Abstract

Background: Previous studies have estimated the size, mass, and population of hypothetical unknown animals in a large oligotrophic freshwater loch in Scotland based on biomass and other observational considerations. The “eel hypothesis” proposes that the anthrozoological phenomenon at Loch Ness can be explained in part by observations of large specimens of European eel (Anguilla anguilla), as these animals are most compatible with morphological, behavioral, and environmental considerations.

Objective: This study expands upon the “eel hypothesis” and related literature by estimating the probability of observing eels at least as large as have been proposed, using catch data from Loch Ness and other freshwater bodies in Europe.

Methods: Skew normal and generalized extreme value distributions were fitted to eel body length distributions to estimate cumulative distribution functions from which probabilities were obtained.

Results: The chances of finding a large eel in Loch Ness are around 1 in 50,000 for a 1-meter specimen, which is reasonable given the loch’s fish stock and suggests some sightings of smaller unknown animals may be accounted for by large eels. However, the probability of finding a specimen upward of 6 meters is essentially zero; therefore, eels probably do not account for sightings of larger animals.

Conclusions: The existence of exceedingly large eels in the loch is not likely based on purely statistical considerations. (Reviewed by the Plan P #PeerRef Community).

JMIRx Bio 2023;1:e49063

doi:10.2196/49063

Keywords

Introduction

Loch Ness is a large oligotrophic freshwater loch located along the Great Glen Fault in Scotland. Since the 1930s, purported sightings of unknown animals in the loch have featured prominently in popular media, but to date, no specimen has been obtained despite numerous efforts, making the probability of such animals unlikely.

The authenticity and interpretations of photographs and films allegedly depicting unknown animals in Loch Ness have been seriously doubted [-]. In the 20th century, systematic searches with submersibles, sector-scanning sonar surveys, hydrophones, underwater photography, long-lining, and trawling undertaken by the Loch Ness Investigation Bureau [], the Academy of Applied Science (AAS) [-], and the Loch Ness and Morar Project [] have returned only ambiguous sonar signals, low-quality photographs, and unidentifiable sound recordings.

In the 1970s, a sample of European eels (Anguilla anguilla) was collected from Loch Ness with baited traps. The distribution of eel masses was skewed, which led biologist Roy Mackal [] to conclude that large eels may exist in the loch. Eel body structure and function are characterized by an elongated body form, a single pair of pectoral fins, strong musculature and high-amplitude winding movement, and a durable integument with a thick epidermis and dark chromatophores [].

Mackal [] noted that large eels would therefore be consistent with eyewitness descriptions of Loch Ness animals including reports of an elongated head-neck, pectoral fins, extreme flexion, and dark integument [,]. This explanation has also been reviewed by naturalists Adrian Shine and David Martin of the Loch Ness and Morar Project, who noted that eels migrate via the River Ness and that most “sightings” occur near river mouths [].

An environmental DNA (eDNA) study conducted at the loch in 2018 detected extraordinary amounts of mitochondrial DNA and nuclear DNA from eels [], prompting principal investigator Neil Gemmell to further suggest the possibility of large eels in the loch []. A large eel-shaped animal was recently filmed in the River Ness by the Ness Fishery Board [].

Simon [] suggests that the physiologically possible maximum length of A. anguilla is 0.5-1.3 meters, which is not particularly monstrous. Using wave mechanics, LeBlond and Collins [] estimated the size of the subject depicted in the infamous “Surgeon’s Photograph” at Loch Ness at 0.6-2.4 meters.

Much larger estimates have been made. Based on their “flipper” photograph, Scott and Rines [] estimated a total body length of 15-20 meters for an unknown Loch Ness animal. These estimates seem inconsistent with Loch Ness biomass calculations. Sheldon and Kerr [,] estimated a population of 156 if the individual mass of a hypothetical unknown animal in Loch Ness is 100 kilograms and suggested just one individual can exist if its mass is 2000-3000 kilograms. Scheider and Wallis [] estimated that the loch may support 157 animals of 100 kilograms each and 10 animals of 1500 kilograms each.

Finally, in an article titled “If there are any, could there be many?” Carl Sagan [] used collision physics and suggested that a population of 300 animals that were 10 meters in size each would be consistent with AAS observations. Thus, if there are any, there may be many. If it’s real, could it be an eel? The aim of this study is to estimate the probability of finding various sizes of eel in Loch Ness based on available catch data.

Methods

Data

Data on the mass distribution of A. anguilla in Loch Ness were taken from 129 eels caught between 1970 and 1971 under the supervision of the Loch Ness Investigation Bureau and described in Mackal [].

Mackal [] does not provide the length distribution for the sample nor did Mackal [] directly regress length on mass. Mackal [] did regress functions of length on mass and maximum circumference but provided only nonlinear equations unsuitable for solving s