Bakker, J.

In the era of “Anthropocene defaunation,” large species are often no longer detected in habitats where they formerly occurred. However, it is unclear whether this apparent missing, or “dark,” diversity of megafauna results from local species extirpations or from failure to detect elusive remaining individuals. We find that despite two orders of magnitude less sampling effort, environmental DNA (eDNA) detects 44% more shark species than traditional underwater visual censuses and baited videos across the New Caledonian archipelago (south-western Pacific). Furthermore, eDNA analysis reveals the presence of previously unobserved shark species in humanimpacted areas. Overall, our results highlight a greater prevalence of sharks than described by traditional survey methods in both impacted and wilderness areas. This indicates an urgent need for large-scale eDNA assessments to improve monitoring of threatened and elusive megafauna. Finally, our findings emphasize the need for conservation efforts specifically geared toward the protection of elusive, residual populations.

Sharks are charismatic predators that play a key role in most marine food webs. Their demonstrated vulnerability to exploitation has recently turned them into flagship species in ocean conservation. Yet, the assessment and monitoring of the distribution and abundance of such mobile species in marine environments remain challenging, often invasive and resource-intensive. Here we pilot a novel, rapid and non-invasive environmental DNA (eDNA) metabarcoding approach specifically targeted to infer shark presence, diversity and eDNA read abundance in tropical habitats. We identifed at least 21 shark species, from both Caribbean and Pacific Coral Sea water samples, whose geographical patterns of diversity and read abundance coincide with geographical differences in levels of anthropogenic pressure and conservation effort. We demonstrate that eDNA metabarcoding can be effectively employed to study shark diversity. Further developments in this field have the potential to drastically enhance our ability to assess and monitor elusive oceanic predators, and lead to improved conservation strategies. 

Abstract:

Recruitment of the sea urchin Diadema antillarum philippi, 1845 was studied on artificial recruitment panels along the leeward coast of the island of Curaçao, southern Caribbean. data were compared with historical data from the same coast that were collected before (1982–1983) and after (1984) the Caribbean-wide mass mortality of Diadema in October 1983. Average recruitment rates observed in 2005 were equal to 2.2 times lower compared to those observed before the D. antillarum die-off (1982 and 1983), but 56.5 times higher than those observed after the die-off in 1984. The increase in recruitment rates between 1984 and 2005 was 5–51 times greater than the increase in abundance of adult individuals over the same period. This suggests that despite the largely recovered recruitment rates of this important reef herbivore, unknown sources of high post-settlement mortality currently prevent a similar recovery of its adult population.