Tiger Shark Report Final

Tiger Shark Report and Dive Recommendations for Cocos Island National Park January 2017 Todd Steiner, Turtle Island Restoration Network and MigraMar, [email protected] Mark Stabb, Turtle Island Restoration Network Cocos Team, [email protected] On November 30, 2017, two divers had a serious encounter with a tiger shark at the Manuelita-Channel dive site in the Costa Rica National Park at Cocos Island, resulting in the death of one recreational diver, a park guest, and serious injury to the other diver, a dive master with the UnderSea Hunter group. This was the First known attack on a diver by a tiger shark at Cocos Island, which has been a popular recreational diving spot for nearly thirty years, and only the fourth recorded shark attack anywhere in Costa Rica1. The Circumstances: A recreational diver and her dive buddy (in this case, one of two divemaster’s with the group) surFaced early and not with the rest of the group. This is the normal dive practice when a group member is running low on air or bottom time. Near the surface, the divers were attacked by a single tiger shark. According to second-hand reports (I did not directly interview the injured divemaster), the divemaster placed himselF between the shark and the other diver, was bitten, pushed aside and the shark went after the other diver who sufFered a fatal bite. Both were pulled out oF the water and were rushed to a mother ship that coincidentally had two medical doctors onboard. The dive master’s bite was successFully treated; however, the recreational diver’s injuries proved too severe For treatment, and she died due to loss oF blood. At the time oF the incident, an “abort dive” signal was relayed to the rest of the divers, they surfaced without incidence and were transported back to the mother ship by a second skiFF sent to the scene. Earlier in the dive, the rest oF the group, led by divemaster Rodrigo Roersch, reported seeing a large Female tiger shark approach the group while they were near the bottom in at least 20 m oF water. The shark made several close passes to the group, causing them to retreat towards nearby rocks, beFore the shark swam away – presumably to then encounter the victims closer to the surFace. Divemaster Roersch, reported the shark’s behavior was very unusual in his [extensive] dive experience at Cocos Island. (“YoYo” Rodrigo Roersch, pers. comm with M. Stabb). We have requested to UnderSea Hunter personnel to know iF any photos or video oF this shark encounter are available, and we await a response. If there are such images, it may be possible to determine iF this individual was the same shark that caused an aborted dive on our expedition the following week (see below). 1 Tiger Sharks: Tiger Sharks (Galeocerdo cuvier) are large top-level predators as adults. Tiger sharks are generally considered solitary hunters that Feed primarily at night near the bottom or diurnally at the surface2. However, their mottled color pattern suggests that they may at times swim near the bottom, where they may remain unseen due to color pattern matching the background. They may then, like white sharks, dash upward to strike an object as the surface such as the red-Footed boobies that Form ‘raFts’ on the surFace around the island (Peter Klimley, pres. comm. w/T. Steiner). No general schooling behavior has been recorded; however, they are known to aggregate around concentrated sources of food including marine mammal carcasses, bird rookeries during Fledgling periods, and at sea turtle nesting site concentrations3. In some locations, tiger sharks are known to make regular, seasonal migrations to coincide with aggregations oF large prey. Tiger shark diets are extremely varied and includes Fish, sea turtles, seabirds, marine and terrestrial mammals, carrion and human Flotsam. A study on tiger shark stomach content in HI4 indicated ontogenic shifts in diet with prey diversity and frequency of large prey items increasing in larger sharks including more sea turtles, marine mammals and birds. Large (human-sized prey) were Found in sharks at >230cm. Yet even large sharks continued to eat bony fish, elasmobranchs (primarily sharks) and crustaceans. The occurrence of these various species do not necessarily reFlect the importance oF these prey to total caloric impact and may underestimate the importance large prey plays in the diet oF medium and large size tiger sharks. In a large study of tiger shark diets in South Africa (n= 628), mammals and birds became an increasingly important Functional part oF the diet, which included humans (Homo sapiens) recorded From the stomachs oF two tiger different tiger sharks (2.1 and 2.3 m).5 This study also indicated seasonal shifts in diets between Fishes and mammals depending on availability oF mammalian prey, and it suggested tiger sharks undergo asymmetric Feeding behavior indicating larger animals eat larger prey, yet