Cocos Island Expedition — August-September 2024
Cocos Log Blog FISH! Part 4
So what are all those sharks eating?
Cocos Island’s high productivity is fueled by upwellings created by the multiple currents that bump up against it seamounts and island habitats bringing nutrients to the surface that feed plankton that in kind feed all kinds of fish, seabirds, whale sharks, giant rays and humpback whales.
There are over 270 species of marine fishes recorded at Cocos and about 10% are endemic (they are found nowhere else on the planet). Another 10% are found only at the other relatively nearby eastern Pacific islands of Galapagos (Ecuador) and Malpelo (Colombia).
Mark, one of our research divers (and TIRN advisory board member), faithfully records every species of fish he sees, and it usually numbers around 90 over the seven days of diving. This trip is no exception— he will record 91 species on this expedition.
But it is not just the amazing richness of species, it is also the incredible abundance. For me, the staggering abundance is a major attraction and I wonder if I am looking back into pre-history, a time before humans depleted and destroyed so many places of their abundant underwater life. Historically, there must have been lots of other places around the world with such incredible abundance, but overfishing and pollution now make this remote location more unique and amazing.
This is more than mere speculation. A 2012 study reported that the average total biomass of reef fishes in the shallow waters around Cocos Island was 7.8 tons per hectare, and the largest in the tropics worldwide.
Whitetip reef shark, creolefish, moorish idols, blue and gold snappers, goatfish and a blue trevally can all be seen in this short clip (by Kaylam Pratt)
One of Coco’s most abundant fish is the Pacific creolefish, plankton-eaters occurring in large schools at all the dives sites, creating food for many predators. In Spanish they are called ‘sandia’ (watermelon) which they resemble with their red coloration and seedy black spots. I have heard divers complain that all these fish get in the way of the shark photography!
Creolefish (called Sandia in Spanish) convert plankton into tasty morsels that are eaten by many predatory fish including the yellowfin tuna seen swimming by (photo by Kaylam Pratt.)
The largest schools of fish seen at a few dives sites are bigeye trevally, one of more than a dozen species in the jack/pompano family. Growing to 2½ feet in length, the schools number in the thousands (possibly tens of thousands) and can stretch vertically 60 feet in the water column and as far as you can see. You can swim into the middle and see nothing but fish. When the fish are mating, the males turn dark black and the trevally swim in pairs—one silver and one black.
Bigeye jack occur in gigantic schools numbering in the thousands (photo by Kaylam Pratt.)
Cocos coral reefs have a myriad of fish of all sizes, ranging from the tiny Cocos seranos to the tiger sharks who patrol its edges.
Some of the most beautiful are blue and gold snappers that swim in mixed schools with similarly colored Mexican goatfish.
Cocos also boasts some 30 species of eels from the massive 7 foot giant moray to the slender, zebra and starry morays. Two of the most common are the fine-spotted moray and yellow-edged moray.
Yellow-Edged Moray Eel (photo by Kaylam Pratt.)
Two Fine Spotted Moray Eels appear to be friends with leather bass in the background (photo by Kaylam Pratt.)
A curious eel checks out Kaylam Pratt, the photographer for many of these images.
One of my favorites are the slender garden eels that poke out of their sandy burrows swaying in the currents mimicking sea grasses. All of these species can be found in the coral reef areas and the sandy ecotone.
Some very strange fish can be found at Cocos too. One is Commerson’s frog fish, which pretends to be and looks like a coral or sponge. This stationary fish which hops instead of swims, uses a lure to attract its prey. The UnderSea Hunter dive masters often know where to find them and point them out to us, otherwise most of us would never notice them.
Commerson’s frogfish with a creolefish in the background (photo by Kaylam Pratt.)
Another fish that is very un-fishlike in appearance and also hops and uses a lure to attract its prey, is the endemic Cocos batfish. We rarely see this as it occurs quite deep ~120 and deeper, usually just out of our safe diving range using the Nitrox 32 blend that we normally dive with.
In Cocos blog Part 5, we will discuss our research objectives for this expedition.
Please take a simple 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.
