Interview: Dr. Blair Witherington on Oil’s Impact on Turtles in the Gulf of Mexico

Dr. Witherington with an oiled Kemp’s ridley turtle in the Gulf of Mexico.

(This blog is part of a series of interviews with scientists who are championing marine research in the Gulf of Mexico.)

A research scientist with more than 24 years of experience in sea turtle biology and conservation, Dr. Blair Witherington has worked with the Florida Fish and Wildlife Conservation Commission’s Fish and Wildlife Research Institute since 1992. He is also an adjunct assistant professor, department of zoology, University of Florida; served as president of the 20th International Sea Turtle Symposium; and is vice chair of the Northwest Atlantic region of the Marine Turtle Specialist Group of the International Union for Conservation of Nature. He has authored or contributed to more than 40 scientific articles, monographs and book chapters. In addition, he has written five books on sea turtles and other natural history subjects.

The BP Deepwater Horizon oil disaster had an immediate and particularly harmful effect on early juvenile sea turtles in the Gulf of Mexico. The worst marine oil spill in history also served to highlight a compelling need for assessments of open-sea habitats – research critically lacking in 2010, yet essential for conservation efforts and restoration planning.

Ocean Conservancy:  Turtle researchers call the post-hatchling and juvenile life stages “the lost years.” With the exception of one species, loggerhead juveniles, the early open-sea surface lives and distribution of juvenile green turtles, hawksbills and Kemp’s ridleys have been poorly understood. Your recent research is changing that. Tell us how.

Dr. Witherington:  Many other researchers have been intrigued by the mystery of the so-called “lost years.” Work I am doing with colleagues Shigetomo Hirama and Robert Hardy builds on the accomplishments of others who have inferred from indirect evidence how the youngest sea turtles disperse from land and live on the open sea. The importance of our recent contribution may be in the opportunity we’ve had to study these turtles directly. We hypothesized where we might find these “lost” turtles, went out to locate them there, and there they were. In observing these turtles within their sea-surface habitat and capturing them to allow measurements and assessment of their diet, we’ve learned definitively how these turtles live.

OC:  How does knowing more about the distribution of juvenile sea turtles in the open waters of the Gulf help guide conservation and protection efforts?

Dr. W.:  One of the most important conclusions we’ve drawn concerns the nature of the habitat that young turtles depend on. The early life stages of at least four sea turtle species in the Gulf are closely tied to surface convergence zones. As the name suggests, these zones are places at the sea surface where water converges and things accumulate. One of the most important accumulations is of the brown alga (a seaweed) called pelagic Sargassum, a floating plant that is the basis for a diverse community of unique organisms. Young sea turtles are closely tied to Sargassum, float within it, and feed on the animals that drift within these open-sea rafts. To fully protect sea turtles it will be essential to understand how Sargassum drift habitat is distributed and what threatens it. Some threats are direct, like the harvest of Sargassum for livestock feed (a practice that has been stopped in U.S. waters). But most threats are indirect and come from the nature of the oceanography where the Sargassum floats—oceanic convergence zones. These zones collect many bad things, including plastics and petroleum. We are just beginning to understand how extensively these pollutants threaten turtles with entanglement and ingestion hazards.

A juvenile green turtle within a seaweed mat in the Gulf of Mexico.
Photo: Blair Witherington

OC:  You spent time in the Gulf rescuing turtles and studying how they were injured by the BP Deepwater Horizon well blowout, the worst oil spill in history. What did you see and learn?

Dr. W.:  The Gulf spill harmed young surface-pelagic turtles profoundly. My involvement with response efforts during the spill was with a team of scientists and field workers whose job it was to rescue turtles at sea in a quantitative way. By this I mean that we carefully recorded our search effort and where we observed turtles so that we could not only tally the numbers of animals affected by the oil, but we could also estimate turtle numbers from places we couldn’t get to. The spill was enormous, and unfortunately we could only get vessels and search teams to a thin fraction of the event’s area and time period. Storms periodically threatened crew safety and much of the area that could be searched was over a hundred miles offshore and inaccessible to the small vessels we had available. What we learned was that our fears of the vulnerability of small sea turtles to oil were well founded. The convergence zones where Sargassum habitat collects and where young turtles live were the areas of surface water where petroleum concentrated the most. In the convergence zones we searched, we collected hundreds of oiled juvenile sea turtles representing four species: green turtles, Kemp’s ridleys, hawksbills and loggerheads. Most were 1- to 2-year-old turtles, about the size of a coconut and a kilo in weight. In the worst cases, the turtles were imbedded in mats of congealed grease and covered so thickly with tenacious oil that they were barely recognizable as turtles. In addition to external oiling, turtles also had oil lining their esophagus. It was pretty bad. The live oiled turtles we rescued were taken to facilities for cleanup and rehab. Hundreds of turtles were saved. It was difficult but rewarding work.

