The Effects of Hypoxia and Anthropogenic Disturbance on the Distribution, Abundance and Recruitment of the Sea Cucumber Holothuria leucospilota at the Mouth of a Subtropical Bay, China

Abstract

Despite the commercial importance of the sea cucumber Holothuria leucospilota increasing in recent years, little is known of its spatial distribution and ecology in the South China Sea. We investigate the distribution, abundance, and recruitment of H. leucospilota from 2017 to 2020 at two sites (S1 and S2) in Daya Bay, a typical subtropical bay which is strongly influenced by human activities in the South China Sea. We report hypoxia to drive H. leucospilota from deeper into shallower waters with higher DO concentrations at the mouth of Dapeng Cove in Daya Bay (S1), particularly during summer. Population size at S1 decreased by 90% from 2017 levels, before this area was opened to the public in 2018; recruitment was not observed by August (summer) of 2020. In contrast, in summers of 2017 and 2020, H. leucospilota abundance at S2, a protected open-water area, increased by 84%, and the proportion of small-sized (recently recruited) sea cucumber in the population increased by 20%. Severe summer hypoxia at S1 could negatively influence H. leucospilota spawning and larval settlement, which combined with depletion of broodstock because of fishing pressure and/or hypoxia-induced mortality, could inhibit recruitment. In contrast, higher DO concentrations and abundant broodstock during summer (the breeding season) favoured recruitment of H. leucospilota at S2. Overall, hypoxia and anthropogenic disturbance impede recovery of H. leucospilota at S1, while at the protected S2, larvae may be released to settle in nearby areas. It is imperative to develop a better understanding of the biology, ecology and conservation of tropical sea cucumbers in China.

Holothurian Fisheries in the Hellenic Seas: Seeking for Sustainability

“…As they collect all kind of holothurians with no size or quantity restrictions, they may easily cause the local depletion of populations. As holothurians are highly susceptible to the "Allee effect" [35] a reproductive failure is quite common in depleted populations beyond a certain threshold due to the sparsity of breeding adults [36][37][38][39]. In such cases, conventional management measures are insufficient for the replenishment of natural stocks and restocking of spawning densities by importing recruits from other source populations may be the only option [37].…”

Effects of Diet on Larval Survival, Growth, and Development of the Sea Cucumber Holothuria leucospilota

“…It is involved in exchange of nutrients and dissolved oxygen between the water column and sediment surface and alters the sediment biota and carbonate dynamics of its habitat. Because this species can endure a wide temperature range, and its coculture in aquaculture systems mitigates the effects of organic pollution (improving environmental quality), it is an ideal species to use in integrated multitrophic aquaculture (IMTA) systems [3,[7][8][9][10].…”

“…The widely distributed edible sea cucumber H. leucospilota occurs in shallow tropical and subtropical Indo-Pacific Ocean waters, where it forms an important part of traditional subsistence fisheries for some Pacific Island nations [ 1 , 3 , 5 , 7 , 8 ]. This species is also a major source of biologically active substances, such as polysaccharides and saponins for biotechnology and medicine, which further increases its market value [ 5 , 8 ].…”

“…Holothuria leucospilota has been severely overexploited and affected by environmental degradation [ 3 , 4 , 7 ] Throughout Asia it is fished in China, Malaysia, Thailand, Indonesia, the Philippines, and Vietnam, and where it is fished, there are few regulations pertaining to its harvesting [ 3 , 7 ]. Because it is accessible and population recruitment is slow, it is particularly vulnerable to overfishing, and once its stocks have decreased dramatically, they can take years to recover [ 2 , 6 ].…”

“…Both its overexploitation and the lack of harvest regulations in Malaysia could jeopardize H. leucospilota locally [ 4 ]. We have reported a population of this species at the mouth of a bay in China to have decreased by 90% over 4 years as a consequence of fishing pressure [ 7 ].…”