not a proof that the parent stock has reached a steady level comparable to that of the 1963 to 1972 period, since variations in catchability of the stock are still largely unexplained. In this context it is recalled that the 1972 catches were unusually large compared to what was expected using the moderate upwelling index for that same year. The low 1979 catches shown by the preliminary statistics may nevertheless be easily explained by the very weak upwelling (close to that observed in 1968) during which the stock was usually hardly vulnerable to the gear independent of its real biomass (Figure 8). The probable positive consequence of this low catchability is to reduce fishing mortality on both juvenile and adults. On the other hand, the weak upwelling will doubtless also have possibly unknown effects (which may be positive or negative) especially as regards spawning and larval mortality.

9.2 Management of the Stocks and Work to be undertaken to manage the Fisheries

As already indicated, the exceptionally high S. aurita catches observed in 1972 cannot be explained by an analysis of duration and intensity of upwelling. It appears that this large catch probably affected the main biomass and probably led to its collapse. A deeper analysis of environmental conditions producing such an exceptional change in the stock’s catchability during 1972 is thus essential. Indeed, it is vital to acquire this knowledge in order to try and prevent a similar collapse in the future. The working party recommends that data relating to the fishery (catches, effort, size composition of catches) and environmental data during 1972 be analysed by half-degree squares and for weekly periods and that the same data be compared with the results for previous years (for instance, 1970 and 1971).

It is still true that the S. aurita stock appears to belong to the biologically unstable stock category, probably even in the absence of a fishery. This major characteristic may be directly linked to the fact that the species appears to reproduce only in precise ecological conditions which the spawners, eggs and larvae are only able to find in certain years (Fontana and Chardy, 1971, FAO, 1979d).

Examples of this type of stock suggest that the result of exerting fishing effort would be to increase the natural variability of the biomass. Variability may therefore be considered an unavoidable feature of these stocks; it is accentuated in the case of S. aurita by changes in the stock’s vulnerability associated with changes in upwelling conditions and probably also with other environmental conditions. In view of the stock’s obviously high degree of instability, the working party recommends that thorough research be carried out to analyse the determining features of these fluctuations in abundance and availability. Variations in recruitment are probably still an important cause of these fluctuations. The working party recommends research on the following:

- Regular estimation of the biomass of the parent stock, in particular, by echo-integration;

- Continuous estimation of the stock’s sexual activity by analysing gonadosomatic indices, and, where possible, egg and larvae abundance;

- Analysis of factors liable to affect larval mortality, especially surface currents moving offshore and potential larval predators of pelagic species in the area of distribution of the larvae.

- Continuation of S. aurita catches at a minimum level;

- Rapid stock regeneration through maintenance of the reproductive stock at an adequate level.

(a) Echo-integration to provide a non-biased and sufficiently precise estimation of the S. aurita biomass;

(b) Fisheries statistics from Ghana and Ivory Coast to be processed and combined in real time so as to have a permanent knowledge of catch levels;

(c) Ivory Coast, Ghana and Togo to take political and administrative measures to limit catches of S. aurita, should this become necessary

It is the working group’s opinion that at least one experienced research worker should analyse existing data as soon as possible in accordance with the preceding recommendations and also improve current data collection. Research should be carried out on a sub-regional level between Ivory Coast, Ghana and Togo. Since assignment of an experienced full-time research worker for this complex subject does not seem to be planned by the coastal countries in the mid-term the working group recommends that the CECAF Project recruit an expert to analyse the issue for a period of about two years. It has become evident that arguments in favour of this recommendation far outweigh the economic interest of the three countries in the region: indeed, the proposed studies may throw new light on the stability of coastal pelagic stocks with variable catchability which is a world-wide problem. On this point, the working group has noted that by virtue of the limited geographic scale of the fishery and of the stock, research undertaken in this sector could provide results more rapidly than in other larger and more complex fisheries.