OC:  What risks are posed by an oil spill to the widely scattered seaweed-raft habitats occupied by these young turtles?

Dr. W.:  Although there is a lot to be learned from massive events like the BP spill, many hundreds of smaller spills occur in the Gulf each year. These releases of petroleum result in floating oil and tar that is carried by surface water into pelagic turtle habitat. We see evidence of this in the years with no headline oil spills, when young turtles on the open sea float amongst tar balls and have their mouths gummed by sticky tar. Apparently, the indiscriminant feeding style of young sea turtles promotes accidental ingestion of many things that are not good for them. We know that large oil spills are disastrous, but the chronic effects from small spills, multiplied by thousands of events over time, may be a greater cumulative threat.

OC:  How does petroleum affect a transient young turtle in the open sea?

Dr. W.:  Oil is sticky. It adheres to the outsides of turtles and if it is extensive enough it can keep them from swimming and feeding, and it can suffocate them. Oil also gets very hot in the sun and can raise the temperature of a turtle into the lethal range. Turtles also ingest oil, which contains numerous toxic compounds that have the potential to affect life functions both in the short and long term.

[See Ocean Conservancy’s “Restoring the Gulf of Mexico: Establishing a Platform for Success” for a statistical snapshot of sea turtles recovered from the BP Deepwater Horizon oil impact area; also “Restoring the Gulf of Mexico: A Framework for Ecosystem Restoration in the Gulf of Mexico” for a deeper dive into Gulf ecology, sea turtle life and more.]

OC:  Have we seen a dwindling of the sea turtle population in the Gulf? Has the health of the population been established for loggerheads, green hawksbills and Kemp’s ridleys? Do we have a baseline for any of the species?

Dr. W.:  You’d think that we would know a lot about that, but we don’t. A population’s health (that is, its ability to persist) has to do with the number of its members at all life stages, their probability of survival and how many new turtles are produced. With this information, population biologists can forecast population changes for better or for worse. We do a pretty good job of counting sea turtle nests on their nesting beaches, but we know much less about survival at sea. For example, Kemp’s ridley sea turtles nest mostly on one stretch of beach in northern Mexico. We know that there were once many tens of thousands of ridleys nesting on those beaches and that the population declined dramatically. Beginning in the 1980s, workers counting nests recorded that numbers were increasing, a trend that has continued until the last few years when the upward arc of nest counts has flattened. These changes likely reflect rates of survivorship and sources of mortality where sea turtles live most of their lives—the sea. If we had measurements of these survival rates that we could depend on, we could predict interruptions to sea turtle recovery… and ideally, do something about it. Right now, we’re a little in the dark and cannot recognize the results of some threats until many years after they occur. As for baselines on any sea turtle population, we have mostly just nest counts on beaches to turn to. These represent an index of just one life stage: adult females. And as baselines go, we find ourselves unsatisfied with the time span of our information. Most nest-count time periods have yet to approach the generation time of loggerheads and green turtles, which is about 45 years. Still, through our brief window in time, we can say that the oldest members of the population have been increasing for green turtles living in the Gulf of Mexico. For ridleys, we’ve seen a similar increase until recently, which may be only a statistical blip. Loggerheads have had nest counts with considerable ups and downs, but have shown no definitive recovery. Hawksbill nest numbers are either up or down depending on which nesting beach one examines.

[See Ocean Conservancy’s “The Gulf of Mexico Ecosystem: A Coastal and Marine Atlas” for maps and information on Kemp’s ridley turtles (pages 94-97) and pelagic Sargassum (pages 36-38). These two maps are also included at the end of the blog.]   

OC:  So what is the state of current Gulf turtle research? What don’t we know but we will need to know to undertake effective Gulf restoration efforts? And what about protection and conservation planning?

