Revealing elasmobranch distributions in turbid coastal waters: insights from environmental DNA and particle tracking

Many sharks, rays, and skates are highly threatened and vulnerable to
overexploitation, as such reliable monitoring of elasmobranchs is key to
effective management and conservation. The mobile and elusive nature of
these species makes monitoring challenging, particularly in temperate
waters with low visibility. Environmental DNA (eDNA) methods present an
opportunity to study these species in the absence of visual identification
or invasive techniques. However, eDNA data alone can be difficult to
interpret for species monitoring, particularly in a marine setting where
its distribution can be influenced by water currents. In this study, we
investigated the spatial and temporal distribution of elasmobranch species
in two Special Areas for Conservation (SAC) off the coast of Wales. We
took monthly eDNA samples for one year (starting September 2020 and March
2022 for the northern and southern SACs, respectively), and used
metabarcoding to reveal the presence of elasmobranch species. We combined
these data with hydrodynamic modelling and particle tracking methods to
simulate the potential origins of the detected eDNA. We detected 11
elasmobranch species, including the Critically Endangered angelshark
(Squatina squatina) and tope (Galeorhinus galeus). Most detections were in
the spring and fewest in the autumn. The particle tracking simulations
predicted that eDNA was shed, on average, approximately 7 km and 15 km (in
the northern and southern SACs, respectively) from the sampling stations
at which it was detected. These results show that the two SACs represent
important areas for elasmobranchs in the United Kingdom and demonstrate
that eDNA methods combined with particle tracking simulations can
represent a new frontier for monitoring marine species.