Despite their economic importance, the ecology of many sea cucumber species is poorly understood and factors influencing their habitat preferences remain largely unexplained. The distribution and habitat preference of two sea cucumber species; Holothuria atra and Holothuria edulis were studied off the north-west coast of Sri Lanka by underwater visual census in October 2008. The relationships between the density of each species and the habitat variables, such as mean grain size, organic content (% of dry weight), gravel (%), silt–mud (%), and depth, were examined using a generalized additive model. All these variables except silt–mud have significant influence (P < 0.05) on the habitat association of H. atra. The shallow water (<10 m) seagrass habitat with sediments characterized by 2–3.5% organic content, 15–25% of gravel and coarse sand (0.7–1.2 mm) were the most preferred conditions by H. atra. High densities of H. edulis were found in the shallow (<10 m) depths of rocky areas with algae and seagrass. Favoured bottom sediment conditions of H. edulis were mainly similar to the conditions preferred by H. atra, except organic content which did not significantly influence the habitat preference of this species. The preference towards the specific habitat characteristics seems to be associated with their feeding and protection. An understanding of habitat preference would be useful to improve the management of these sea cucumber populations and enable more precise stock assessment.

The sea cucumber stocks off the east and northwest coasts of Sri Lanka were estimated bysurveying 1307 km2 and 1779 km2 by an underwater visual census (UVC)in June and October 2008 respectively. The presence of twenty-five sea cucumber speciesbelonging to seven genera; Actinopyga, Bohadschia, Holothuria, Pearsonothuria,Stichopus, Thelenota and Acaudina was identified in fivehundred sampling sites randomly selected in each area. The overall average density (± SD) of sea cucumbers was higher in the northwest (350 ± 648 ind ha-1) than inthe east (90 ± 130 ind ha-1) and low-value species were predominant in bothsurvey areas (79 ± 125 ind ha-1 in the east, 244 ± 488 in the northwest) whencompared with medium (10 ± 34 ind ha-1, 105 ± 175 ind ha-1,respectively) and high-value category (<2 ind ha-1).The estimated total biomass in the northwest (13 024 t) was roughly four times greaterthan the east (3027 t). Holothuria edulis was the most abundant speciesin numbers while Holothuria atra had the highest stock biomass in bothareas. Sea cucumber densities were significantly different among the habitat(p < 0.001) and depth categories. The highestdensity was reported in rocky habitat associated with seagrass and / or macroalgae. Thedensity of low-value species was significantly high in 1–10 m depth category(p < 0.001) in the northwest while othercommercial groups were dominant in 20–30 m depth range in both areas. Temporal banning ofthe fishery for high and medium-value species would be a better management option to avoidfurther stock depletion.