Revealing Elasmobranch Distributions in Turbid Coastal Waters: Insights From Environmental DNA and Particle Tracking

Abstract

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 1 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 the 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.

Keywords: eDNA; hydrodynamic modelling; marine; metabarcoding; sharks and rays.

FIGURE 1

Sampling stations in the two Welsh Special Area for Conservation (SAC)s considered here: Carmarthen Bay and Estuaries (CBAE) (green area) and Pen Llŷn a'r Sarnau (PLAS) (blue area). The inset map in the bottom left illustrates the sites in the wider UK context. Each of the ten sampling stations in the two areas are indicated by black markers, and the abbreviated station names are given in full in the supporting information.

FIGURE 2

Elasmobranch detections by species and month across (a) CBAE SAC in 2022/23 and (b) PLAS SAC in 2020/21.

FIGURE 3

(a) Detections of Mustelus asterias in CBAE SAC (June 2022) and (b) detection of Galeorhinus galeus in CBAE SAC (June 2022). Black dots represent all sampling stations and triangles around a dot indicate an eDNA detection of the species at that station (in June 2022). The density score represents the percentage of particles present in each 1 km² grid cell over the 3‐day backtracking simulation.

FIGURE 4

Detections of Squatina squatina in PLAS SAC in March and May 2021. Black dots represent sampling stations and triangles around a dot indicate an eDNA detection of the species at that station. The density score represents the percentage of particles present in each 1 km² grid cell at every 5‐min time interval in the 3‐day backtracking simulation. The areal extent of the simulated particle dispersal covers 10 km² (March) and 42 km² (May).

FIGURE 5

Detections of G. galeus in PLAS SAC in April, May, July and August 2021. Black dots represent sampling stations and triangles around a dot indicate an eDNA detection of the species at that station. The density score represents the percentage of particles present in each 1 km² grid cell over the 3‐day simulation period. The areal extent of the simulated particle dispersal covers 7 km² (April), 65 km² (May) (across two detections), 27 km² (July) and 34 km² (August).

FIGURE 6

Connectivity heatmaps showing the backtrack particle mixing between stations for (a) CBAE SAC and (b) PLAS SAC for 3 days at 1 km² resolution. The colour scales indicate the number of particles as a percentage of the total particles released from each of the 10 stations. Stations are numbers 1–10 are arranged in a clockwise (CW) direction as follows: (a) TS, SA, PE, FE, SI, PB, BU, LL, PO and CA and (b) PO, CA, PE, BL, HA, DY, BA, TY, AB, BO (see Figure 1).

FIGURE A1

Number of detections for each elasmobranch species per month in CBAE SAC. Months are numbered according to the order January to December.

FIGURE A2

Number of detections for each elasmobranch species per month in PLAS SAC. Months are numbered according to the order January to December.

FIGURE A3

Species accumulation curves with 95% confidence intervals for elasmobranch species detection from eDNA in our surveys.

FIGURE A4

The maximum distances travelled (a) and surface area covered (b) by backtracked particles from each elasmobranch detection in CBAE SAC.

FIGURE A5

The maximum distances travelled (a) and surface area covered (b) by backtracked particles from each elasmobranch detection in PLAS SAC.