Effects of sandfish (Holothuria scabra) removal on shallow-water sediments in Fiji.

Abstract

Sea cucumbers play an important role in the recycling and remineralization of organic matter (OM) in reef sands through feeding, excretion, and bioturbation processes. Growing demand from Asian markets has driven the overexploitation of these animals globally. The implications of sea cucumber fisheries for shallow coastal ecosystems and their management remain poorly understood. To address this knowledge gap, the current study manipulated densities of Holothuria scabra within enclosures on a reef flat in Fiji, between August 2015 and February 2016, to study the effects of sea cucumber removal on sedimentary function as a biocatalytic filter system. Three treatments were investigated: (i) high density (350 g m−2 wet weight; ca. 15 individuals); (ii) natural density (60 g m−2; ca. 3 individuals); and (iii) exclusion (0 g m−2). Quantity of sediment reworked through ingestion by H. scabra, grain size distribution, O2 penetration depth, and sedimentary oxygen consumption (SOC) were quantified within each treatment. Findings revealed that the natural population of H. scabra at the study site can rework ca. 10,590 kg dry sediment 1,000 m−2 year−1; more than twice the turnover rate recorded for H. atra and Stichopus chloronotus. There was a shift towards finer fraction grains in the high treatment. In the exclusion treatment, the O2 penetration depth decreased by 63% following a 6 °C increase in water temperature over the course of two months, while in the high treatment no such change was observed. SOC rates increased ca. two-fold in the exclusion treatment within the first month, and were consistently higher than in the high treatment. These results suggest that the removal of sea cucumbers can reduce the capacity of sediments to buffer OM pulses, impeding the function and productivity of shallow coastal ecosystems.