Phase III – Seagrass and Sea Urchin Interactions Overgrazing and Resource Use in the WIO Region

This project aimed at assessing causes and consequences of sea urchin overgrazing in order to provide coastal managers with effective and sustainable options to deal with events of overgrazing. The research project had the following objectives: (i) To document impacts of overgrazing by urchins in seagrass habitats, (ii) To investigate possible causes for the population outbreaks of the sea urchin Tripneustes gratilla, (iii) To determine population trends of T. gratilla, (iv) To identify social-ecological benefits of sea urchin collection, as well as losses in areas characterized by overgrazing, and (v) To propose mitigation and monitoring strategies and make management recommendations. This collaborative multidisciplinary research project was implemented in three countries within the WIO region namely Kenya, Tanzania and Mozambique in partnership with Swedish counterparts. Methodology for this project included literature review, field surveys and experiments, interviews and questionnaire surveys. The project targeted local community, the scientific community, coastal managers, decision and policy makers as a whole as beneficiaries.

Results of the review suggest that urchin overgrazing is a global phenomenon, ranging from temperate to tropical coastal waters and involving a variety of seagrass and sea urchin species. Results further suggest that overgrazing can have a range of large, long-term indirect effects such as loss of associated fauna and decreased sediment stabilization. A range of drivers behind overgrazing have been suggested, including bottom-up (nutrient enrichment), top-down (reduced predation control due to e.g. overfishing), “side-in” mechanisms (e.g. changes in water temperature) and natural population fluctuations. Preliminary data on epiphytic composition and biomass revealed no significant differences between fertilised and unfertilised sites. It was also found that seagrass fertilisation did not significantly affect the biomass of seagrass. The main predators on sea urchins included asteroids (Protoreaster linkii), fish (trigger fish and wrasses) and gastropods (Cypraecassis rufa). Preliminary findings further showed that juveniles were mostly found in seagrass (Thalassia hemprichii) sites especially where seagrass grew within a rocky environment of rubbles. Surveys conducted along Dar es Salaam coast recorded ten species of sea urchins, with Echinometra mathaei being the most abundant followed by Tripneustes gratilla. Total sea urchin abundance was significantly different among the study sites. Seagrass–sea urchin interaction was depicted by significant negative correlations between sea urchin densities with seagrass biomass, canopy height, shoot density and percentage cover, suggesting that grazing by sea urchins might have contributed to the reduction of above ground seagrass biomass in locations with higher sea urchin densities. Findings from the questionnaire survey conducted in Mozambique reveal that the collection of T. gratilla is for its gonads and the collection depends basically on the reproductive time. The collected sea urchins gonads are for family consumption as well as for commercial purposes. All ages and sexes are involved in the harvesting of the sea urchins.

Future research should focus on (1) identification and quantification of ecosystem and societal scale effects of overgrazing; (2) assessment of the relative importance and interactions of different drivers; and (3) development of a holistic proactive and reactive long-term management agenda.