Maturity and spawning pattern of Sardinella aurita in relation to water temperature and zooplankton abundance off Ghana, West Africa

Studies on the maturity and spawning pattern of Sardinella aurita Valenciennes in relation to sea surface temperature (SST) and zooplankton abundance were carried out during the period 1986–91 in Ghanaian waters (West Africa). Both sexes exhibited synchronous seasonal maturation patterns. The mean length at first maturity occurred at approximately16.7 cm for males and 17.1 cm for females. The mean length at first maturity (L₅₀) increased in comparison with that observed in the 1970s. Three spawning seasons were identified: a major season which extends from July to October, and two minor seasons which occur in March and December. Significant correlations were found between the female gonadosomatic index and SST, between egg abundance and SST, and between egg abundance and zooplankton abundance. The SST seems to be an important factor in species maturation. There is a time lag between attainment of peak gonad maturation and onset of spawning. Spawning appears to commence in response to the start of the coastal upwelling which is characterised by a sharp decrease in SST and an increase in zooplankton abundance.     

Diatom taphocoenoses in the coastal upwelling area off South West Africa

The diatom floral composition of 124 sediment samples from the South East Atlantic records the influence of coastal upwelling on sediment composition off South West Africa. Inner shelf samples between 19° and 24°S are rich in diatom valves and the patterns of diatom species distribution in these samples are related to the coastal upwelling process. Comparison with recent phytoplankton data shows that the sediment assemblages preserve many of the important species of the diatom biocoenoses, including Chaetoceros (resting spores), Delphineis karstenii, Thalassiosira eccentrica, and Thalassionema nitzschioides. Delphineis karstenii, a pioneer species in enriched coastal water, occurs nearshore and Chaetoreros resting spores are widespread, with highest relative abundance values in some more offshore samples. The abundance of the Thalassiosira eccentrica group and of Thalassionema nitzschioides in sediment samples in and near Walvis Bay reflects the recurrence of intense upwelling off this part of the coast. An abundance of large centric species has been reported in hydrological conditions characteristic of newly upwelled waters and, correspondingly, Actinocyclus octonarius and some large Coscinodiscus species occur in the sediments in nearshore patches or belts.     

Zooplankton Spatial and Temporal Distribution in a Tropical Oceanic Area off West Africa

Tropical instability vortices are believed to modify the trophic food web by affecting plankton production. Such instabilities have been evidenced in a tuna fishing area of the Atlantic (10°–20° W and 2°–5° N). However, the origin of the tuna abundance remains uncertain as no data on zooplankton communities on which tuna preys are supposed to fed upon are available. This study was focused on short term spatial and diel variations of mesozooplankton communities sampled at 2° N during and after the passage of an instability wave evidenced from satellite imagery, at 4° N and at 0°, i.e. in the Equatorial divergence. Samples were collected with an opening–closing multisampler and with a WP2 net (200 μm). Copepods prevailed (86–92% of the total zooplankton) with Clausocalanidae, Oncaeidae, Corycaeidae, Calanidae, Eucalanidae as dominant families. At 0° and at 2° N during the wave, large-sized organisms constituted over 50% of total dry weight (DW). These percentages decreased after the wave and at 4° N. Zooplankton DW and densities were highest at 0° and lowest at 4° N. At 2° N after the wave, we observed a strong increase in the abundance of zooplankton which surpassed that at 0°. This increase mainly resulted from an increase in small copepods and copepodites. Our results suggest that the high production linked to the upwelling of nutrients caused by the geostrophic divergence is partly advected northward by the instability wave. The advection did not reach 4° N which displayed the characteristics of a typical tropical structure.     

Sero-genetic studies on the San of South West Africa.

The San, a physically, culturally and linguistically distinctive people, have been shown by archaeological records anciently to have inhabited the whole of Eastern and Southern Africa. They, in common with the Khoi, the other members of the Khoisan race, are confined now to Southern Africa and principally to Botswana and South West Africa, though a number are also found in Angola. Sero-genetic data concerning seven South West African groups are presented in this study, and confirm a shared overall genetic profile characteristic of the San in general, slightly different from that of the Khoi and in significant contrast with that of the Negroes.     

Sero-genetic studies on the Caucasoids of South West Africa/Namibia

Gene marker studies on the Afrikaans and German populations of South West Africa/Namibia reveal that both very closely resemble their parent European stocks but that the Germans show appreciable evidence of having received a genetic contribution from non-Caucasoid, probably Khoi or Negro, sources.     

Biology Teaching in West Africa

Simulation games designed as training exercises have been used in a wide variety of fields. With the exception of medical education they have as yet been little used in teaching science. Lebemo is a game simulating research in bacterial genetics. The player has to manipulate data, interpret results, plan an overall strategy, and decide upon appropriate experimental protocols; a combination of features found in research and project work but rarely in traditional laboratory classes. In its present form Lebemo is self-instructional and takes a few hours to play. Lebemo and games like it might prove useful adjuncts to traditional laboratory classes and could perhaps replace more time-consuming project work.     

