Habitat utilisation by juveniles of 13 commercially important fish species
was studied in five habitats located in Chwaka Bay, Zanzibar: mangrove
creeks, mangrove channel, sand/mud flats, a seagrass area close to
mangroves, and a seagrass area far from mangroves. Fish samples were
collected from each habitat using a seine net, and fish abundance and size
were measured to determine habitat utilisation. The seagrass beds near to
mangroves showed the most diverse fish assemblage of all habitats, possibly
because it functions as a corridor between the mangroves and deeper parts of
the embayment. Juveniles of Cheilio inermis, Hipposcarus harid,Leptoscarus vaigiensis, and Scolopsis ghanam inhabited seagrass beds only.
Juveniles of Gerres filamentosus and Monodactylus argenteus were mainly found in the mangrove habitats. Lethrinus variegatus, Pelates quadrilineatus and
Siganus sutor were found in more than two habitats, with highest abundances in seagrass
beds. Juveniles of Gerres oyena, Lethrinus lentjan, Lutjanus fulviflamma and Sphyraena barracuda were the most generalist species and were found
in all studied embayment habitats. Visual census surveys supported the seine
net data showing that most fishes in the embayment habitats were juveniles
or sub-adults. In terms of habitat utilisation by different size classes,
five of the 13 species (Lethrinus lentjan, L. variegatus,
P. quadrilineatus, Siganus sutor and Sphyraena barracuda) were found as small-sized individuals
in shallow and turbid mangrove areas, whereas large-sized individuals were
observed in deeper and less turbid seagrass beds. A possible explanation for
this pattern could be an ontogenetic shift in habitat utilisation, although
this could not be proven. The patterns observed in the present study show a
high similarity to those observed in marine embayments in the Caribbean,
indicating that similar mechanisms are at work which make these systems
attractive juvenile habitats.