Corresponding author: Steven Crookes ( stecrookes@googlemail.com ) Academic editor: Marcela Uliano-Silva
© 2020 Steven Crookes, Tej Heer, Rowshyra A. Castañeda, Nicholas E. Mandrak, Daniel D. Heath, Olaf L. F. Weyl, Hugh J. MacIsaac, Llewellyn C. Foxcroft.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Crookes S, Heer T, Castañeda RA, Mandrak NE, Heath DD, Weyl OLF, MacIsaac HJ, Foxcroft LC (2020) Monitoring the silver carp invasion in Africa: a case study using environmental DNA (eDNA) in dangerous watersheds. NeoBiota 56: 31-47. https://doi.org/10.3897/neobiota.56.47475
|
Biodiverse habitats are increasingly subject to an intensification of anthropogenic stressors that may severely diminish species richness. Invasive species pose a dominant threat to biodiversity and biosecurity, particularly in biodiversity hotspots like Kruger National Park, South Africa. The invasive silver carp, Hypophthalmichthys molitrix, was introduced into the Olifants River and may experience range spread owing to favorable environmental conditions. Intensive monitoring protocols are necessary to effectively manage invasions of species like silver carp. Unfortunately, tropical and sub-tropical aquatic systems are difficult to monitor using conventional methods (e.g., netting, electrofishing and snorkeling) owing to a range of factors including the presence of dangerous megafauna. Conservation of such systems may be advanced by the adoption of novel methods, including environmental DNA (eDNA) detection. Here, we explore the utility of environmental DNA (eDNA) to conduct safe, reliable and repeatable surveys in dangerous watersheds using silver carp as a case study. We conducted eDNA surveys at 12 sites in two neighbouring watersheds, and determined that the species has expanded its range within the Olifants River and to the south in the Sabie River. Expansion in the former is consistent with the presence of suitable spawning conditions. We discuss the implications of this survey for biodiversity monitoring in similar aquatic systems in the tropics and advocate an integrative approach to biomonitoring in these ecosystems.
Biomonitoring, hazardous sampling, invasive species, Asian carp, species detection
Africa is home to some of the most diverse habitats on the planet, encompassing myriad climatic, geologic and biotic zones (
River systems are threatened by direct modification through channelization (
Monitoring the biotic component of river systems is essential to effective management of watersheds. For fishes, conventional monitoring of lotic habitats has traditionally relied upon methods including nets (e.g. seine, fyke, gill) or traps, angling, direct observation (SCUBA-diving or snorkeling), electrofishing, and telemetry and acoustic monitoring (
The Olifants River in southern Africa is home to dangerous aquatic megafauna animals including the common hippopotamus (Hippopotamus amphibius (Linnaeus, 1758)) and Nile crocodile (Crocodylus niloticus (Laurenti, 1768)) (
Silver carp (Hypophthalmichthys molitrix (Valenciennes, 1844)) was first introduced to South Africa in 1975, when individuals from a German population were donated to the Marble Hall experimental fish farm adjacent to the Olifants River (
Silver carp is a highly invasive fish extensively introduced from its native range in eastern Asia to Europe, North America, and southeast Asia (
We identified 12 sites in the Olifants watershed (Table
Sampling information and results of the qPCR analysis for the presence of silver carp eDNA at twelve sites in the Olifants watershed ‘PCR’ column indicates how many duplicate reactions for each of the three biological replicates were positive. The final column shows the mean Cq values across all positives (omitting non-amplifications) and their standard deviation (SD).
Site Code | Date | Site Description | River | Geographic Co-Ordinates | PCR | Mean Cq ± SD |
---|---|---|---|---|---|---|
O1 | 16/06/15 | Olifants River Weir | Olifants | -24.055824, 31.720335 | 0/0/0 | N/A |
O2 | 16/03/15 | East of Olifants Camp | Olifants | -23.982616, 31.775594 | 2/2/0 | 34.622 ± 0.688 |
O3 | 16/03/15 | Olifants/Letaba Confluence | Olifants | -23.989445, 31.826483 | 2/0/1 | 31.744 ± 0.087 |
O4 | 17/03/16 | Olifants River Gorge | Olifants | -23.985550, 31.848714 | 2/2/2 | 32.434 ± 1.267 |
O5 | 17/03/15 | South Africa Border | Olifants | -23.956183, 31.881781 | 0/1/2 | 34.884 ± 1.737 |
O6 | 17/03/15 | Letaba River Weir | Letaba | -23.942911, 31.731429 | 2/2/2 | 33.809 ± 4.149 |
O7 | 19/03/15 | Upper Olifants, Above Dam | Olifants | -23.943567, 31.902952 | 2/0/2 | 33.914 ± 2.261 |
O8 | 19/03/15 | Massingir Dam, Pelagic | Olifants | -23.921727, 31.956548 | 0/2/2 | 30.615 ± 2.161 |
O9 | 20/03/15 | Massingir Dam Wall | Olifants | -23.873332, 32.145614 | 1/0/2 | 33.549 ± 0.639 |
O10 | 21/03/15 | Coromana, Mozambique | Sabie | -25.184227, 32.033023 | 1/2/0 | 35.038 ± 0.091 |
O11 | 21/03/15 | Coromana, South Africa | Sabie | -25.185171, 32.031348 | 2/2/1 | 34.238 ± 1.619 |
O12 | 21/03/15 | Upper Sabie, South Africa | Sabie | -25.183838, 32.030184 | 2/0/1 | 36.208 ± 2.248 |
Map of sampling locations for silver carp eDNA within the continent of Africa (A). All sampled sites within southeast Africa (B) are shown as red circles. The two areas sampled are the Olifants system (Olifants and Letaba Rivers, grey inset) and the Komati (Sabie River, red inset). Also shown is the site of introduction and escape of silver carp in South Africa (red star) and the location of the Massingir dam, Mozambique (yellow star). Dotted line delimits the South Africa – Mozambique border.
