Cocos Island Expedition — August-September 2024
Cocos Log Blog Part 3 Sharks and Rays
Scalloped hammerhead surrounded by a myriad of smaller fish (photo by Kaylam Pratt.)
The iconic species everyone comes to Cocos to see is the scalloped hammerhead shark. Occurring in large schools, sometimes 200 or more, these aggregations are almost all females, averaging 8 feet in length. Many of the “wallpaper” photos from below looking up were taken at Cocos, and this is a photographer’s dream shot.
Unfortunately, while we didn’t see the perfect wallpaper view on this trip, we did see lots of hammerheads.
Video by Luis Fallas.
At Cocos, one gets the opportunity to see hammerheads up close as they come into “cleaning stations” to have parasites and dead skin picked off by “cleaner” fish, mostly butterflyfish, but also king angels and occasionally juvenile hogfish. The sharks’ behavior is to slow down and enjoy the spa treatment, but they can be wary of diver’s bubbles. In summer, the female hammerheads have recent mating scars and the butterflyfish seem to concentrate on feeding on the dead skin in these scar areas.
A school of butterflyfish at a hammerhead cleaning station (photo Kaylam Pratt.)
In the video below, the yellow dots are the butterflyfish, but on this day the hammerheads were not slowing down enough to get a ‘good’ cleaning.
Video By Diego Jiménez.
There are also cleaning stations for Galapagos and blacktip sharks at Cocos. Blacktips seem the shyest of all and move away shortly after divers appear.
Video by Luis Fallas.
On the other hand, Galapagos are not concerned with the presence of divers, and close encounters at the cleaning stations are common.
Video by Marta Torne.
Tiger sharks are also seen cruising mostly on the sandy-coral reef ecotone, but their abundance may vary seasonally. On this trip, only one juvenile was observed swimming by a school of hammerheads over the sandy bottom in the Coral Garden.
(See me tagging a large tiger shark in this short documentary by The Economist film crew on an expedition in 2019 https://seaturtles.org/economist-short-doc-highlights-cocos-galapagos-swimway/ )
The most common and most ignored shark are the whitetip reef sharks that can be seen on almost every dive hanging on the sandy bottoms or swimming by, sometimes in small schools. They can easily be approached for close-up photos, or in our case to secure tissue samples for research purposes.
The most commonly seen shark at Cocos is the whitetip reef shark often laying on a sandy bottom (photo by Kaylam Pratt.)
Other sharks seen occasionally include whale sharks, silvertip and silky sharks, the latter two were seen on this trip.
Cocos is also home to four common rays.
Marbled rays, a type of stingray, beautiful and graceful, with beautiful black, brown and white mottling on their circular disk, are common at Cocos. Females can exceed 9 feet in size and their tails exceed the length of their bodies. If it is mating season, it is common to see many smaller males chasing females. On this trip, we only observed individuals.
A marbled ray rests on the sand (photo by Kaylam Pratt.)
Video by Marta Torne.
Several spotted eagle rays were also seen on this trip, gracefully swimming along the sandy bottoms, likely surveying for their food source, mollusks and crustaceans.
Eagle ray and school of blue and gold snapper (photo © Nonie Silver, from previous expedition.)
One sickle-finned devil ray was observed while diving on this trip, and several giant mantas. Both of these species are filter feeders and can reach enormous sizes, up to 22 feet for the giant manta and 12 feet for the devil ray.
A giant manta glides by overhead followed by some jacks (photo by Kaylam Pratt.)
While the abundance and biomass of sharks and rays are spectacular at Cocos, so is the abundance and diversity of fishes that provide food for these and other predators.
The amazing diversity of other fishes of Cocos is explored in the next Cocos Log Blog Part 4. In Part 5, we will discuss our research objectives for this expedition.
Again, if you haven’t yet, please take action to protect the wildlife of Cocos by clicking link and letting the Costa Rican government know we need more action to protect the migration route of Cocos endangered marine life.
