The fish grazing of rocky shore algae in the Gulf of Guinea

Abstract

There is a conspicuous absence of large algae on five breakwater systems surveyed along the northern shore of the Gulf of Guinea. Such algae are a distinctive feature of most natural, rocky areas in the region and their absence on these artificial structures may be accounted for by heavy sedimentation and pollution inside the harbours, and as a result of fish grazing on the outside. Herbivorous pomacentrids ( Abudefduf, Pomacentrus), parrot fish ( Pseudoscarus) and surgeon fish ( Acanthurus) are common along parts of the breakwaters sheltered from direct wave action. Preliminary experiments and observations show the importance of fish grazing and indicate diurnal differences in its intensity. The ‘broken-up’ nature of the breakwaters with moderate wave action is the probable reason for the occurrence of large fish populations and hence the presence largely of grazing-tolerant algae.