At each site, three 2 L water samples (3 × biological replicates) were collected in sterile (10% bleach solution (6% w/v sodium hypochloride)) polycarbonate plastic Nalgene bottles. Unless access was difficult, all sampling was conducted by reaching from the bank to extract a sample from the top 5 cm of surface water in the littoral zone. Using single-use gloves for each sampling event, each sterile bottle was swept through the surface until filled. Each sample was immediately placed in a bleach-sterilized cooler and held at 4 °C during transportation back to the laboratory. Where direct access to the riverbank was difficult (e.g., large stretches with high embankments and prolific scrub vegetation), the site was accessed by boat and water was collected from as close to the shoreline as possible. At each site, a single Nalgene bottle containing 2 L distilled water (environment blank control) was opened and exposed to the environment before the top was resealed.
All water samples were filtered immediately upon return from the field in a central bleach-sterilized laboratory located in Skukuza, Kruger National Park, or in an ad-hoc, bleach-sterilized field laboratory near Massingir, Mozambique. Water was vacuum pumped through 1.2 μm pore glass fibre filters (47 mm diameter, VWR 696-filter). The filtration set-up included a tripartite manifold system of three funnels, each provisioned with a magnetic seal that securely clasped a filter between the funnel and the pump. Each biological replicate was filtered simultaneously in each of the three funnels (3 × filters per 2 L sample). After each sample was filtered, the entire apparatus and surrounding area was bleached sterilized, wiped with distilled water, and left to dry before proceeding with the next sample. After each filtration event, a separate set of sterile forceps was used to submerge each filter in a 2 ml Eppendorf tube containing 95% ethanol for storage at -20 °C. All samples were shipped to Canada for eDNA detection analysis.
eDNA extraction was performed in a dedicated extraction space. Using a protocol adapted from
The Asian carp invasion in North America has resulted in development of numerous eDNA assays to detect all four problem species, including two in the genus Hypophthalmichthys (
All qPCR reactions were performed in a laboratory with no previous history of Asian carp tissue or DNA samples. For each pooled eDNA extract, two duplicate technical replicates (PCR reactions) were performed. For all 12 sites, this tallied to 72 reactions in total, six per site/location and two per biological replicate. Alongside the target reactions (and environmental blank – one per location), two no-template controls were run to control for qPCR reagent/sample contamination. All reactions were performed on a single reaction plate, thereby eliminating inter-run variance in PCR results. All reaction volumes were 20 μl, consisting of 200 nM of each primer (0.4 μl), 10 μl of PowerUp SYBR green mastermix (Applied Biosystems, USA) and the remaining volume (9.2 μl) made up of eDNA extract providing the template for the reaction. To determine whether PCR inhibition may impede positive detection, we reassessed each sample using a separate internal positive control (IPC) assay that consisted of a primer and probe set that amplify a unique, and not found in nature, manufactured DNA sequence. We used the Taqman-probe based IPC developed by
We initially performed a post-hoc power analysis to confirm that the sampling design was sufficient to correctly reject the null hypothesis of no detection, thus boosting confidence in any negative finding. We used
where y = probability of a false negative (upper boundary of 0.05 (alpha)), j = number of samples to be subject to qPCR, and dˆ is the proportion of samples (out of three biological replicates) that yield ≥ 1 eDNA positive qPCR detection, to determine the predicted minimal number of samples necessary to achieve 95% power, predicated on our observed data.
Following the adoption of eDNA as a proxy of occupancy in habitat occupancy models (e.g.
