5.1 General
5.2 Hydrography and its influence on the life cycle of demersal fish stocks in the area
5.3 Migrations and reproduction
5.4 Latitudinal stratification of stocks
An inventory of data on the biology of the principal species is presented in Appendix 6. Although biological data on the species are on the whole scanty, there is some information available on length-weight relationship, growth and migration. Information on length-weight relationships of the principal species in the Senegambia area has been published by Franqueville and Freon (1976). Nguyen Xuen Loc and J. Wojciechowski (1972) have also published length-weight relationships for Dentex macrophthalmus, D. maroccanus, D. congoensis, D. filosus and D. angolensis. Apart from the species listed in Appendix 6, there are others for which biological data are available, namely:
(i) Coastal soft bottom species: Albula vulpes, Sphyraena piscatorum and Lutjanus spp.(ii) Intermediary (soft/hard): Diplodus sp. (Senegambia and Southern Mauritania) Lethrinus atlanticus (off Guinea), Dactylopterus sp. (off Guinea and Senegal) flat fishes and Balistes capriscus
(iii) Edge of slope: Scorpaenidae, Epigonus spp., triglidae, Synagrops microlepis, Merluccius spp., Paracubiceps sp., Chloropthalmus sp. and Macrouridae. Some information is also available on the migration of Sphyraena sp., Lutjanus sp. and Pleuronectes sp.
The main hydrographie features of the northern sector (above lat. 9°N) affecting the life cycle of demersal fish stocks can be described as follows:
In February, the 24°C isotherm taken as an indicator of the thermic front is situated in the Cape Verde area. This isotherm moves northward and is in the Cape Verde area in May, off Saint Louis in June and in the Cape Timiris area from August to November. The reproductive cycle and migration of fish stocks in the zone are matched with the displacement of the thermic front.
Based on detailed analysis of catches of the artisanal fisheries in Kayar and Saint Louis (Senegal) since 1972 and also on surveys of the LAURENT-AMARO since 1970 between lats. 11° and 21°N, Champagnat and Domain (in press) have worked out migrations of some demersal fish species between lats. 10° and 24°N.
These migrations are described in terms of two groups of demersal species, i.e., those with northern and southern affinities. These are summarized as follows:
(a) Species with northern affinity
Champagnat and Domain (in press) considered the following:
Pomadasydae: Diagramma mediterraneum, Pomadasys incisus, Parapristipoma octolineatum;These species are dispersed in the area between 20° and 23°N from August to October. Starting from November, they start moving south and stabilize between 10° and 16°N in February-March. There is latitudinal stratification of species in this area. Within species there is also stratification by age, the juveniles being more southernly distributed than the adults. Prom April, as the waters get warmer, northern movement of these species begins. In general, this displacement coincides with their reproductive activities which gets to the peak from mid-June to mid-July in the area 19°-21°N. There are important nurseries in this sector especially between Cape Timiris and Cape Blanc. The juveniles stay in this area until they reproduce for the first time south of 190N, particularly in October-November, before joining the adults.
Sparidae: Pagrus ehrenbergi, Pagellus coupei, Dentex filosus, D. canariensis;
Serranidae: Epinephelus aeneus, E. goreensis, E. gigas, E. caninus, Cephalopholis taeniops;
Carangidae: Lichia vadigo, L. amia
Pomatomidae: Pomatonus saltatrix
Sciaenidae: Argyrosoma regium, Umbrina canarienais
Muraenesocidae: Cynoponticus ferox (Phyllogramma regani)
(b) Species with southern affinity
Migrations of these species are less clear and they affect a number of species mainly in the Sciaenid community described by Longhurst (1963). Nevertheless, Champagnat and Domain (in press) described the general movements of the following species: Pomadasys jubelini, Scyris alexandrinus, Caranx carangus, Sphyraena piacatorum. S. sphyraena, S. dubia and Rachycentron canadum. These movements can be summarized as follows: from January to June the populations are concentrated in the coastal area off Senegal and in the complex estuarine area extending from Sine-Saloum to Guinea-Bissau where they have their first spawning. In June there is a rapid northern movement to the Cape Timiris area for their second spawning. In December, after spawning, they return to the estuarine areas. The spawning periods of some demersal species off Senegal are presented in Table 1.
Some species are stratified by size from south to north. It is observed that, for the hard bottom species especially the sparid communities, sizes increase from south to north. This phenomenon may also be true for other species, but there is not much data to support such assumption.