The effects of long-term exploitation of demersal fish populations off the coast of Sierra Leone, West Africa

The present condition of the inshore sciaenid fauna in Sierra Leone, based on the data collected in 1985–86 from commercial inshore trawlers and accumulated records, is compared with the first assessment of the virgin stocks conducted in the 1950s. This has enabled the effects of the long term on this tropical multispecies fishery to be assessed. In addition to the changes in the species composition, the estimation of growth and mortality from both time periods have allowed the population characteristics of the most common dermersal species to be compared.     

Limnological studies of two contrasting but closely linked springs in Nigeria,West Africa

ABSTRACT

An account is presented of the chemistry and algal flora of two streams in Ekiti State, Nigeria. The two streams, one of which is much warmer than the other, are separate upstream but subsequently merge. A comparison of the springs revealed that while they differed markedly in temperature, colour, turbidity, conductivity, solids, total alkalinity, total hardness, Ca, SiO₃, SO₄, Mg, Cl^- and dissolved oxygen, they showed some similarities in pH, Fe, Na, K, PO₄ and NO₃. In the cold spring, while total alkalinity, conductivity and silicate increased in the wet season, total hardness decreased slightly and other variables exhibited fluctuations with season. For the warm spring, except for increased total alkalinity and total hardness, dissolved oxygen and pH decreased in the wet season. However, most parameters of the warm spring were unaffected by season. Floristically, the springs were rich in species with a total of 84 taxa: 56 in the warm spring, 33 in the cold spring, 44 at the confluence and 40 beyond the confluence. The warm spring was dominated mainly by blue-green algae such as Lyngbya birgei, Synechococcus aequalis and Oscillatoria spp.; the cold spring was dominated by Rhizoclonium hieroglyphicum and Navicula spp. The confluence was dominated by Terpsinoe musica, R. hieroglyphicum and Lyngbya birgei, while the site downstream of the confluence was dominated entirely by R. hieroglyphicum. In most cases, the confluence and downstream area beyond the confluence of the springs exhibited intermediate characteristics between the two springs. This study is a contribution to the dearth of information on streams in West Africa.     

The Foraging Ecology of the Endangered Cape Verde Shearwater,a Sentinel Species for Marine Conservation off West Africa

Large Marine Ecosystems such as the Canary Current system off West Africa sustains high abundance of small pelagic prey, which attracts marine predators. Seabirds are top predators often used as biodiversity surrogates and sentinel species of the marine ecosystem health, thus frequently informing marine conservation planning. This study presents the first data on the spatial (GPS-loggers) and trophic (stable isotope analysis) ecology of a tropical seabird—the endangered Cape Verde shearwater Calonectris edwardsii–during both the incubation and the chick-rearing periods of two consecutive years. This information was related with marine environmental predictors (species distribution models), existent areas of conservation concern for seabirds (i.e. marine Important Bird Areas; marine IBAs) and threats to the marine environment in the West African areas heavily used by the shearwaters. There was an apparent inter-annual consistency on the spatial, foraging and trophic ecology of Cape Verde shearwater, but a strong alteration on the foraging strategies of adult breeders among breeding phases (i.e. from incubation to chick-rearing). During incubation, birds mostly targeted a discrete region off West Africa, known by its enhanced productivity profile and thus also highly exploited by international industrial fishery fleets. When chick-rearing, adults exploited the comparatively less productive tropical environment within the islands of Cape Verde, at relatively close distance from their breeding colony. The species enlarged its trophic niche and increased the trophic level of their prey from incubation to chick-rearing, likely to provision their chicks with a more diversified and better quality diet. There was a high overlap between the Cape Verde shearwaters foraging areas with those of European shearwater species that overwinter in this area and known areas of megafauna bycatch off West Africa, but very little overlap with existing Marine Important Bird Areas. Further investigation on the potential nefarious effects of fisheries on seabird communities exploiting the Canary Current system off West Africa is needed. Such negative effects could be alleviated or even dissipated if the ‘fisheries-conservation hotspots’ identified for the region, would be legislated as Marine Protected Areas.     

The distribution of Branchiostoma senegalense (Acrania,Branchiostomidae) in the off-shore shelf region off North West Africa

During investigations on the macrobenthos between 17° N and 26° N on the North West African continental shelf, abundant stocks of Branchiostoma were found off Spanish Sahara (23° N-25° N), on the d'Arguin Bank (21° N-20° N) and off Gambia (12° N-13° N). The lancelets were identified as Branchiostoma senegalense (WEBB, 1955) by means of statistical analyses. The lancelet populations occur at dephts down to about 40 m. Deeper regions are probably avoided because of the excessively high detritus content. Favourable environmental factors such as temperature, composition of the sediment and food supply permit high lancelet abundancies. Up to 9000 animals'm² were found in the population off Spanish Sahara. Rough calculations showed that the lancelet population comprises about 6 × 10¹² individuals with a biomass of 1.5 million tons. The potential natality amounts to about 45 × 10¹⁵ larvae a year.     

Food selection by a sit-and-wait predator,the monkfish,Lophius upsicephalus,off Namibia (South West Africa)

Synopsis We investigated prey selection by the monkfish, Lophius upsicephalus. This sit-and-wait predator preferentially selected prey types that would be expected to react to the lure of the illicium. Prey size selection was dependent on predator size, with larger predators feeding on a wider range of prey sizes.