Dr. W.:  Scientists concerned with sea turtles in the Gulf have begun to consider how an organized network of research locations could contribute the information that population biologists, resources managers and regulators need to assess populations and bring about their recovery. Sea turtles live in many locations, and each of these is likely to be biologically unique. A hope we have is that a network of index sites could contribute the information needed for a collective understanding of populations—information like how quickly turtles grow and their probability of survival to the next life stage. Of course, we also need to understand the factors that dictate growth and survival, like availability of food resources and intersections with threats.

OC:  Given that so much more awaits research, how would you map the research that needs to be carried out the most urgently? What would that look like and why is it needed?

Dr. W.:  This is where the population biology of the animal helps a bit. Because very few sea turtles live the decades required to mature and reproduce, these older turtles on the cusp of maturity are the most valuable members of the population. Losing them hits the population hard. So understanding how to reduce threats that take these subadult turtles is critical. The chief threat in the Gulf to this life stage, especially in loggerheads and Kemp’s ridleys, is incidental drowning in trawl and long-line fisheries. But some threats of high magnitude to younger life stages are also critical to conservation. For all life stages it will be important for regulators to understand where sea turtles intersect with threats we can manage. Technology that allows both tracking of turtles and monitoring of our own behavior (fishing, boating, release of pollutants) should be in our long-term plan.

OC:  Which species of turtles are found in significant numbers in the area of the Deepwater Horizon spill? You propose in your research that the open waters of the northern and eastern Gulf are of “unique importance” to Kemp’s ridleys. So do we know yet precisely how they have been harmed?

Dr. W.:  The Gulf is unique for its role in fostering the Sargassum drift community of organisms, which includes young sea turtles. I’ve mentioned four species (green turtle, Kemp’s ridley, hawksbill, loggerhead), but a fifth species also occurs in the Gulf. This is the leatherback turtle, which remains in the open sea for its entire life. Harm from the BP spill on all of these species is still being assessed, and as one might imagine, the task is under intense scrutiny. It’s also cloaked within a legal veil that does not permit me to say much about it. Sorry.

OC:  When you investigated the diets of turtles occupying mats of Sargassum you found they had ingested marine animals and Sargassum, etc., but you also reported an “alarming indicator of mortality,” which was that approximately 8 to 21 percent of the dry mass of ingested material was “debris,” and mostly plastic.

Dr. W.:  Yes, this was an alarming result. Sea turtles eat floating plastic. It can reduce their growth and kill them, but we don’t fully know the magnitude of this threat. Post-hatchling loggerheads and green turtles that strand dead following tropical cyclones show plastics in about 90 percent of necropsied specimens. The plastics are typically shards of plastic containers degraded in sunlight and broken up by waves. The shards stack up in the intestine, block the gut, and puncture it. Unfortunately, the sea is full of plastic and the floating bits accumulate precisely where little turtles live—in the convergence zones I described.

OC:  You also found that the mortality rate of these turtles is difficult to gauge, pointing out that there are few “strandings” of dead turtle carcasses on shorelines. Thus, understanding threats to early juveniles and post-hatchlings has been a missing element in management efforts. How is this to be remedied?

Dr. W.:  This is tricky. One way to measure mortality is to put marks on lots of turtles and make repeated observations of how many you see again. We are currently putting tiny injectable encapsulated radio tags into the juveniles we catch, but we have had so few returns we can’t say anything about survival. We simply have not tagged enough. Another method may be to give turtles broadcasting tags (satellite transmitters) that are small enough not to be a burden and affect survival. Then, turtles that go missing could be assumed to be dead (given a numerical model to account for error). We’ve put satellite tags on small pelagic juveniles, and we’ve gotten interesting movement data, but because the tags sink if they pop off the turtle, we can’t conclude much about turtles that go off the air. More advanced floating tags that archive data will need to be developed that are small enough to not cause trouble for these small turtles. I look forward to that future.

OC:  Without this research into the mortality risks posed by plastics and petroleum is it even possible to assess damage from catastrophic oil spills?

Dr. W.:  Yes. Given measurements of where turtles are, how many there are, and extent of oiling observed, and given assumptions about mortality given the intersection of turtles and oil, we can estimate mortality within intervals of statistical confidence. It’s much more helpful to our understanding of the disaster than simply claiming we don’t know.

Pelagic Sargassum map.

Kemp’s Ridley sea turtle

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