To assess if the Olifants River is suitable for silver carp spawning, a preliminary assessment was completed using methodology developed by
Every site except for O1 was positive for silver carp eDNA in at least one technical replicate across three biological replicates (Table
The percentage of successful biological replicates yielding at least one detection (dˆ) was 69.40 %. Post-hoc power analysis revealed that the minimal predicted number of biological replicates per site should be two for this system, assuming 95% power, thus providing confidence that non-detection of silver carp at site O1 was unlikely due to a lack of appropriate sampling intensity. An occupancy analysis resulted in a high level of detection of silver carp in the study area. Estimated global probability of detection (p) was 0.849, inferred site occupancy (Ψ) was 0.892, and sample detection probability (θ) was 0.754. An analysis of variance indicated a near-significant effect of watershed and sampling site upon the levels of silver carp eDNA (F-value = 2.910, p = 0.0657; and F = 2.005, p = 0.0652, respectively) (Figure
Levels and variance of qPCR detection (y-axis) of silver carp DNA among sampling sites within and between rivers in the Olifants system and Sabie River (x-axis).
Preliminary spawning assessment indicates that the Olifants River was a minimally appropriate habitat in 2017–2018 (Figure
Results of a silver carp spawning suitability model for the Olifants River, July 1, 2017-June 30, 2018 A growing degree days (base 15). 633 GDD15 (red dashed line and solid red vertical line) is required for maturation and 900 GDD15 (blue dashed line and solid blue vertical line) is required for initiation of mass spawning B mean daily water temperate. Minimum temperature required (red dashed line) is always exceeded C unimpeded river length required for egg hatching to occur D mean daily water velocity. Flow spike required for high suitability is not achieved.
Environmental DNA detection confirmed the presence of silver carp eDNA throughout the sampled areas of the Olifants River and neighbouring systems (where sampled), except for Site O1. Our study adds to the mounting evidence confirming this technique as a sensitive tool to monitor highly invasive Asian carps generally (
eDNA detection levels were high (except Site O1), with at least half of all qPCR reactions positive for silver carp. Notwithstanding the likely nonlinear relationship between eDNA levels and population densities, the fact that silver carp was detected in half of all qPCR reactions at three sites for which the IPC (200 copies per reaction) was completely inhibited is consistent with a large, established population constantly shedding a lot of eDNA into the surrounding environs. These results indicate that silver carp is pervasive in the Olifants watershed, supporting the proposition that this species is established within South Africa (
One criticism of eDNA utility in lotic systems, however, is the potential systemic bias introduced by the directional flow of water. Until recently, the conveyance of eDNA downstream was thought to operate primarily on relatively small spatial scales (up to 12 km) varying by hydrology, season and target organism (e.g.
Due to the long unimpounded distance of the Olifants River upstream of Massingir Dam, along with high temperatures, this system is suitable for silver carp spawning. Temperature data in the river indicate that maturation can occur by early October and mass spawning by early November based on a July 1 winter start date. However, silver carp maturity can be reached within approximately three months, indicating that spawning could potentially occur at any point in the year. The minimum temperature required for spawning (17 °C) was always met in this system. There exists some uncertainty with this approach, primarily due to the estimate of velocity based on weir dimensions and the assumption of linearity in the hatching distance.
Our findings are consistent with those derived from ecological niche modelling, in which the middle-lower Olifants River was assessed to have a high predicted climatic suitability for silver carp establishment (>75%) (
The adoption of eDNA detection methods is especially pertinent in areas in which conventional monitoring can be inefficient or dangerous. These areas are often in the tropics, and harbor much of the world’s biodiversity, including charismatic freshwater megafauna (
The success of this pilot project to investigate the utility of using eDNA detection to identify the presence of the silver carp in two watersheds in southern Africa cannot be disputed, at least for AIS. However, at-risk species (SAR) show similarities with invasion-front species in that often their a priori distribution is unknown or merely suspected, and whose populations are fragmented and characterized by few individuals. Yet, eDNA has been shown to be just as effective at targeting SAR as it has AIS (e.g.
We recommend that eDNA detection be used as part of the conservation biologist’s toolbox when considering the management of invaders in dangerous aquatic ecosystems in the tropics and elsewhere. Moreover, future investigations should take into account the complexities of hydrodynamics when monitoring rivers, potentially by using hydrodynamic models (e.g.
We thank Pauli Viljoen for logistical and field support. We thank South African National Parks for financial and logistical support. OLFW acknowledges support received through the National Research Foundation – South African Research Chairs Initiative of the Department of Science and Innovation (Grant No. 110507). The CIB/DSI Centre of Excellence for Invasion Biology (CIB) for continued support. LCF acknowledges support from the DSI-NRF Centre of Excellence for Invasion Biology, Dept. of Botany and Zoology, Stellenbosch University, and the National Research Foundation (of South Africa, Project Numbers IFR2010041400019 and IFR160215158271). NEM, DDH and HJM were supported by NSERC Discovery grants and the NSERC CAISN Strategic Network, while HJM also was supported by a Canada Research Chair in Aquatic Invasive Species.