What did the 2018 eDNA study of Loch Ness find and how definitive are its results?
Executive summary
The 2018 environmental DNA (eDNA) survey of Loch Ness collected 250 water samples and generated hundreds of millions of DNA sequence reads to inventory life in the loch, finding no genetic evidence for a population of Jurassic‑age reptiles or other large unknown vertebrates but detecting unusually high levels of European eel DNA and DNA from many land‑based species [1] [2] [3]. The results narrow some long‑standing hypotheses about “Nessie” but are not an absolute disproof: methodological limits, sampling windows, and gaps in reference databases leave qualified room for alternative explanations [4] [5] [6].
1. What the team actually did and why it mattered
Professor Neil Gemmell and collaborators sampled Loch Ness in 2018, taking roughly 250 water samples from surface to depth and sequencing environmental DNA to create a broad biodiversity snapshot, comparing millions of sequence reads to reference genome databases to identify species present in the water [1] [7] [8].
2. The headline findings: eels, everyday animals, and no plesiosaurs
The survey catalogued thousands of taxa and reportedly found evidence consistent with some 3,000 species in and around the loch, including fish, amphibians and large amounts of DNA from humans, dogs and livestock, but it did not recover DNA that matched plesiosaurs, sharks or other large marine reptiles that would match the classic “monster” image [9] [3] [8] [10].
3. The eel signal and what scientists said about it
One of the clearest signals was a substantial amount of eel DNA across many sampling locations, leading Gemmell and others to say a large eel population is the only unruled‑out biological explanation consistent with the eDNA data, and thus a plausible natural explanation for many sightings [3] [11] [12].
4. Strengths of the study: scale and modern methods
The project applied contemporary eDNA metabarcoding at scale — hundreds of samples and on the order of hundreds of millions of sequence reads — producing a detailed inventory that both reaffirmed expected species and revealed surprising terrestrial signatures, illustrating eDNA’s power for rapid ecosystem surveillance and invasive‑species detection [8] [1] [4] [13].
5. Important limitations that prevent absolute conclusions
eDNA cannot directly measure individual size or rule out rare, transient or deep‑dwelling organisms missed by sampling; DNA signals in water dissipate in days to weeks, sampling occurred in specific months, and some recovered sequences lacked matches in reference databases, leaving an unresolved fraction and the theoretical possibility of undetected organisms [4] [5] [6]. Critics have flagged that an unidentified percentage of sequences — variously reported by commentators as around 20–25% — constitutes a gap that the team’s public statements downplayed, and that absence of evidence in these samples is not proof of absolute absence at all times [5] [6].
6. Alternative readings and the media narrative
News outlets emphasized the “no plesiosaur” angle and the intriguing eel result, while specialists noted more nuance: eDNA effectively rules out many large‑animal hypotheses (e.g., sharks, sturgeon) based on the samples analyzed, but it cannot confirm that any eel is enormous — sequence data cannot specify organism size — nor can it rule out non‑matched or extremely rare taxa that were not represented in reference libraries or not shedding detectable DNA at sampling times [8] [2] [11] [4].
7. Bottom line: how definitive are the results?
The 2018 eDNA study is decisive in eliminating several specific biological candidates for Nessie (notably plesiosaurs and large marine reptiles) within the limits of the samples and databases used, and it compellingly points to abundant eels as a plausible natural source for many sightings; however, it is not an absolute, time‑independent disproof of every hypothetical creature because of sampling windows, environmental DNA decay, and remaining unidentified sequence data [10] [3] [4] [5].