The Fish Community of an East African Mangrove: Effects of Turbidity and Distance from the Sea

Abstract

Mangroves are often reported as nursery grounds for fish. Fish may enter mangroves
in order to avoid predators, but may not need to do so if turbidity provides a sufficient predator
refuge outside the forest. This study assessed the effects of turbidity in the field and laboratory
on mangrove fish community structure and behaviour. The extent to which fish penetrate into
mangroves has received little attention. This study also looked at differences in fish community
structure at mangrove sites near (6m) and far (200m) from the mangrove/sea boundary.
Twelve field samples were taken at approximately monthly intervals from replicate 25m2
landward and seaward plots, in a Sonneratia alba stand at Gazi Bay, Kenya. A total of 25 species
of fish were caught, 15 in seaward plots and 13 in landward ones. Mean abundance for all plots and
sampling times was 2.15 (equivalent to 0.09 m-2). Seaward plots had a total mean abundance more
than twice that of landward plots (2.75 ± 1.9 S.D. vs. 1.23 ± 0.33 S.D. respectively). There was
no relationship between abundance and turbidity. Laboratory experiments showed no significant
changes in behaviour of three common species in response to turbidity. The low density of fish
recorded concurs with previous work, and probably reflects conditions in Gazi Bay as a whole,
rather than unusual features of the mangrove environment there.