Loch Ness: a century of surveys and what each one measured

Loch Ness has been surveyed more thoroughly than almost any body of fresh water in Europe, and the reason is not limnology. Each generation has brought its best available instrument to the loch, which makes the case an unusually clean record of how the answer changes with the method.

Photography, and its collapse

The 1934 image known as the "surgeon's photograph", credited to the London gynaecologist Robert Kenneth Wilson, established the long-necked silhouette in the public imagination for sixty years. In 1994 it was reported as a staged photograph, based on an account given by Christian Spurling, who described a model built on a toy submarine.

The 1970s brought underwater photography by the American patent lawyer Robert Rines, whose "flipper" images were published alongside computer enhancement. The degree of enhancement applied, and what the unenhanced frames actually show, has been contested ever since.

Sonar

Operation Deepscan in 1987 remains the most ambitious sweep attempted: a line of boats moving down the loch in formation with echo sounders, covering it as a curtain rather than a series of tracks. The operation logged a small number of contacts at depth that were not accounted for.

Environmental DNA

In 2018 a team led by the geneticist Neil Gemmell sampled water throughout the loch and sequenced the DNA suspended in it, an approach that detects organisms from shed cells without needing to see or catch anything.

The survey found no reptilian DNA and nothing matching a large unknown vertebrate. It did find European eel DNA in quantity, at essentially every sampling point. Gemmell was careful about what that permits: it is consistent with eels being abundant, and it does not establish that any individual eel is unusually large.

That is where the evidence currently sits. The most-reproduced photograph is accounted for, the sonar returns are unexplained but unattributed, and the most sensitive survey yet conducted found a loch full of eels.