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Peaceful coexistence between people and deadly wildlife: why are recreational users of the ocean so rarely bitten by sea snakes?

Citation

Udyawer, Vinay (2021), Peaceful coexistence between people and deadly wildlife: why are recreational users of the ocean so rarely bitten by sea snakes?, Dryad, Dataset, https://doi.org/10.5061/dryad.z8w9ghxbd

Abstract

1) Research on interactions between humans and deadly snakes has focused on situations that result in high rates of snakebite; but we can also learn from cases where snakes and people coexist peacefully. For example, coastal bays near Noumea, in the Pacific archipelago of New Caledonia, are used by thousands of tourists and snakes, but bites are rare.

2) Our long-term studies clarify reasons for this coexistence. Although 97% of snakes encountered in standardized snorkel surveys were a harmless species (Emydocephalus annulatus), we recorded dangerously venomous taxa often enough (one snake per eight hours snorkelling) that we would expect many risky human- snake interactions in these crowded bays. However, the risk is reduced by low overlap between humans and snakes in the timing of activity, both seasonally and on the diel cycle. Mate-searching male snakes, the group most likely to approach divers, enter the bays only in cooler months of the year when few beach users are present. Also, snakes tend to be active by night whereas people are not.

3) Risk is further reduced by spatial divergence: bare-footed beach users stay in sandy areas rather than the adjacent coral-reef areas that are preferred by snakes. The response of snakes to disturbance is important also: most sea snakes are reluctant to bite even when harassed. Water currents frequently push sea snakes against hard objects, perhaps explaining why the snakes do not interpret brief contact with a human as an attack. The ability of snakes to flee is increased by uniformly high body temperature, and a complex three-dimensional aquatic environment.

4) Thus, the danger of snakebite for recreational users of these popular beaches is reduced by aspects of human and snake behaviour that (i) decrease encounter rates, and (ii) render snakes unlikely to bite even if contacted. The risk to snakes is also reduced, because snakes are more difficult to detect and kill underwater than on land. As a result, thousands of snakes and people coexist harmoniously within these small bays.

Methods

This repository has two sets of data: 

1) Beach survey data of usage and behaviour of beachgoers at two tourist beaches in Noumea, New Caledonia

On five days in January 2019, four days in October 2019, and four days in January 2020 we walked along the beachfront of both bays at hourly intervals (daylight hours only during good weather) to record numbers of people in areas where the substrate was dominated by sand versus by fringing coral reef. These months represented seasonal periods where we expect high human-snake interactions, when numbers of recreational users are high (December-February) and when snake movements are at their peak during the winter mating season (August-October). The numbers of people in each ~200 m section of that transect were scored, as were their locations (in water vs on the beach), and on some surveys we also estimated age groups (children < ~10 years old, vs older people) and clothing (barefoot or not) as a function of activity (walking, swimming, snorkelling, etc.) and substrate type (sand vs coral). Our surveys encompassed the entire diel period of human use of the water, with no people recorded in the water at night during occasional nocturnal surveys on foot (as above) nor during 24 nocturnal observations (hourly, 1800 h to 0600 h over two nights) from a hotel balcony overlooking one of the bay (to verify absence of people in the water).

2) Acoustic telemetry data from telemetered sea snakes monitored within two bays in Noumea, New Caledonia

We hand-captured 4 Aipysurus duboisii, 2 A. laevis and 16 Hydrophis major in the Baie des Citrons and Anse Vata in January and October 2017, and a qualified veterinarian surgically implanted acoustic transmitters (V9P-2H; Vemco Ltd., Bedford, Nova Scotia) into the snakes under gaseous anesthesia. Transmitters weighed <1% (mean ± SE; 0.65 ± 0.17%) of snake body mass and were neutrally buoyant; snake behavior and locomotor ability were assessed during a 24-hour post-surgery recovery period and appeared to be unaffected by the implantation. The animals were released at their sites of capture the day after surgery, and their movements over the following 349 days were monitored with an array of 18 acoustic receivers that recorded detections of individuals when they were within detection range of each receiver. The detection range of receivers within the array was tested by comparing the expected and observed number of detections from range-testing tags placed at multiple locations within the array over a period of 1 week at the start of the study, and was measured to be on average 150 m. Receiver stations were arranged ~300 m apart through relatively shallow (<3 m depth) areas of the two bays, with all receivers remaining fully submerged and active across the full tidal range within the study site. We also obtained hourly data on tidal height (data from Service Hydrographique de la Marine Nationale) from which change in tidal height (henceforth hourly tidal range) and direction of flow (i.e., rising vs. falling tide) was calculated for each 60-minute observation period coinciding with movement records of telemetered snakes.

Usage Notes

Beach survey dataset is an excel spreadsheet with two worksheets a) Number of beach users; and b) beach users habits.

Acoustic telemetry data is in a R data object in the .RDS format that can be opened and accessed in the R statistical environemnt. The R object contained within is an ATT object to be used with the Animal Tracking Toolbox in the VTrack packge (https://github.com/rossdwyer/VTrack). The ATT object is a list object containing three components a) Raw tag detection data from the acoustic array in Noumea, b) tag metadata from animals tracked in this study, and c) receiver array metadata for the array used to monitor the telemetered snakes. See the following vignette on how movement metrics and activity space metrics can be obtained using this ATT object: https://vinayudyawer.github.io/ATT/docs/ATT_Vignette.html 

Funding

PADI Foundation, Award: 28454

Australian Research Council, Award: FL120100074

Laboratoire d'Excellence Corail

Laboratoire d'Excellence Corail