Fish and benthic communities as indicators of mangrove ecosystem recovery in Gazi bay Kenya

Abstract

The possibility of mangrove ecosystem recovery following artificial reforestation of mangrove trees was investigated in Gazi Bay, Kenya. This study was conducted to establish the status of mangrove habitats 10 years after reforestation as indicated by faunal composition, abundance and diversity. A semi-pristine and a degraded site were included in the study as controls. The aim of the study was to provide baseline data, which can be used in understanding the ecology of mangrove forest floor before and after reforestation in order to provide better management options. Comparisons were made between habitats of the natural (semi pristine), reforested and degraded mangroves. Meiofauna sampling was done using plastic corers of 3.6 cm diameter to a depth of 20 ern. Quantitative sampling of fish inside the mangroves was achieved using stake nets, gill nets and malema traps (traditional fish traps). Sediment grain size, organic matter content, pigments, temperature, salinity and dissolved oxygen in three plots were measured using standard methods. Feeding interaction between fish and mangrove benthic fauna was elucidated via gut content analysis of the fish. Data was collected between February 2004 and April 2005. Results indicated that sediment grain size, pigments, organic carbon and total nitrogen and phosphorus affected the distribution of meiofauna in the mangroves. A total of 18 meiofauna taxa were encountered with the highest density being recorded in the reforested site (1774 ind.10 ern") followed by natural (1650 ind.10 cm-2). The Shannon- Weaver diversity of all meiofauna taxa for natural and reforested sites was H'=0.20 and H' = 0.21 respectively. Nematodes comprised 90 % of all meiofauna recorded and comprised 24 families in all three habitats. Multi-dimensional scale (MDS) plots showed that all three replicates of the natural site and two replicates of the reforested site were similar in nematode genera composition while the degraded sites had different communities. For the fish, Gerres oyena (27.3 %), Chanos chanos (26.2 %) and Pelales quadrimaculatus (7.9 %) were the most abundant species encountered. There was no significant difference in fish density and diversity between sites (p< 0.05). Of the 18 meiofauna taxa recorded in the study 44.4 % were found in the stomach contents of the fish examined. One-way ANOVA showed significant difference between sites (natural, reforested and the degraded) (p < 0.05), in terms of gut content. Although the reforested site was verysimilar to the natural site in fish and meiofauna community composition, still, small differences were found pointing to a period of more than 10 years after reforestation in order to come to a full recovery of the associated mangrove ecosystem.