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3. SUMMARY OF FINDINGS AND DISCUSSION

The survey covered mainly the shelf areas of the islands, but with some course lines extending off the shelves and in-between the islands. The larger shelf Maio - Boavista was covered two times. Observations of bottom conditions showed that only the Maio - Boavista shelf had appreciable areas suitable for trawling. An interesting feature of the findings of the oceanographical observation is the indication of upwelling on the western side of the João Valente Bank. This is judged to be an effect of the prevailing current- and wind systems and since these are active over large parts of the year, it seems likely that the upwelling is a process of significant importance for the biological production in this area. About 3/4 of the total fish biomass estimated for the whole archipelago of about 100 thousand tonnes was found over the shelf areas of Maio - Boavista - Sal. The findings of small pelagic and the demersal fish was almost exclusively located to the island shelf areas. Over the deeper waters off the shelf only very limited resources of mesopelagic fish was observed. It should be noted, however, that resources of larger pelagic fish such as tunas are not covered by this type of survey.

The acoustic survey indicates that the total biomass of 100 thousand tonnes consists of demersal and small pelagic fish in a 50/50 proportion. An estimate of the stock of demersal fish based on the trawl swept area method suggests that this category may have been somewhat overestimated by the acoustic survey, and that true proportion may be approximately represented by about 65 thousand tonnes og small pelagic fish and 35 thousand tonnes of demersal species. The bulk of the pelagic fish consists of scads. These were also the only species caught in considerable quantities by bottom trawl, up to about 5 tonnes per hour's tow.

A report of previous exploratory resource surveys carried out with the trawler “Ernst Haeckel” of the German Democratic Republic was made available by the Department of Fisheries, Praia (Anon 1976). In about two weeks surveys in each of April and October 1976 good fishing grounds for bottom trawl were located on the shelf around Boa Vista and on the João Valente Bank. Good catches were obtained with the bottom trawl with an average of 5-6 tonnes per hour and ranging up to 30 tonnes per hour. About 50% of the catches was reported to consist of the horse mackerel species Trachurus picturatus. Not a single specimen of this fish was identified in the survey reported here. Boops boops was the next common species in the 1976 surveys with about 19% of the total catch in the October coverage. The high catch rates of Trachurus picturatus by the “Ernst Haeckel” suggests a considerable abundance of this species in the archipelago at that time. Its absence in the autumn of 1981 indicate that the occurrence of this horse mackerel which otherwise is known from the Azores, Madeira and Canary Islands systems and from Morocco is periodical in the Cape Verde archipelago.


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