Boom‐and‐bust cycles of holothurian (sea cucumber) populations in the Chagos Archipelago: An indication of poaching or natural processes?

Abstract

Abstract The Chagos Archipelago is a remote Indian Ocean atoll of international conservation significance. Holothurians (sea cucumbers) are a major resource and influence reef health. Past populations have fluctuated from poaching and natural events. Visual censuses of holothurians were conducted in 62 shallow‐water transects within four atolls of Chagos in 2014, 4 years after creation of the Marine Protected Area, to determine its possible influence on holothurians. Comparison with data from the same locations in 2006 and 2010, using the same methodologies, showed significant overall population decline at Diego Garcia, recovery at Salomon (significant) and Peros Banhos (not significant), and no significant change on the Great Chagos Bank. The median abundance of Holothuria atra and Bohadschia sp(p.) decreased at Diego Garcia, while that of Stichopus chloronotus increased at Diego Garcia and Salomon atolls. Changes for other species were less marked. Diego Garcia, Salomon and Peros Bahnos showed a decline in diversity (no. of holothurian taxa and/or Shannon index, H), but diversity has been relatively stable on the Great Chagos Bank. All atolls, except Diego Garcia, are uninhabited and illegal fishing of holothurians, principally by Sri Lankan fishers, appears to have eased since 2005/2006, based on evaluation of 299 patrol survey reports. This may have led to the recovery or stabilization of populations in the outer atolls of Salomon, Peros Banhos atolls and the Great Chagos Bank. The reasons for holothurian decline at Diego Garcia, where exploitation is also prohibited (but cannot be ruled out), are unclear. Population patterns on all islands may reflect both natural fluctuations and disturbance events, including changing exploitation pressure. Conservation requirements include periodic re‐censusing of holothurians and ongoing monitoring of illegal fishing to help safeguard populations and the integrity of the Marine Protected Area.