A new scientific paper (with a long name), Multidecadal underwater surveys reveal declines in marine turtles by a collaboration of scientists and institutions, including Turtle Island Restoration Network, shows a strong correlation with 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, utilized two datasets, including a 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, and a more recent data set by Turtle Island and CREMA from 2009-2014 in which 129 turtles were captured and tagged during approximately 400 dives.
The long-term data set found a 26% decrease in the relative abundance of turtles each year that was correlated with the occurrence of tiger sharks at Cocos. Tiger sharks were not regularly seen at Cocos until around 2010, their appearance corresponds to the significant decrease in turtle sightings.
Interestingly, the data showed that for each additional tiger shark present, there was a 43% decrease in the relative abundance of turtles and this is supported by data and observations from the more recent TIRN/CREMA dataset. A correlation with warmer water temperatures and a decrease in turtle sightings was also established, and may be a reflection of the larger climate changes facing the Earth, a result of burning fossil fuels.
We know industrial fishing is devastating populations of sharks and sea turtles in the Eastern Tropical Pacific, with the largest impact affecting highly migratory species, because even if 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 multi-species migratory route. Data now indicates six species of sharks and turtles move along the proposed Swimway, including endangered scalloped hammerhead and whale sharks, and green and leatherback sea turtles. Ecuador has expanded the border of the Galapagos Marine Reserve to include part of its side of the Swimway, but to date Costa Rica has failed to protect its part of the Swimway.
You can help by signing an Action Alert to Costa Rica here!
Tiger sharks started being seen regularly at Cocos around 2010 and at the same time turtles sightings started to diminish. We observed tiger sharks trying to consume turtles. Turtles becoming prey can definitely impact populations.
Another phenomena called the “fear factor” has been recognized by scientists as playing a significant role in transforming landscapes, and in this case ocean-scapes as well. It is the idea, now supported by data, that the presence of predators can alter the behavior of prey species by changing their behavior and the habitats they utilize.
So, the question becomes have the turtles left Cocos or have they just moved to sites we are not observing (i.e., away from the regular dives sites at Cocos)?
We expect both are partially true and we hope to refine our understanding by looking at the finer movements of turtles and tiger sharks using acoustic data we have been collecting, but not yet analyzed. Another new technology we hope to employ is eDNA. Collecting water samples from many different locations around Cocos, we can determine if turtles are still present, but in areas that we don’t dive.
You can learn more about our most recent Cocos Island research expedition here and learn more about participating by reaching out to me at TSteiner@tirn.net.
Photos by Elpis Joan.
