Structure and dynamics of demersal assemblages on the continental shelf and upper slope off Ghana, West Africa

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Using two-way indicator species analysis and detrended correspondence analysis, species on the continental shelf and upper slope of Ghana were classified into 6 assemblages. The first 2, namely the Sciaenid and Lutjanid assemblages, are predominantly suprathermocline whilst the 2 Sparid assemblages begin at the thermocline depth (~40 m) and extend offshore. The last 2 (deep shelf and upper slope) assemblages occur well below the thermocline. The Sciaenid community is associated with the soft, muddy substrate found in shallow waters generally <40 m deep, and is made up mainly of species of the Pseudotolithus and Galeoides genera. Lying beyond this belt is a wide area of mixed-to-hard bottom, with which the Lutjanid and the Sparid assemblages (shallow and deep parts) are associated. There were clear faunal discontinuities around 30–40, 100 and 200 m depth. The first ecotone is closely related to bottom depth and the presence and depth of the thermocline, 10 the second to a steep shelf drop, and the third to significant division between shelf and slope assemblages. The structure of the assemblages is determined primarily by depth and type of sediment on the seabed whilst the dynamics of the assemblages are influenced by mainly temperature, salinity and dissolved oxygen.

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Dr. Kwame A. Koranteng

E-mail: kwamek@africaonline.com.gh