Bertarelli-Annual-Report-2024-LR - Flipbook - Page 40
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Bertarelli Foundation
Grey reef shark © George Duffield
Environmental DNA
Helps Reveal Reef Shark
Distribution Across a
Remote Archipelago
Dunn, N., Curnick, D.J., Carbone, C., Carlisle, A.B.,
Chapple, T.K., Dowell, R., Ferretti, F., Jacoby, D.M.P.,
Schallert, R.J., Steyaert, M., Tickler, D.M., Williamson,
M.J., Block, B.A., Savolainen, V. (2023) ‘Environmental
DNA helps reveal reef shark distribution across a
remote archipelago’ - Ecological Indicators https://doi.
org/10.1016/j.ecolind.2023.110718
Environmental DNA assessments revealed that Illegal,
Unreported and Unregulated (IUU) fishing is having
a significant impact on the shark community in the
northern atolls of the Chagos Archipelago, depleting
local reef shark populations.
Approximately one-third of elasmobranchs (sharks, skates
and rays) are threatened with extinction as a result of
overfishing and overexploitation. The Chagos Archipelago
is encompassed by a large no-take MPA, but continued
IUU fishing activity has resulted in reef shark population
declines. Presently, IUU activity is focused around the
northern atolls of the MPA, as the presence of a military
base on Diego Garcia atoll (ca. 200 kilometres from the
northern atolls), effectively deters IUU activity in the
local vicinity.
As many elasmobranch species are highly mobile and
elusive, having a versatile methodological toolkit for
species monitoring has become increasingly important
in the face of these declines. Environmental DNA
(eDNA) methods are being increasingly used in proof-ofconcept studies to detect shark species and are widely
seen as the future of biodiversity monitoring, but have
yet to become fully established. This study assessed
whether species-specific eDNA methods could infer
the distribution of the two shark species – grey reef
shark and silvertip shark (Carcharhinus albimarginatus) –
around the atolls of the Chagos Archipelago.
Water samples were collected from Diego Garcia atoll
in the south and from the northern atolls (Peros Banhos,
Salomon atolls and Nelson Island on the Great Chagos
Bank), complemented by visual dive surveys and acoustic
receiver data from previous tagging studies. DNA was
extracted and assayed using species-specific primers.
Shark eDNA detections were significantly reduced and
sporadic around the northern atolls, which are under high
pressure from illegal fishing. By contrast eDNA detections
of both species were ubiquitous and consistent around
the highly protected atoll of Diego Garcia. In the northern
atolls visual and acoustic telemetry techniques did not
correlate with eDNA detections, suggesting further
optimisation of eDNA methods is needed for detecting
sharks. However, both species were detected by eDNA
in sites where they were not observed, showing that
eDNA methods can be used to fill gaps in data from more
established monitoring techniques. These findings suggest
that eDNA methods are a useful tool but should be used in
combination with other techniques to provide a complete
picture of shark distribution so that threatened species can
be better protected.
