Post-Irmageddon in the Ten Thousand Islands

On Sept. 12, 2017, Hurricane Irma made landfall on the southwest coast of Florida as a Category 3 storm. Six months later there are still signs of Irma’s fury in the urban landscape: many homes without pool cages, many more roofs with blue tarps and huge stumps from fallen trees. Less apparent, or perhaps less sensational, is the damage to the remote coastal ecosystems in this region.

The Conservancy of Southwest Florida’s biologists have been conducting long-term research on the Kemp’s ridley turtles who inhabit the Ten Thousand Islands. Their research goal at the end of 2017 was to deploy satellite transmitters on Kemp’s ridleys as part of ongoing investigation of their behavior in south Florida waters during the winter months. Marine turtles are cold-blooded and will migrate to warmer waters if the water temperature drops below a certain threshold, typically around 63° F.

The Conservancy's tracking efforts thus far have demonstrated Kemp’s ridleys in Southwest Florida may or may not undergo seasonal migrations depending upon a given year’s climate; however, the effects of Hurricane Irma thwarted the research plans and appear to have impacted the feeding grounds of one of the most endangered marine turtle species in the world.

The waters of the Ten Thousand Islands are littered with mangrove debris which have become festooned with barnacles and tunicates (sea squirts). These “twigs,” as they became affectionately known, fouled the webbing of the biologists' nets every time they tried to catch a turtle. The debris also changed the structure of the turtles’ feeding habitat on the seafloor.

The researchers' first clue of something amiss in the study area was the relatively low abundance of marine turtles in the aftermath of Irma. Turtles congregate in the passes between islands as they ride the tidal flow in and out. Researchers can usually observe multiple turtles surfacing to breath on a given day provided the weather and water conditions are favorable. The researchers have been lucky to see one or two turtles a day during recent trips, sometimes going a whole day without a sighting.

Another clue came from the crab traps used to assess the availability of food as part of the Kemp’s ridley diet studies. Blue, stone, calico and spider crabs have been collected in these waters, but traps were coming up empty following the hurricane.

Kemp’s ridleys in the Ten Thousand Islands are immature (i.e. non-reproductive) and remain at these feeding grounds for several years. Turtles reach maturity at around 24 in. and move to adult habitat in offshore waters of the Gulf of Mexico. Prior to Irma, the shell lengths of Kemp’s ridleys captured in the study area ranged 12–22 in. with most in the 16–20 in. size range. The few Kemp’s ridleys observed after Irma were considerably smaller, only 11 in. or less.

A possible explanation for the size difference is that the larger ridleys with experience feeding in this region have moved to a different area in search of food. The small ridleys the biologists observed were new recruits and have yet to become accustomed to the feeding grounds.

One of the small Kemp’s ridleys observed after Irma deserves recognition as it was particularly elusive, earning the name “Houdini." It appeared to remain in the study area from late-October through mid-December.

During each of the week-long trips, in a backwater area dubbed “Ridley Cove,” a very small turtle was observed loafing at the surface. The net was deployed around the turtle following each sighting, but each of the four attempts were unsuccessful. The turtle was observed in the net during two of the attempts, but did not become adequately tangled given its very casual demeanor, and apparently slipped through the webbing given its small size.

The researchers came across Houdini again on their last day, bobbing at the surface without a care and taunting them as if saying “Catch me if you can," but the team resisted the temptation to deploy the net a fifth time. Instead, the Conservancy will let this turtle grow a little larger and perhaps its luck will run out in the not too distant future.

Continuing efforts will document the recovery of the Ten Thousand Islands and how the Kemp’s ridleys adapt to catastrophic changes in their feeding grounds.

To learn more about the Conservancy of Southwest Florida’s environmental science work visit conservancy.org.