Mogos Islands mark the highest waters of Golfo Dulce.
By Brooke Bessesen
While Jorge and I both loved working on the water, the results of our research brought the greatest rewards. Golfo Dulce is a true bio-gem—one of Costa Rica’s preeminent riches. Several hundred Green sea turtles, critically endangered Hawksbill sea turtles, Olive Ridley sea turtles and (reportedly) Pacific Leatherback sea turtles, rest, feed, mate and nest in the gulf. A rare xanthic colony of pelagic sea snakes resides around the inner basin. Both Northern and Southern Hemisphere Humpback whales enter the inlet to give birth and possibly provide sanctuary for young calves. Whale sharks aggregate in Golfo Dulce. Resident dolphins and other toothed cetaceans breed and raise offspring. Scalloped hammerhead sharks are born there and needlefish spawn. What a remarkably vibrant bionetwork!
The name “Brown pelican” belies the attractive hues of a mature bird.
I’m sure it comes as no surprise that during our 400+ hours of observation in Golfo Dulce, Jorge and I witnessed an astonishing array of marine life. Indeed, we were astounded by the intense biodiversity revealed to us during our research. In addition to the animals I’ve already blogged about in this series, many more are worth mentioning. Some were officially documented, others were not, but all helped define our emerging portrait of Golfo Dulce.
Brown pelicans (Pelecanus occidentalis) are commonly seen in Golfo Dulce and we located a year-round communal roosting area along the banks of Piedras Blancas National Park in the upper half of Golfo Dulce. We saw many other marine birds, too, including Brown boobies, magnificent frigates, osprey, several species of gulls, terns, swallows, herons, ibis and dozens more wading and estuary birds.
Side by side, you can begin to see the characteristics that visibly differentiate the three sea turtle species we documented inside Golfo Dulce.
When we began our research, nobody expected us to find very many sea turtles inside Golfo Dulce — most sea turtle activity was thought to occur on the Pacific side of the Osa Peninsula. It turned out that chelonids were the most frequently seen family of animals, accounting for 38 percent of our total sightings. Discovering such significant numbers of sea turtles was one of our most important findings. Sadly, fishermen with many years of experience in Golfo Dulce say the sea turtles there have declined at least 30 percent in recent years.
Jorge and I documented three species: Pacific Black sea turtles, still commonly referred to as “Greens” (Chelonia mydas agassizii), Olive Ridley sea turtles (Lepidochelys olivacea) andHawksbill sea turtles (Eretmochelys imbricata). Locals also reported seeing near-extinct Pacific Leatherback sea turtles (Dermochelys coriacea) inside Golfo Dulce. That’s four endangered species of sea turtles utilizing the embayment. Amazing!
Our biseasonal data show Golfo Dulce to be a year-round feeding and breeding area for endangered Green sea turtles. We logged over a hundred sightings of them between both surveys. This species, by far the most common, was usually observed in the upper regions of the gulf resting at the sea surface. But we also documented Green sea turtles mating in all four quadrants of the inlet, so their use of the fiord waters appears widespread.
Dolphin B43 shown alongside a rendered outline of its dorsal fin. We saw this individual five times.
An unexpected but delightful result of our survey work in Golfo Dulce was the identification of about 80 individual Bottlenose dolphins (Turciops truncatus), some of which can be seen in the Appendix of my 2010 report.
How does one go about identifying dolphins? Well, pioneering biologists studying various species discovered ingenious ways to distinguish individuals. Jaguars have unique spots. Gorillas have unique nose prints. Dolphins have unique dorsal fins. By examining the shape, natural markings, scars and trailing edge, a dorsal may appear as distinct as a fingerprint. Of course dolphins don’t sit quietly at the surface while you study the intricacies of their dorsal patterns, so ID work is best done through photos. Luckily, we managed to get photographs for almost 90 percent of our dolphin sightings.
These three photos show a baby Humpback whale next to its resting mama – TOP: nursing; MIDDLE: breathing; and BOTTOM: spyhopping
A variety of whale species may be found in the eastern Pacific Ocean off the coast of Costa Rica, including Byrde’s whale (Balaenoptera edeni), Fin whale (Balaenoptera physalus), Sperm whale (Physeter macrocephalus) and Killer whale (Orcinus orca). But the most commonly seen whale inside the Golfo Dulce is the Humpback whale (Megaptera novaeangliae), aspecies that annually migrates from colder feeding grounds near the magnetic poles to warmer equatorial breeding and birthing grounds.
In fact, the area around the Osa Peninsula is a really special place for Humpbacks — it is the only known place in the world where the migration paths of whales from both the northern and southern hemispheres cross over. Because the Arctic and Antarctic seasons are opposing, Humpbacks from the North Pacific Ocean spend time inside the Golfo Dulce during January and February while Humpbacks from the Southern Ocean normally arrive during July and August. That means whales are coming and going from the gulf throughout much of the year.
We logged 25 Humpbacks during our surveys, mostly mothers with young calves. The whales were often seen traveling up into the embayment, possibly seeking sanctuary from the open sea. We even witnessed a baby Humpback nursing — an experience Jorge and I will never forget! That sighting plus birthing events reported to us during both seasons suggest that embayment may even be a haven for Humpback nativity. Isn’t that remarkable? Indeed, the more we learn about the role the Golfo Dulce plays in the whales’ ecology, the more important it appears.
It is incredible to think how far Humpback whales travel to visit the Golfo Dulce and we can only hope the calm gulf waters will always offer those gentle titans safe retreat at the end of their long journey.
Brooke Bessesen conducted Marine research at the Osa in 2010 and 2011 as a recipient of the Greg Gund Memorial Fellowship. Check out her Golfo Dulce report on our website.