Shark encounters in the Molokai Channel have been noted since International Marathon Swimming Hall of Famer Greta Andersen attempted a Molokai Channel crossing in 1958. Towards the end of her attempt, she was reluctantly forced out due to tiger sharks. More recently, a 2016 tiger shark encounter with Ranie Pearce was captured on video (see below).
And in 2019, Captain Mike Twigg-Smith was escorting three Molokai Channel swimmers in a single season who had encountered or been attacked by sharks during a five-month period.
Channel swimmer Dr. Steven Minaglia and open water swimming biostatician Melodee Liegl looked into this issue and wrote a scientific paper “Moonless night sky increases Isistius species and live human contact” that was recently published on PLOS ONE – see here.
Their abstract reads, “The nocturnal feeding behavior and zoogeographical habitat of cookiecutter sharks Isistius brasiliensis and Isistius plutodus (Isistius spp.) greatly reduces interaction of this species with live humans. Attacks on live humans are exceedingly rare with 7 cases reported worldwide, 6 of them in Hawai‘i, and 5 of these occurring among channel swimmers.
Published research suggests that periods of bright moonlight may increase Isistius spp. contact with live humans and does not otherwise identify significant trends or risk factors.
Yet 5 of the 6 Isistius spp. bites on live humans in Hawai‘ian waters occurred with the moon set and after nautical twilight end and before nautical twilight start.
From 1961-2023 in Hawai‘i, 129 successful solo channel crosses and 5 Isistius spp. related injuries in the habitat of cookiecutter sharks were analyzed across two groups:
- one where both the moon and sun were set (dark group) and,
- one where the moon and/or sun was in the sky (light group)
There was a significant difference for swimmers bitten by Isistius spp. in the dark 4 (12%) versus light groups 1 (1%), p=0.012, RR 12.6 (95% confidence interval: 1.5-108.9). Swim start time and year was also significant (Pearson correlation 0.566, p <0.001).
Swimmer gender and use of shark deterrent devices and artificial illumination were not significant.
The growing popularity of channel swimming in Hawai‘i and swim start times have contributed to an increasing likelihood of live human and Isistius spp. contact and a moonless night sky is a significant risk factor for this interaction.
Dr. Minaglia writes, “The data is small, but I think it is timely and will influence behavior in a positive way. It shows a striking risk profile (1-in-8 chance of cookiecutter shark bite with moon and sun set with no light in the sky versus a 1-in-101 chance with moon and/or sun in the sky) when crossing the deepest part of the deep channels in Hawaii. The paper is worth disseminating at this time because of its commentary on human safety.
Cookiecutter Sharks
Dr. Minaglia and Liegl write in their paper, “The two species of cookiecutter sharks, Isistius brasiliensis and Isistius plutodus (Isistius spp.) are best known for the characteristic wound they leave on their prey items and for their “hit and run” feeding behavior recently witnessed by live humans. These pelagic sharks remove circular plugs of flesh by sucking onto their prey using specialized pharyngeal muscles and fleshy lips, inserting their upper jaw teeth, and sweeping their proportionately larger lower jaw teeth through the flesh often twisting in a circular motion to remove and ingest their feed. The resultant circular, concave wound is a characteristic sign of an Isistius spp. bite. Cookiecutter sharks are thought to undertake diel vertical migrations with a daytime depth range between 1,000–3,500 meters in primarily warm, coastal waters near islands. They typically rise near the surface at night to feed on squid, small fish, and a variety of large pelagic species and then return to the bathyal zone when daylight resumes [5–7]. They possess a well-developed lateral line system, bioluminescence, and large eyes that give them a competitive advantage over their prey especially in low-light settings.”
Citation: Minaglia S, Liegl M (2024) Moonless night sky increases Isistius species (cookiecutter shark) and live human contact. PLoS ONE 19(2): e0291852. https://doi.org/10.1371/journal.pone.0291852
Editor: Daniel M. Coffey, Texas A&M University Corpus Christi, United States
Received: September 6, 2023; Accepted: January 18, 2024; Published: February 21, 2024
Acknowledgements
Carl Kawauchi, Michael Twigg-Smith, Bill Goding, Ivan Shigaki, Mike Spalding, Adherbal de Oliveira, Eric Schall, Isaiah Mojica, Andy Walberer, Steven Munatones, Dr. Harry Huffaker, Dr. Patrick Pedro, Dr. Darrell Takebayashi, and the swimmers of the Hawaiʻian channels provided information necessary to complete this research.
Publication copyright: © 2024 Minaglia, Liegl
© 2024 Daily News of Open Water Swimming
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