Population structure of three commercially important species in the Gulf of Guinea

The Gulf of Guinea Large Marine Ecosystem (LME) extends from the Bissagos Islands to Cape Lopez and takes in the maritime waters of all countries between Guinea Bissau and Gabon. The ecosystem is very productive and the fisheries sector is of great economic importance. This thesis uses molecular markers (mitochondrial DNA and microsatellites) to provide a compara­tive study of the population structure of three commercially important species in the region: Trachurus trecae (Cunene horse mackerel), Pagellus bellottii (Red pandora) and Sepia offici­nalis (Common cuttlefish). T. trecae showed evidence of population subdivision within the Gulf of Guinea (Fst=0.056) which was explicable by temporal (Fst=0.048), as opposed to spatial (^ST=0.001), structuring. Thus contemporaries from the same length cohort showed genetic similarity, regardless of geographic proximity. A significant correlation (correlation coefficient D: r=0.93, p=0.01) was found between the cohort length and Tajima’s D. P. bellottii likewise showed little evidence of spatial subdivision within the Gulf of Guinea (/st=0.009), however four individuals from a single trawl showed high sequence variation from all other samples (and when included in the analysis FsT=0.095). Both fish species displayed bimodal length frequen­cies for some trawls and when split according to cohort length there was evidence of within trawl heterogeneity, indicating that shoals are an aggregation of smaller groups. S. officinalis revealed no spatial subdivision in the the Gulf of Guinea (Fsr=0.00), though four individuals showed highly atypical allele sizes. Possible evidence of selection at one microsatellite locus was found. When compared with outgroups from southwest Africa and Europe T. trecae and S. officinalis showed great differentiation (Fst=0.642 and Fst=0.301 respectively). Comparative results across species therefore indicate (i) that the Gulf of Guinea is a well defined LME and (ii) there are no major oceanographic structures within the LME that have caused spatial pop­ulation subdivision. Given such a lack of spatial subdivision, management needs to operate at a regional level for these species. These results were found for three species with very differing life histories, so they may also be applicable to other marine species in the region