Hydrography
The surface temperatures were recorded continously by a thermograph. The temperatures for the period and region surveyed are shown in Figures 8-11. Between Agadir and Dakar the situation is rather uniform with temperatures between 16 and 18 C on the shelf. The cold water front is located outside the Bissagos islands in late February, and in the area south of Dakar an upwelling situation is indicated.
Fish distribution
The distribution of fish during the survey is shown in Figures 12-15. The figures show the distribution at three levels, scattered, slightly gathered and dense, measured by the acoustic system. The dominating species in the area, when identified by fishing have been marked with a species code at the position of the fishing station. The nearby and not fished area has been found to hold mostly the same species. This assumption is based on the echo traces and is valid mainly for the pelagic species. The following comments on fish distribution begin in the south and proceed northwards and deviate somewhat from the cronological order of the survey as this did not follow a straight one-directional pattern.
The northern part of the Sierra Leone shelf seems to be an important nursery ground for juvenile fish. A dense patch outside Freetown was found to consist of post-larval fish of the species Sardinella maderensis, Engraulis encrasicholus and Selene dorsalis. These were also present outside the dense patch. Apart from this the investigated area in Sierra Leone is by far dominated by Brachydeuterus auritus.
From Conacry north to the Bissagos Islands the dominating species was the triggerfish Balistes capriscus forming easily detectable pelagic formations in the area. This coincides well with the findings in May - June 1981 in the same area. In contrast to earlier patterns for the triggerfish it was, however, not detected at all between Bissagos islands and Cape Roxo but found again in several trawl hauls further north. The phenomenon may perhaps be explained by the cold-water front dividing the distribution of trigger fish in two and trapping the northern part in upwelling water. Apart from the trigger fish the area Conacry-Bissagos Islands held some interesting concentrations of Sardinella maderensis in shallow waters. Schools were also frequently counted by the sonar in shallow waters. Not all registrations were identified by trawling, but were nevertheless assumed to be Sardinella maderensis. These findings represent a confirmation of those of the May-June survey last year.
Outside the Bissagos Islands good concentrations of big Sardinella aurita, Trachurus trecae and Decapterus rhonchus were found to have aggregated between the southward passage (16 February) and the northward passage (25 February) through the area. At the latter time a Soviet fleet was also located in the area.
In the area between Bissagos islands and Cape Verde the shelf was dominated by scattered registrations of Brachydeuterus auritus, various sparids, trigger fish, Trachurus trecae and various demersal species. Outside the outlet of the Casamanche river Ilisha africana and various croakers (Sciaenidae) were identified in good concentrations. Schools of anchovy were also registrated in the area. In Senegal waters north of Cape Verde (Figure 13) scattered pattern of mostly demersal species was found. A dense patch of Brachydeuterus auritus was detected at N 15°15'.
In Mauritania dense patches of horse mackerel (Trachurus trecae) were detected, especially along the continental edge and in the area around Cape Timiris. North of Cape Timiris the horse mackerel was mixed with Sardinella aurita and anchovy. South and west of Nouadhibou the acoustic registrations were good and were found to consist mostly of squid and juvenile (post-larval) fish.
In the waters outside West-Sahara no fishing was carried out by the vessel, but the registrations of fish were assumed by their pattern to consist mainly of horse mackerels. The registrations were comparatively dense, especially in the area between N 21°30' and N 25°00'. North of this the registrations were scarce or nil. The shelf outside West-Sahara seems to hold a considerable abundance of fish biomass exploited by trawlers (Figure 6).
The shelf from Cape Juby to Agadir was covered twice; in the period 23-30 March and 1-3 April (Figure 15 a and b respectively). The fish distribution changed between the two coverages. During the first survey dense registrations of sardines were located at N 28°15' close to the shore, and further north at N 29 30', then together with mackerel (Scomber japonicus).
Apart from this the shelf was covered with scattered registrations of sardine and mackerel, together with horse mackerel and various demersal species. During the second survey (Figure 15 b) the sardine had moved closer to the shore. Sardine fishing was only observed in the area west of Agadir. The dense registrations of sardine further south seemed to be unexploited at the time.