small prey (Fishes) remains common in their diet. Tiger Sharks and Humans: Tiger sharks (Galeocerdo cuvier) are curious when in contact with humans6 and oFten approach divers closely but do not linger and then move on (pers. obs., T. Steiner). Tigers sharks are second only to white sharks (Carcharidon carcharias) in recorded attacks on humans with 111 (32 Fatal) recorded worldwide7. In Costa Rica (1907-present) only four shark attacks of any species have been recorded8. Tiger Sharks Agonistic Behavior: An aggregation oF as many as 16 diFFerent tiger sharks have been recorded feeding on a blue whale carcass in New Caledonia9 in the South Pacific. Here, several agonistic behaviors between sharks were recorded For the First time, including ‘”Give way,” “Stand back,” and “Tail slapping” behavior, as well as “feeding frenzy” behavior (see Table 1 for description of behaviors). During these observations, agonistic behavior was also sometimes directed at the vessels observing the shark feeding. “On several occasions, some of the largest sharks came straight to the platform of the main research vessel or the motor of the small tender, showing some ‘jaw gapping’ and ‘flank displaying’ (Video clip 6) as an agonistic behaviour toward the boat and people on board,” and “Most of the time, they would then ‘ram with their snout’ before backing off either with a ‘rapid withdrawal’ (Table 1, 2 Video clip 7) or by violently ‘slapping’ the boat or the motor with their tail (Table 1, Video clip 8). “ Tiger Sharks at Cocos Island: Tiger shark observations at Cocos were rare before 2007 and their numbers seem to be increasing yearly (Figure 1). Though not recorded in the primary reference on the fishes of Cocos Island10, we know of one early mention oF a tiger shark in Chatham Bay From the 192511, and a second report by Hans Haas in 195412. Since 2011, Cocos Island Research program of Turtle Island/CREMA, in association with MigraMar, have acoustically tagged 8 tiger sharks. The majority oF visual observations and acoustic recordings have been around the “Manuelita-Channel” and “Manuelita-Out” dive sites, but tiger sharks have been recorded at most of the regularly frequented dives sites at Cocos Island (Figure 2). Furthermore, one individual tagged shark, moved between as many as Four diFFerent dive sites on the same day and visited six diFFerent dives sites over a <7-month period, (Figure 3). With longer time-series records, the number oF individual sharks using multiple dive sites would likely increase. In recent months, anecdotal inFormation From long-time divemasters at Cocos has included seeing tiger shark aggregations at Cocos,: six sharks together at Manuelita Channel (Geovanny Castro, pers. comm with T. Steiner), seeing three tiger sharks consume a sea turtle near Silverado (“YoYo” Rodrigo Roersch, pers. comm.) though no accompanying whale or other animal carcasses. Diving Cocos after the incident: The Park Service immediately closed the Manuelita- Channel and adjacent dive sites Manuelita-Out and Manuelita-Coral Garden and also cancelled all night dives. Our research team arrived at Cocos a Few days later and was granted permission to dive these sites during daylight hours. We limited our diving at these sites to four total, allowing us to collect/service and redeploy two acoustic receivers at Manuelita-Channel and Manuelita-Out, and attempt to tag hammerhead sharks (primary species planned in advance For this mission). We dove these sites without incident, including observing one tiger shark at Manuelita-Out, near the sandy bottom that did not approach the research dive team. On the first day oF diving, while the research team was servicing equipment at the Manuelita-Out dive site, the other skiFF completed a dive at the Pajara islet a Few hundred meters to the southwest. Upon return toward the mother ship anchored at Manuelita island, the skiff was approached by a large tiger shark at the surface. The shark circled the boat – somewhat unusual behavior – and was photographed. A unique, characteristic ‘white spot’ was observed on the front of the dorsal fin of this shark. On the afternoon dive on Day 3 (12/7/17, at ~1515 hrs) a dive team made up oF a divemaster and a combination oF eight research permitees and recreational diver assistants, dove Ulloa (the islet just to the north west oF the “Lobster Rock” dive site shown on many maps), located approximately 2 km ESE oF the closed dive sites. On this dive, a large tiger shark was First sighted near the surFace when the dive team was at about 20 m deep. The shark rapidly descended to the depth of the divers, approaching the group multiple times getting closer with each pass, eventually approaching within 3-4 meters. The dive group tightened Formation, retreated to the rocky slope of Ulloa, and exited the water as quickly as was saFely possible.

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