The Impact of Human Activities on the Ephibenthic Bivalve Community in Protected and Unprotected Marine Areas at the Kenyan Coast

Abstract

A comparative survey of bivalve fauna found in reef flat, sea grass zones and shallow lagoons «2m) was undertaken in protected and unprotected marine areas to determine the density, diversity and species richness and the possible impacts of human activities on these mollusks. Data was collected by laying 3 transects of 400m2 in each site at the three areas. All bivalves observed were identified, their shell lengths measured and evidence of any kind of human interference recorded. Data was analyzed through use of species richness, diversity and similarity indices, coefficients of dispersion and covariance. These parameters were used to compare and describe species composition at each study site. Human activities were expressed in percentage frequencies of occurrence. The most prevalent human activity was then used to show the impact on the most vulnerable bivalve species A total of 17 bivalve species belonging to 13 families were observed in both the protected and unprotected areas of the Kenya coast from Oct. 1997March 1998. Their distribution showed low densities (2 bivalves/m2), low diversity (1.26 ± 0.005) in species composition and distribution (0.06475 ± 0.03) in the different substrate types. Modiolus auriculatus (0.473 ± 0.02) and Pinna muricata(0.45 ± 0.03) were the most prevalent. There was a significant difference in species diversity between Malindi and Mombasa reef flats at P<0.10. Shallow lagoons had very low relative density (0.49 ± 0.05) and diversity (0.068 ± 0.022) as compared to that of the reef flat which exhibited the highest density of (0.691 ± 0.017). This was attributed to the high deposits of shell, coral, and sand. Species composition classified up to family level showed Mitilidae (54% ad 44%) as the most represented in Kanamai and Malindi areas respectively; and Pinnidae (36%) in Mombasa. Abundance similarities were low (65 %) but for the lagoon (86.9%) and reef flat (90.0%) of Mombasa. Most bivalve species were clumped together (11421) or uniformly distributed (0- 0.395), with a few showing random distribution (1). Swimming, goggling/ SCUBA diving, walking/ trampling and turning of rocks were identified as the main forms of human activities causing disturbance. Frequencies of occurrence of these activities varied in the three areas with Kanamai exhibiting the highest. Results showed that the distribution of bivalve fauna in the protected and unprotected areas is bivalve density independent and not only influenced by human activities and management strategy but by other biological and environmental factors such as substrate type, tide range and wave activity. Human activities however affect those bivalves with fragile shells such as P. . muricata, through trampling resulting in injury and / or death. Therefore decentralisation of human activities within the marine parks is recommended to reduce their impacts. These activities can be carried out in the reserves. Beside marine protected areas are insufficient alone as they are not isolated from other critical environmental impacts and thus should' be complemented with strong conservation efforts outside the parks.