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 [1-5]. 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 [6], the Academy of Applied Science (AAS) [7-12], and the Loch Ness and Morar Project [13] 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 [6] 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 [14].

Mackal [6] 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 [15,16]. 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 [16].

An environmental DNA (eDNA) study conducted at the loch in 2018 detected extraordinary amounts of mitochondrial DNA and nuclear DNA from eels [17], prompting principal investigator Neil Gemmell to further suggest the possibility of large eels in the loch [18]. A large eel-shaped animal was recently filmed in the River Ness by the Ness Fishery Board [19].

Simon [20] 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 [21] 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 [7] 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 [22,23] 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 [24] 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 [25] 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.

The length distributions for the Loch Ness and Zeeschelde eel samples are shown in the upper plots of Figure 1. The fits were similar for both skew normal and generalized extreme value distributions. The distribution for Loch Ness is evidently skewed toward lower lengths, similar to the length distribution of European eels presented in Figure 2 of Melia et al [29] for eels captured in the Camargue lagoons of France. The Zeeschelde distribution has notably less skew. The length distribution in a sample of eels in Vistonis Lake, Greece was skewed toward higher lengths (see Figure 2 of Macnamara et al [30]), suggesting much variability in eel length skew across different environments.

The probabilities of finding an eel at least as big as L in Loch Ness and in Zeeschelde are shown in the lower plots of Figure 1. Loch Ness has a comparatively low probability of finding larger eels 0.6-0.8 meters in length, and the probability of finding an eel in excess of 1 meter in length is very low for both environments.

Table 1 contains the estimated probabilities for specific lengths of interest. Results were somewhat similar for skew normal and generalized extreme value fits, and for both Loch Ness and Zeeschelde.

While the chance of finding a large eel approximately 1 meter in length in Loch Ness is low (around 1 in 50,000), this is certainly possible given the eel population of the loch: assuming a standing fish stock of 0.55 kg ha⁻¹ for Loch Ness [22,23] and given a surface area of 5600 hectares, the total standing fish stock of the loch is approximately 3080 kg. Further assuming that half of this stock is eel mass (plausible given the Loch Ness eDNA study [17]), this would imply 1540 kg of eel in the loch. The average Loch Ness eel mass is 0.1857 kg [6]; therefore, there are over 8000 eels in Loch Ness at a given time. Over the course of a few generations, an eel 1 meter in length may be expected.

However, this is not quite the “monster” postulated. Indeed, the probability of finding a 6-meter eel in Loch Ness is essentially zero—too low for the software used to provide a reliable estimate. Thus, while large eels may account for some eyewitness sightings of large animate objects rising to the loch surface, they are unlikely to account for “sightings” of extraordinarily large animals, which may instead be accounted for by wave phenomena, the occasional stray mammal, or other reasons.

This study used data on the distribution of European eel (A. anguilla) masses in an oligotrophic freshwater loch in Scotland to estimate the probability of finding an eel of extraordinary size there. Similar to other eel populations in Europe, the average eel length in Loch Ness is relatively small and highly comparable to biometric data on silver eels (sexually mature A. anguilla) collected in the same decade at the Girnock Burn fish trap on the River Dee in Aberdeenshire, Scotland (see Table 5.3 of Bašić et al [33]), suggesting Loch Ness eels are similar to those found elsewhere in the country. The findings of this study suggest that the chance of finding a 1-meter eel in the loch (1 in 50,000) is reasonable given the standing fish stock, and so some eels may account for purported sightings of somewhat large animals at the loch surface. For comparative purposes, Figure 2 shows an image of a female silver eel 1.05 meters in length. Other images of European eels in excess of 1 meter in length are provided on the web [28].

However, these analyses suggest that larger eels upward of 6 meters are highly improbable; therefore, “super” eels are an unlikely explanation for eyewitness reports of the very largest alleged animals at Loch Ness. Marine Scotland Science has reported growth rates of eels on the Girnock tributary of the River Dee in Scotland as high as 35.2 mm yr⁻¹ [33]. Assuming a linear rate of growth throughout the life cycle of an eel (in reality, the rate of change of an eel’s total length is an exponential decay curve, as described by Tesch and Thorpe [34]), it would take an eel in Scotland almost 30 years at such a growth rate to reach the 1-meter size. Given that the oldest eels can live for a number of decades [34], again it seems likely that approximately 1 meter is a realistic maximum length for eels in Loch Ness; a 6-meter specimen would need to live with constant high growth for almost 200 years, an age close to the longest-living fish, the Greenland shark (Somniosus microcephalus).

Though one European eel reportedly (unverified) lived to the grand age of 155 years [35], that specimen did not grow to a remarkable size because eel growth is nonlinear, slowing in older ages. Furthermore, the “breaching” behavior attributed to unknown Loch Ness animals (swimming upward and out of the water) is not a behavior that is characteristic of eels during migration or otherwise [34], especially as such behavior would represent unnecessary energy expenditure in a cold environment with relatively little food.

This analysis is limited by several factors. First, the Loch Ness eel sample used was relatively small at 129. Larger samples across longer time periods may provide more accurate estimates. Second, the assumption of a skew normal distribution would not hold if, for example, a larger sample revealed a bimodal distribution of eel lengths with a small peak at higher lengths. Third, this analysis is based on purely statistical considerations; the biological mechanism behind the physiological possibility of much larger eels is beyond the scope of this study. Some authors have suggested one such mechanism as neoteny [6] (ie, uncontrolled growth of the leptocephalus larva in A. anguilla preceding subsequent stages of development [36]). Fourth, environmental conditions such as temperature and available biomass impact eel growth and length; therefore, comparisons to other environments such as Zeeschelde may not be appropriate (ie, some of the data cited may not be relevant to the relatively cold waters of Loch Ness).

In conclusion, while Sagan [25] found that if there are any, there may be many, this study shows that if it’s real, it could be an eel, but not a very large one.