Cayman Reef Sharks

That shark you saw last summer at Little Cayman may be the same shark you saw the summer before on Grand Cayman. That's one bit of information from a nearly decade-long study of Cayman sharks discovered, and there are many more surprises found by the researchers.

Grand Cayman and Little Cayman are 59 nautical miles apart, and Little Cayman is just 3.5 nautical miles (nm) from Cayman Brac. They have narrow coastal shelves - 43 percent are Marine Protected Areas - dropping to more than 2000 meters deep.

Beginning in 2010, researchers tagged 66 reef sharks off Little Cayman (60 percent), Grand Cayman (26 percent), and Cayman Brac (14 percent). They placed 57 acoustic receivers at various spots, covering about 7 percent of the coastal shelf. Through 2019, divers periodically retrieved the receivers, and the data was downloaded and the receivers reattached to their mooring lines. The last data downloaded was in 2019. Data yielded 77,651 shark detections, 11 percent at Grand Cayman, 54 percent at Little Cayman, and 35 percent at Cayman Brac.

Residency

The sharks were classified by their behavior: Passer-by, Transient, Pseudo-resident, and Resident. Only 13 individuals were found to be residents, with a linear home range averaging 11 nm, although more than half traveled less than 5.5 nautical miles. Six sharks (11 percent) were detected at more than one island, and three were detected at both Grand Cayman and the Sister Islands. All inter-island movements occurred from April to September; a mature male and a mature female repeated their inter-island movement in two non-consecutive summers.

Sharks appeared to concentrate more north and southeast of Grand Cayman, northwest, west, and south of Little Cayman, and east and south of Brac. The reasons may be food availability and avoiding human activity. On Grand Cayman, boat traffic and water sports are concentrated along the west and northwest, so the sharks avoid the area. On the south side of the Sister Islands, they may seek more exposure to wave action and strong currents. Many areas were identified as the primary site of only one individual.

This study recorded the first documented movement by Caribbean reef sharks over distances greater than 27 nm, traveling as far as 80 nm across the ocean deeper than 6,500 feet. This suggests reef sharks likely travel to Cuba (77 nm from Cayman Brac) and Jamaica (112 nautical miles). Most Caribbean reef sharks are either migrants that come and go or nomads that pass by once, spending only a limited amount of time in one area. The Caymans may be a "navigation point" or "pit stop" en route from one Caribbean region to another.

Some Caribbean reef sharks undertook movements far beyond the usual home range. Movements by mature individuals could result from foraging, social interactions, reproductive requirements, or investigating potential foraging grounds while avoiding the territories of others. Immature sharks moved strictly between Little Cayman and Cayman Brac, following a direct path along the upper reef with less exposure to potential predators.

Females were detected more frequently on receivers that did not detect males (on Grand Cayman and Cayman Brac) and were detected less frequently on receivers that detected both sexes (on Little Cayman), suggesting that females might seek to avoid male encounters to mitigate energy demanding mating activities outside of the mating season. In contrast, male sharks were more mobile than females, perhaps a mate-searching behavior. Surprisingly, despite having strong site fidelity, females in this study made long-distance movements more often than males, indicative of possible migrations to more favorable areas for parturition and survival of pups.

All long-distance movements between islands occurred between May and August, peaking in July. Only mature sharks moved between Grand Cayman and the Sister Islands, and a few sharks made multiple trips between islands. This behavior coincided with the months when mating scars on female Caribbean reef sharks are evident and diver observe newborn Caribbean reef sharks.

Mature individuals of both sexes undertook repeat trips between islands in non-consecutive years. The annual, or in females, the biennial reuse of a particular area has been linked to the apparent biennial reproductive cycle of females in most large-bodied shark species in which the gestation is thought to be approximately one year; females can give birth every two years.

Finally, the sharks showed no significant patterns of regular (diel) occurrence or movement behavior. This was a surprise because it was expected that the number of individuals detected and their space use would increase at night because of greater night foraging activities. Diel behavior is widespread in sharks due to feeding behavior, predator avoidance, energetic advantages, and environmental conditions. Caribbean reef sharks have been reported to exhibit a diel shift in habitat, with sharks tending to occur deeper (e.g., outer reef, deeper depth) during the day and shallower (e.g., inside lagoons, shallower depth) during the night, behavior that has been linked to both foraging success and predator avoidance.

The study provided an understanding of the movements of Cayman reef sharks, which is essential for the conservation and management of the species. Although the Marine Protected Areas (MPA) have proven beneficial for reef fish, this study shows that individual sharks' home ranges extend to areas outside the protected zones, where they can be exposed to recreational fishing. The realization that existing MPAs were unlikely to provide adequate protection for threatened shark species was key to passing legislation introduced in 2015, giving blanket protection to all sharks throughout Cayman waters. MPAs now comprise 48 percent of the Cayman shelf.

Last year, scientists from the Department of Environment estimated that there are 180 Caribbean reef sharks and 336 nurse sharks in the Cayman Islands. Hammerheads, lemon, blacktip and tiger sharks are too few to estimate populations.

Citation: Kohler J, Gore M, Ormond R, Johnson B, Austin T (2023) Individual residency behaviours and seasonal long-distance movements in acoustically tagged Caribbean reef sharks in the Cayman Islands. PLoS ONE 18(11): e0293884. https://doi.org/10.1371/journal.pone.0293884 (This work was supported by funding from the UK government, and the Save the Seas Foundation. A small portion of the funds came from the Cayman Islands Brewery sale of 'Whitetip Lager, which is donated to the Cayman Islands Department of Environment's shark research.