Marine Conservation, Science and Research

Osa Conservation Supports Research in Golfo Dulce: More Species, More Understanding

By Brooke Bessesen

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.

Several American crocodiles (Crocodylus acutus) and Caimans (Caiman crocodylus) were observed in the estuary of Platanares and up into Rio Esquinas. And two more crocs were documented in the open waters of Golfo Dulce. These species are known to inhabit the brackish waters and mangrove roots of the Osa region and are said to be in all rivers that feed into Golfo Dulce. We also spotted a beautiful tree boa among some riparian mangroves.

Male needlefish make the water frothy with their sperm, and then females deposit eggs along the rocky beach.

Two species of needlefish (Tylosurus spp.) are known to spawn at certain beaches inside Golfo Dulce, so one evening we went to observe a spawning event. Every month beginning five days after the full moon and continuing for three consecutive days, T. acus pacificus amass to lay their eggs. It was incredible to see the frothy white sperm on the water surface and then find the fertilized eggs strewn along the exposed beach after the tide fell. During the survey we also saw flying fish, roosterfish, parrotfish, tuna, bonitas, ballyhoo, snappers, jacks, saboles, sardines, mullets, a variety of rays, eels, puffers and a plethora of species specific to coral reefs.

Whale sharks, Rhinocodon typus—the largest fish in the world—are seasonal visitors to Golfo Dulce. Although we did not record any first hand sightings, we did have reports from local fishermen and boat guides. Members of this species are generally described as swimming close to the coast and remaining in the embayment for several days at a time. Although normally seen singly or in small numbers, historical aggregations of whale sharks in Golfo Dulce have reached upwards of 20 individuals!

The Golfo Dulce

In 2010-2011, we recorded several shark sightings, representing three species: Tiger shark (Galeocerdo cuvier), Bull shark (Carcharhinus leucas) and Scalloped hammerhead shark (Sphyrna lewini). Most were juveniles, supporting claims that Golfo Dulce is a critical habitat for young sharks. Several species of rays were also observed during the survey period, including Manta rays, Spotted eagle rays, stingrays and cownose rays. And we saw an extensive assortment of invertebrates, including Portuguese Man-of-wars, sea jellies, sea stars, corals, crabs, oysters, conches and other mollusks.

While most studies focus on particular species, the goal of our research was to collect and share data that might provide an overview of Golfo Dulce as a dynamic marine habitat. Clearly, abundant and varied sea life does symbolize the ecology of the embayment.

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 .

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