Fifteen Years Ago Cocos Island Swarmed with Turtles. Why Have Many Disappeared?

A new scientific paper (Multidecadal Underwater Surveys Reveal Declines in Marine Turtles) by a collaboration of scientists and institutions, including Turtle Island Restoration Network, shows a strong correlation between the decline of sea turtles and the appearance of tiger sharks at Cocos Island National Park, Costa Rica.

The research, published in Conservation and Science, (https://tinyurl.com/bdhxk66z) used two data sets:

1. Long-term data collected by UnderSea Hunter from 1997 to 2019, which recorded 12,479 marine turtles and 3,194 tiger sharks across 35,706 dives

2. More recent data from Turtle Island and The Threatened Marine Species Restoration Center Association (CREMA) in which 129 turtles were captured and tagged during approximately 400 dives between 2009 and 2014

Tiger sharks were not regularly seen at Cocos until around 2010. The long-term data set collected by UnderSea Hunter divemasters revealed that as the sharks increased, there was a 26% decrease in the relative abundance of turtles. We observed tiger sharks trying to consume turtles. Turtles becoming prey can definitely impact populations.

A correlation between warmer water temperatures and a decrease in turtle sightings was also established, and it may reflect the more extensive worldwide climate changes resulting from burning fossil fuels.

Industrial fishing is devastating shark and sea turtle populations in the Eastern Tropical Pacific, with the most significant impact affecting highly migratory species. Though they are protected at one location like Cocos National Park, they face fishing threats during their migrations.

Turtle Island has spearheaded a movement to create the "Cocos-Galapagos Swimway" to connect these two marine protected areas by a no-fishing zone along this migratory route. Data now indicates six species of sharks and turtles move along the proposed Swimway, including endangered scalloped hammerhead and whale sharks, as well as green and leatherback sea turtles. While Ecuador has extended the Galapagos Marine Reserve border to include the Swimway, Costa Rica has not protected its part of the Swimway.

The "fear factor" plays a significant role in transforming ocean habitats. Researchers have discovered that just the presence of predators can alter the prey species' behavior, and the habitats they utilize.

When I first got to Cocos in 2007, and for several years thereafter, we would see many sea turtles per trip, catching as many as seven on a single dive. I haven't seen one on any expeditions in recent years. The tigers are there, and the turtles are gone. Now, a diver is incredibly lucky to see a single green turtle and maybe a rare hawksbill occasionally transiting through but not sticking around.

So, the question is, have turtles left Cocos, or have they just moved to sites other than the regular dive sites at Cocos?

We suspect both our reasons and hope to refine our understanding by looking at the finer movements of turtles and tiger sharks using the acoustic data we have been collecting. We hope to employ a newer technology, eDNA. By collecting water samples from different locations around Cocos that we don't dive, we can determine if turtles are still present.

- Todd Steiner, (Founder)
Turtle Island Restoration Network

You can learn more about our most recent Cocos Island research expedition at https://tinyurl.com/2zxzu7xx/ and learn more about participating by contacting me at TSteiner@tirn.net.

You can help by signing an Action Alert to Costa Rica here.