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Loggerhead Sea Turtle
Juvenile loggerhead sea turtles seem more prone than other species to barnacle growth on the carapace and the skin.
Watch a short video of the Odyssey crew collecting data and cleaning barnacles off of the loggerhead sea turtle -   56k   200k
Photo & Video - Chris Johnson

February 23, 2005
Real Audio Report

Log Transcript

This is Genevieve Johnson speaking to you from the Odyssey in the Canary Islands.

We sight cetaceans around the Canary Islands almost every day. However, one of our most frequent encounters is with Caretta caretta - the loggerhead sea turtle.

With its broad, shield like, red-brown carapace (shell), thick neck and robust beak and head, the loggerhead turtle is easily identified at sea. It is a large turtle reaching up to 1.2 meters at maturity.

The majority of the animals sighted in the Canaries are juveniles with a carapace length of 40-50 centimeters. If the opportunity arises, we periodically bring sea turtles onboard Odyssey to weigh and measure them and also record their sex. This is a harmless procedure for the animal but adds significantly to what is known about the movement of this species around the Canary Islands - a region where they are not known to nest.

Sometimes the crew of the Odyssey come across stressed or injured animals. The other day, Bridget, the Deckhand of the Odyssey, called down from the observation platform while scanning the horizon for the blows of sperm whales to report the sighting of a small turtle. The turtle was a juvenile loggerhead. Its carapace covered in a dense mop of barnacles.

Genevieve Johnson & Loggerhead Sea Turtle
Genevieve Johnson with the loggerhead sea turtle after the barnacle have been removed.
Photo - Chris Johnson

We picked the turtle up easily from the sea in a large net. Its carapace and neck were carpeted in stalked gooseneck barnacles and it appeared quite thin and lethargic. Barnacles and algae often attach to turtles, and, although they don't feed on the animal as parasites or disease organisms do, they may still have a detrimental effect on the turtles health. The burden of dragging the excess weight of the barnacles through the water requires the turtle expend more energy. Excessive barnacle growth may also slow the animal down, preventing it from catching food as effectively. It may also be that the animal is already unhealthy, thus making it easy for barnacles to settle. Whatever the cause, it appears both scenarios reinforce each other in a vicious cycle.

Once onboard, the crew swiftly removed the barnacles. During this process, we noticed some other 'hitchhikers' in the form of Columbus crabs. It is estimated that more than 80% of juvenile loggerheads in the North Atlantic carry these little crabs. Columbus crabs usually travel in male/female pairs, living at the base of the turtles tail. In exchange for the security of its mobile home, the Columbus crab performs a valuable service for the turtle by eating small barnacles and parasites from its shell and skin. It seems the crabs we found on this turtle were ineffective or joined the turtle when the barnacles were already too large or too numerous for them to consume. Interestingly, these crabs cannibalize any additional members of their own species that attempt to colonize their turtle.

In less than ten minutes, we collected the necessary data and returned the turtle to the water with a clean carapace. We watched her swim briefly beneath the clear, calm surface before she disappeared into the depths.

There are seven known species of sea turtle worldwide. The loggerhead is the most likely of the hard-shelled species to drift into cooler seas of 15 degrees Celsius or less. The giant leatherback turtle (Dermochelys coriacea) - the only species lacking a hard external shell, is also known to occur in the cooler waters around the Canary Islands. However, it is far less frequent with an average of about one sighting per year.

The next day was unusually calm and we sighted three loggerhead turtles. The third sighting was a juvenile basking at the surface less than five meters off our bow. We saw several jellies throughout the day and one happened to drift a few feet ahead of the turtle. With a single sweep of its fore flippers it glided toward the hapless jelly and swallowed it.

Loggerhead eating jellyfish
Sea jellies are often consumed by the juveniles of some turtles species such as the loggerhead and adults of others like the giant leatherback.
Photo - Chris Johnson

The diet composition of the loggerhead changes as it grows and develops more powerful jaws. Adults are capable of crushing heavy shells and readily consume echinoderms, (sea stars) molluscs, (shellfish) and crustaceans (crabs, lobsters). Younger animals yet to develop the strength of the adults are more opportunistic, readily consuming an easy meal such as a sea jelly.

Unfortunately, this exposes sea turtles to one of the most devastating man-made threats throughout the world's oceans - plastic pollution. From the turtle's perspective, a jelly floating on or beneath the surface of the sea is absolutely indistinguishable from plastic debris, particularly plastic bags. Juveniles of some species, including the loggerhead turtle and adults of others primarily feed on jellies. We sighted this turtle within five miles of land drifting among a vast sea of plastic debris. Plastic shopping bags, bait bags, water bottles, bottle tops, the list is as endless as the ocean of plastic in which it now swims. Watching the turtle consume the jelly with a dozen pieces of plastic within 50 meters of Odyssey, made us wonder how long it will be until this little turtle makes a fatal mistake in food selection.

The endless sighting of plastics in every ocean and every port we visit constantly disheartens the crew. Plastic debris causes considerable, widespread mortality of marine mammals, birds and turtles through entanglement or ingestion. Of course, most victims succumb far from sight, therefore we may never know the true extent of the tragedy.

Fortunately there is some hope. If we can each take responsibility for the plastics we use, we will make a difference to the lives of marine creatures. Reduce the amount of plastic we purchase, recycle plastic whenever possible and dispose of plastic garbage responsibly.

Plastic Bag
To a sea turtle, a plastic bag is indistinguishable from a jelly.
Photo - Chris Johnson


Log written by Genevieve Johnson.

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