Environmental DNA and Hydroacoustic Surveys for Monitoring the Spread of the Invasive European Catfish (Silurus glanis Linnaeus, 1758) in the Guadalquivir River Basin, Spain

Abstract

The European catfish (Silurus glanis Linnaeus, 1758) was introduced into the Ebro Basin in Spain in 1974 for recreational fishing. Since then, the species has spread throughout the country's river basins, reaching the Iznájar Reservoir (Guadalquivir River Basin) in 2011. This area is of great ecological and economic relevance, especially as it includes the Doñana National Park, one of the most important nature reserves in Europe. Recently, the presence of catfish has been reported in the lower reaches of the river. In this work, we used non-invasive vertical and horizontal hydroacoustic surveys and environmental DNA river sampling to unravel the actual distribution and dispersal pattern of the species in the lower Guadalquivir River. The hydroacoustic profiles and the species-specific detections by real-time PCR (qPCR) and droplet digital PCR (ddPCR) showed that these non-invasive methods allow the detection and quantification of catfish and provide valuable information on the species' presence. We have confirmed the presence of catfish in most of the study area, including downstream areas of the Guadalquivir Basin. The results suggest the possibility of other isolated introductions and/or human-mediated movements of specimens, and imply that a coordinated catfish prevention and mitigation strategy is therefore urgently needed.

Keywords: Doñana National Park; eDNA; early detection; echosounder; monitoring.

Figure 1

Detection and quantification of Silurus glanis using eDNA and hydroacoustic surveys in the Guadalquivir River Basin. (A1) Iberian Peninsula showing the Andalusian region. (B1) The Andalusian region, showing the Guadalquivir River Basin and including the Doñana National Park location. (C1) Detailed map of the Guadalquivir River Basin in the province of Sevilla showing the locations sampled for hydroacoustics and catfish eDNA (qPCR and ddPCR). (C1.1) Detail of the El Gergal Reservoir in Sevilla. (D1) Detailed map of the Iznájar Reservoir in the Guadalquivir River Basin, province of Córdoba. In this figure, the squares represent catfish eDNA qPCR quantification results while the circles around them represent catfish eDNA ddPCR quantification results. For both molecular methods, a heatmap from color white to color red dots gradient indicates the number of copies obtained. Hydroacoustic data (aggregate counts) is also represented by a heatmap from white to green indicating the numerical density of individuals greater than 1 m, while the number of individuals per stretch is given numerically in a yellow circle. In (C1.1) each individual is displayed as a yellow dot (some of them overlap in the middle) and white dots indicate the location of the two negative water samples taken for eDNA analyses.

Figure 2

The abundance patterns of Silurus glanis using hydroacoustics and catfish eDNA qPCR/ddPCR dual mapping. (A2) Principal component analysis (PCA) biplot based on the correlation matrix of catfish eDNA qPCR and ddPCR copies in 2 L of water, hydroacoustic catfish detections (aggregated counts) upstream of the eDNA water sampling points ranging from 3 to 10 km and finally distances from river mouth (km) for each of the values. Spearman rank correlation test plot between (B2) distance from the river mouth and the hydroacoustic detection at 6 km of eDNA sampling, (C2) distance from the river mouth and the eDNA qPCR detections, and (D2) distance from the river mouth and the eDNA ddPCR detections. Different colored dots represent different correlation tests.