Time filter

Source Type

Nentwig V.,University of Munster | Bahlburg H.,University of Munster | Monthy D.,Seychelles National Parks Authority
Pure and Applied Geophysics | Year: 2015

The Seychelles, an archipelago in the Indian Ocean at a distance of 4,500–5,000 km from the west coast of Sumatra, were severely affected by the December 26, 2004 tsunami with wave heights up to 4 m. Since the tsunami history of small islands often remains unclear due to a young historical record, it is important to study the geological traces of high energy events preserved along their coasts. We conducted a survey of the impact of the 2004 Indian Ocean tsunami on the inner Seychelles islands. In detail we studied onshore tsunami deposits in the mangrove forest at Old Turtle Pond in the Curieuse Marine National Park on the east coast of Curieuse Island. It is thus protected from anthropogenic interference. Towards the sea it was shielded until the tsunami in 2004 by a 500 m long and 1.5 m high causeway which was set up in 1909 as a sediment trap and assuring a low energetic hydrodynamic environment for the protection of the mangroves. The causeway was destroyed by the 2004 Indian Ocean Tsunami. The tsunami caused a change of habitat by the sedimentation of sand lobes in the mangrove forest. The dark organic rich mangrove soil (1.9 Φ) was covered by bimodal fine to medium carbonate sand (1.7–2.2 Φ) containing coarser carbonate shell fragments and debris. Intertidal sediments and the mangrove soil acted as sources of the lobe deposits. The sand sheet deposited by the tsunami is organized into different lobes. They extend landwards to different inundation distances as a function of the morphology of the onshore area. The maximum extent of 180 m from the shoreline indicates the minimum inundation distance to the tsunami. The top parts of the sand lobes cover the pneumatophores of the mangroves. There is no landward fining trend along the sand lobes and normal grading of the deposits is rare, occurring only in 1 of 7 sites. The sand lobe deposits also lack sedimentary structures. On the surface of the sand lobes numerous mostly fragmented shells of bivalves and molluscs were distributed up to 150 m from the coastline. Intact bivalve shells were mostly found positioned with the convex side upwards. On small ledges of a granitic body at 130–150 m from the shore mostly fragmented and gravel sized shells were deposited at different elevations up to 4 m above sea level. This implies a run up height of at least 4 m above sea level up to 150 m from the present shoreline. © 2014, Springer Basel.

Wilson S.K.,Marine Science Program | Wilson S.K.,University of Western Australia | Graham N.A.J.,James Cook University | Fisher R.,Australian Institute of Marine Science | And 6 more authors.
Conservation Biology | Year: 2012

Disturbance plays an important role in structuring marine ecosystems, and there is a need to understand how conservation practices, such as the designation of Marine Protected Areas (MPAs), facilitate postdisturbance recovery. We evaluated the association of MPAs, herbivorous fish biomass, substrate type, postdisturbance coral cover, and change in macroalgal cover with coral recovery on the fringing reefs of the inner Seychelle islands, where coral mortality after a 1998 bleaching event was extensive. We visually estimated benthic cover and fish biomass at 9 sites in MPAs where fishing is banned and at 12 sites where fishing is permitted in 1994, 2005, 2008, and 2011. We used analysis of variance to examine spatial and temporal variations in coral cover and generalized additive models to identify relations between coral recovery and the aforementioned factors that may promote recovery. Coral recovery occurred on all substrate types, but it was highly variable among sites and times. Between 2005 and 2011 the increase in coral cover averaged 1%/year across 21 sites, and the maximum increase was 4%/year. However, mean coral cover across the study area (14%) remained at half of 1994 levels (28%). Sites within MPAs had faster rates of coral recovery than sites in fished areas only where cover of macroalgae was low and had not increased over time. In MPAs where macroalgae cover expanded since 1998 there was no recovery. Where coral was recovering on granite reefs there was a shift in relative prevalence of colony life-form from branching to encrusting species. This simplification of reef structure may affect associated reef fauna even if predisturbance levels of coral cover are attained. ©2012 Society for Conservation Biology.

Cariglia N.,Northumbria University | Wilson S.K.,Marine Science Program | Wilson S.K.,University of Western Australia | Graham N.A.J.,James Cook University | And 5 more authors.
Aquatic Conservation: Marine and Freshwater Ecosystems | Year: 2013

In tropical regions all over the world, holothurian (sea cucumber) fisheries are an important source of income for local communities.Almost all fisheries targeting holothurians are considered fully exploited or in decline, and several fisheries have collapsed.This study examined the differences in abundance of commercially exploited and ecologically important coral reef holothurian species between long established marine protected areas (MPAs) and unprotected areas in the inner islands of the Seychelles.Over a period of 1 month, 21 sites across the inner islands of the Seychelles were surveyed, nine of which were within MPAs.The probability of observing holothurians in areas protected from fishing (~80%) was twice that in areas subjected to fishing.The probability of observing holothurians of high or medium commercial value in counts was 10-fold greater inside rather than outside MPAs.Habitat was an important determinant of holothurian presence: occurrence of high and medium value holothurians was associated with rock and coral habitats.MPAs may help conserve high densities of holothurians of economic importance. © 2013 John Wiley & Sons, Ltd.

Kaiser-Bunbury C.N.,TU Darmstadt | Kaiser-Bunbury C.N.,University of Aarhus | Mougal J.,Seychelles National Parks Authority | Valentin T.,Seychelles National Parks Authority | And 2 more authors.
Applied Vegetation Science | Year: 2015

Questions: Which physical or chemical removal method is most effective in controlling woody invasive alien plant (IAP) species in sensitive conservation areas, while minimizing the impact on different life stages of native plant communities? Is there a distinct influence of removal method on successional trajectories of post-treatment seedling communities? Location: Mid-altitude inselbergs, Mahé, Seychelles, Indian Ocean. Methods: We conducted two field experiments. First, to test the effects of four IAP control methods (cut, 5%, 10%, 15% Tordon 101 herbicide concentration) on ten woody IAP species, we treated a total of 320 adults and monitored mortality for 18 wk. Second, we set up five blocks each with four plots (4 m × 4 m) at three inselberg sites (N = 60 plots) and recorded the response of seedlings, saplings and adults of all native and IAP species to four Tordon 101 (a.i. picloram and 2,4-D) herbicide and manual control treatments (cut, cut-stump, foliar spray, control) over 22 mo. Treatment effects were analysed with GLMM models, and community responses were investigated with NMDS and multivariate ANOVA. Results: The first experiment showed that IAP mortality (98.3%) was similar across all three herbicide concentrations, but markedly higher than mortality of cut plants (42.5%). In the second experiment, abundance, but not diversity, of native seedlings declined with herbicide application in the first 6 mo after treatment. Ordination analysis showed that seedling communities experienced high turnover driven by early successional IAP species and abundance, and that between-plot variation was determined by site-specific differences, with only a weak treatment effect. Foliar spray resulted in high mortality of native saplings (52.7%) and adults (18.3%), and both herbicide applications reduced adult growth over a post-treatment period of 12 mo. Conclusion: IAP removal controlled plant invasion processes and altered community composition in the short term, regardless of removal method. Post-treatment trajectories of seedling communities are determined by site-specific differences. Chemicals can strongly increase the efficacy of IAP control, but short-term detrimental effects on native saplings and adults may be expected. As re-invasion by IAPs is likely, a long-term restoration plan is required. Management strategies that aim to conserve β-diversity should employ a landscape approach to habitat restoration to buffer against site idiosyncrasies. © 2015 International Association for Vegetation Science.

Discover hidden collaborations