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

M Huxham,E Kimani,J Augley

doi:10.4314/wiojms.v7i1.48254

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.

Highlights

Many studies have shown that mangrove habitats can support high abundances and diversities of juvenile fish, suggesting that they can be important nursery sites (e.g. Chong et al, 1990; Robertson & Duke, 1990; Williamson et al, 1994; Sheaves, 1995; Vance et al, 1996; Nagelkerken et al, 2000a; Lugendo et al, 2005)
Huxham et al (2004) reported low densities of fish within mangrove habitats at Gazi, a finding repeated in the current work
The overall mean density of 0.09 m-2 is almost two orders of magnitude less than the highest densities reported from mangrove sites (Thayer et al, 1987) and amongst the lowest reported in the literature

Summary

Introduction

Many studies have shown that mangrove habitats can support high abundances and diversities of juvenile fish, suggesting that they can be important nursery sites (e.g. Chong et al, 1990; Robertson & Duke, 1990; Williamson et al, 1994; Sheaves, 1995; Vance et al, 1996; Nagelkerken et al, 2000a; Lugendo et al, 2005). Many studies have shown that mangrove habitats can support high abundances and diversities of juvenile fish, suggesting that they can be important nursery sites Refuges against predators, such as narrow spaces between pneumatophores, and by obscuring the visibility of the prey species. Since the juveniles of most fish species that utilise mangroves at the current field site, Gazi Bay, are planktivorous or benthic feeders (Kimani et al, 1996; Huxham et al, 2004), and are preyed upon by larger piscivores, this differential effect of turbidity should provide an advantage in turbid habitats. It has long been argued that turbidity attracts juvenile fish because of the potential predator refuge it provides (Blaber & Blaber, 1980; Abrahams & Kattenfeld, 1997; Maes et al, 1998). The attraction of mangroves might be explained by the high turbidity of the waters in which many are found

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Conclusion