The lack of data makes it difficult to go into details for this group. During the surveys of “Dr. Fridtjof Nansen” and “Kattegat” little attention was paid to the larger pelagic species. However, on both vessels visual sightings of surface schools were recorded, and these appear in Fig. 8.1. The schools were not usually detected by the sonar as they were confined to very near the surface. Some of the surface school observations consisted of species belonging to the group “small pelagic species”, mostly scad.
Table 8.1. Composition of the long-line catches of “Aelita” (% weight).
|
TUNAS |
44.7 |
|
|
Thunnus albacores |
|
34.1 |
|
T. obesus |
|
6.1 |
|
T. alalunga |
|
4.0 |
|
BILLFISH |
27.3 |
|
|
Makaira ampla |
|
24.0 |
|
Xiphias gladius |
|
3.0 |
|
Makaira mazara |
|
0.3 |
|
SHARKS |
23.0 |
|
|
Prionace glauca |
|
8.5 |
|
Carcharhinus limbatus |
|
5.9 |
|
Alopias vulpinus |
|
2.8 |
|
Carcharhinus longimanus |
|
2.1 |
|
C. melanopterus |
|
1.4 |
|
C. albimarginatus |
|
1.3 |
|
C. leucas |
|
0.6 |
|
C. brevipinna |
|
0.4 |
|
OTHERS |
5.0 |
|
Among the billfishes the marlin (Makaira ampla) dominated the catches. Also for these species the best catches were taken off Sofala Bank. The most important sharks caught were the blue shark (Prionace glauca) and grey shark (Carcharhinus limbatus).
Spanish mackerel
The most abundant of these species caught on the “Dr. Fridtjof Nansen” surveys was the narrow-barred Spanish mackerel (Scomberomorus commerson). It occurred frequently as by-catch both in bottom and pelagic trawls and catches up to 60 kg/hour were obtained. “Professor Mesyatsev” had catches up to 137 kg/hour at Sofala Bank in July. The distribution area of the narrow-barred Spanish mackerel is indicated in Fig. 8.1. It was only recorded in water shallower than 50 m at Sofala Bank and at the Boa-Paz Bank. Other Spanish mackerel, such as S. guttatus, S. maculatus and S. lineolatus, were only caught at Sofala Bank and appeared to have a more near-shore distribution than S. commerson; this can be seen in Fig. 8.1. These species were not usually observed in water deeper than 30 m. The major part of the surface schools recorded over the shallower part of Sofala Bank probably consisted of Spanish mackerel. These schools were frequently seen in the distribution area of the buccaneer anchovy on which they feed.
Juvenile Spanish mackerel of about 2-3 cm were found in April off Cabo Delgado. In May and September specimens of 5-7 cm were caught at Sofala Bank. These observations indicate two peaks in the spawning activity, namely in April and August.
Fig. 8.1. Observations of larger pelagic species and surface schools.
Sharks
Table 8.2 lists the most important sharks identified on the cruises of “Dr. Fridtjof Nansen”. They were accidentally caught in trawl hauls and some very good catches were obtained by gill-net and longline. Sharks were frequently observed swimming at the surface and were especially abundant off the mouth of the Zambezi River. There is without doubt a significant resource of sharks which at present are only lightly exploited. They are caught as by-catch in the shrimp fishery and in the bottom trawl fishery of the Soviet trawlers.
Table 8.2. Sharks identified on the cruises of “Dr. Fridtjof Nansen”.
Tunas
A significant proportion of the surface schools observed by “Dr. Fridtjof Nansen” in more offshore waters probably consisted of tuna. “Kattegat” in November-December 1977 made several sightings of surface schools of yellowfin tuna in the area from Angoche to north of Pemba (Fig. 8.1).
Others
Talang queenfish (Scomberiodes commersonianus) and cobia (Rachycentron canadus) were caught in small numbers at some trawl stations on the shallower part of Sofala Bank. In the same area there were also a few accidental catches of the eastern little tuna (Euthynnus affinis) and the oriental bonito (Sarda orientalis).
