ABSTRACT1. Data are analysed from visual censuses of shallow-water holothurians (sea cucumbers) in 72 shallow water transects 100 m  2 m within four atolls of Chagos. Mean holothurian abundance in Diego Garcia, where harvesting is absent, was 18.5 individuals/transect (all transects) and 55.4 individuals/transect (only those containing holothurians). In the three exploited atolls, mean abundance did not exceed 3.5 and 5.2 individuals/ transect, respectively.2. Comparison with data collected during this study and an earlier investigation reveals a marked decline over four years in both mean and maximum density of commercially valuable Stichopus chloronotus and Holothuria atra in Salomon and Peros Banhos, both exploited atolls, and also for Holothuria nobilis in the latter.3. Holothurian counts were also made along an extensive transect (21 km  4 m) encircling Salomon atoll. Abundance showed highly significant negative correlation with fishing pressure, the latter estimated using an ordinal (0-3) scale (R s 5 À0.605, Pp0.01). Harvesting effects were not discernible using data from 200 m 2 transects.4. While recent studies have shown Chagos is virtually pristine regarding contaminant levels, its holothurian resources are under increasing pressure. Results from this study, and examination of Sri Lanka's fishing activity in distant waters, point to heavy and illegal harvesting.5. Stronger measures are needed to control the illegal fishery, to prevent holothurian abundances falling to the nonsustainable levels now prevalent across much of the Indo-Pacific, and to ensure that Chagos remains a biodiversity hotspot and environment of international renown. Use of smaller surveillance vessels would facilitate this.

We examine relationships between species richness (S), rarity (R) and average taxonomic distinctness (Δ + ) from analysis of a comprehensive dataset for benthic marine algae (including Cyanophyta). This comprises 2894 species from 66 sites across the Indian Ocean. Ranked values for the sites, determined according to the 3 metrics, show significant positive correlation (p ≤ 0.01); Mauritius, India and Aldabra emerge as biodiversity 'hotspots', while Indonesia (Nias Island), Maldives (Male Atoll) and the Gulf of Aden are 'coldspots'. Concordance between metrics was unexpected, given their disparity in robustness to sampling rigour and particularly since Δ + is conceptually unrelated to S and R. Lack of significant latitudinal correlations was also evident except for Δ + , which increased towards temperate waters in the southern hemisphere. This contrasts with the variable patterns observed with longitude, for which significant correlations (negative, i.e. towards the west) were prevalent only for S (algae overall and separate categories except Phaeophyta), evident for R (Cyanophyta only) and absent for Δ + . Hence, use of one floral category as a surrogate for biodiversity in another is not guaranteed. Aquatic biodiversity patterns are complex, in accordance with recent findings derived mainly from faunal datasets. Relationships between different metrics can depend on both the group(s) selected and the environmental or geographical factor(s) examined. Our study is based on analysis of extensive but low resolution (presence/absence) data (Silva et al. 1996) collected from sites of variable size that were not sampled evenly. We address these constraints, but acknowledge the possibility that some patterns may prove to be artefacts, pending analysis of data from recent and ongoing studies. However, we do not expect this to significantly affect our overall conclusions.KEY WORDS: Biodiversity · Species richness · Rarity · Average taxonomic distinctness · Latitudinal and longitudinal patterns · HotspotsResale or republication not permitted without written consent of the publisher

ABSTRACT1. This study examines the perceptions of 500 Sri Lankan fishers about influences on the outcome of the 2004 Asian tsunami. It is based upon analysis of questionnaire data on 13 natural environmental and development risk factors, in relation to human deaths and house damage (impact indicators).2. Mangroves, coral reefs and sand dunes afforded protection against tsunami damage (67-94% of fisher responses), as did housing and roads.3. Fishers overall believed rivers/estuaries, concave coastlines and hotels exacerbated impacts. However, a significantly greater proportion of fishers living within 100 m of the coast reported that rivers/estuaries had a protective role than those living further inland. Rivers seemingly diverted 'tsunami water' far inland, where it overflowed and caused damage.4. Risk and damage are multi-faceted concepts and measurable in different ways. Findings are considered in the light of ecological studies and modelling, with special reference to mangroves, whose alleged protective role has become equivocal during post-tsunami research.5. Insights of fishers and other communities with intuitive knowledge add a valuable perspective to the understanding of natural disasters and environmental change. This approach is seen as complementary rather than an alternative approach to purely 'scientific' research.