Population trends of remote invertebrate resources in a marine reserve: trochus and holothurians at Ashmore Reef.

Abstract

Marine protected areas (MPAs) have a high capacity to protect fish and invertebrate resources, given adequate surveillance and enforcement. Ashmore Reef National Nature Reserve (Ashmore Reef) was closed to commercial fishing and harvesting of invertebrates such as trochus (Trochus niloticus) and holothurians in 1983. We evaluate population trends in trochus and holothurians during eight years of monitoring, focusing largely on the differences between their populations before and after a lapse of surveillance. The trochus population increased in density from 1998 to 2005, followed by a slight decline in all surveyed habitats in 2006. This decline followed approximately five consecutive months without surveillance. Amongst populations of 18 species of holothurians, densities declined in five, and remained relatively stable in the others. Densities of commercially valuable holothurians (primarily Holothuria whitmaei and H. fuscogilva) were too low to allow the detection of trends. Continuous enforcement of the fishing closure is important to ensure successful conservation of Ashmore Reef, as are standardized monitoring techniques to enable temporal trends to be detected with confidence.

Biophysical Habitat Features Explain Colonization and Size Distribution of Introduced Trochus (Gastropoda)

“…Highest densities of R. nilotica should be expected on shallow reefs with a relatively consistent depth across the reef front and complex surface habitat, corroborating earlier speculation in the literature (Smith, 1987;Rees et al, 2003;Purcell et al, 2009;Dolorosa et al, 2010). R. nilotica have previously been found to inhabit the higher-energy reef crest in their highest abundance with declining numbers onto the outer reef slope into deeper waters (Ceccarelli et al, 2011). The present study found an apparent gradation in the distribution of different-sized individuals, with smaller animals having higher abundance in shallow depths, especially on the reef crest.…”

“…The topshells were positively associated with reef surface complexity and this may be owing to their cryptic nature of requiring physical refugia (Gosliner et al, 1996;Dolorosa et al, 2010;Soekendarsi, 2018). The surface structural complexity of the reef is considered as an important factor dictating the distribution of marine invertebrates including R. nilotica (Ceccarelli et al, 2011;Dumas et al, 2013;Fabricius et al, 2013). The most suitable habitat for promoting high densities of R. nilotica are those with a high degree of structural complexity.…”

Tectus (Trochus) niloticus search for suitable habitats can cause equivocal benefits of protection in village-based marine reserves

“…In this case study, we investigated the quantitative effects of VMBR on the highly prized topshell trochus ( Tectus niloticus) . Trochus constitute spatially explicit structured resources whose heterogeneous distribution changes within small distances, under the combined—and sometimes antagonistic—influence of environmental conditions, biological interactions (intra/interspecific) and anthropogenic activities [ 32 – 34 ]. At the full reserve scale, which is usually the only considered in conservation or fishery management approaches, our results failed to demonstrate any positive effect of protection.…”

Small-Scale Habitat Structure Modulates the Effects of No-Take Marine Reserves for Coral Reef Macroinvertebrates

“…Yet, in the latter case, the synergistic effects of protection and punctual translocation of adult trochus individuals inside the reserve by local fishers may enhance reserve outcomes. On the other hand, decades may be required before some of the positive effects eventually become visible for severely depleted, vulnerable species exhibiting low, erratic reproductive success, such as giant clams, holothurians or large gastropods [39]–[42].…”