Dr. Jeff Seminoff is a marine ecologist and leader of the Marine Turtle Ecology and Assessment Program at NOAA’s Southwest Fisheries Science Center. Dr. Seminoff’s current research uses innovative approaches such as skeletochronology, stable isotope analyses, biotelemetry, animal-borne imagery, and aerial surveys to learn more about the life history of marine turtles throughout the Pacific Ocean.
How did you get involved in sea turtle research? Was this always what you wanted to do?
My initial interest was in the marine realm studying reef fishes, although I always loved herpetology. While studying reef fishes in graduate school I had the opportunity to do some work with sea turtles and absolutely loved it. So, I switched gears to focus on sea turtles. My first work involved a journey along the coast of Mexico to all of the sea turtle nesting beaches to evaluate the effectiveness of different conservation strategies.
Can you tell us a little about the new research techniques you’re using to study sea turtles?
Our lab used two distinctive techniques discussed in the Turner Tomaszewicz paper. The first is skeletochronology, which uses a humerus bone from a turtle that has stranded to determine the animal’s age when it died. Much like dendrochronology (tree ring dating) there are growth rings that occur in these humerus bones. We can take a slice of the humerus from a deceased turtle and if we decalcify and stain the bone in a certain way and look at it under a microscope we can see the different growth rings. The presumption is that those rings equate with annual growth—one ring equals one year of growth.
That’s how we age the animals, but the other part that we’re particularly proud of here at our lab is the isotope chronology component of this research. We know there is a natural continuum in the composition of carbon and nitrogen isotopes in surface waters of the ocean. Nitrogen values in surface waters are much more depleted farther off shore due to a predominance of lower-trophic organisms in surface food webs, and likewise, carbon signatures change the farther offshore you get. Nearshore or coastal waters have a very different signal or isotopic value compared to the high seas (such as the ocean north of Hawaii).
We know that when hatchling sea turtles leave the beach they spend some period of time offshore. We don’t necessarily know how long; the grandfather of sea turtle ecology, Archie Carr from the University of Florida, termed this period of time the “lost years”. He’d see the small hatchlings leave the beach, but then he’d never see hatchlings in the water. The next time he saw sea turtles, they would be the size of dinner plates.
So where do these turtles go and how are you able to determine this with skeletochronology and isotope analysis?
Newly hatched sea turtles leave the beach and then some time passes (between 2 and 8 years) and they come back into coastal habitats. Green turtles, which were the main focus of this study, usually live in coastal estuaries like San Diego Bay, Chesapeake Bay, Long Island Sound, or any of the lagoons in Florida. We know that an animal, wherever it is living, is eating the food available in that area. An offshore green turtle is assimilating the nutrients from its food source and actually assimilating the local isotopic profile of the offshore water. When it moves to coastal waters, which is typical for green turtles, its diet and behavior will shift as it starts to incorporate the local isotopic signals of the nearshore habitat. There is a dichotomy between the offshore and nearshore signals. If we know sea turtle humerus bones grow like tree rings, putting on new growth each year, then we should expect that an animal in the high seas offshore habitat is putting down growth rings built on the nutrients from those offshore waters. Once that animal recruits to nearshore waters, it has a whole new set of nutrients and it starts laying down a nearshore signal in those concentric growth rings.
To do the isotope analysis, we take a second slice of humerus bone and put it on the microscope with a dental drill. We take the age information we’ve gathered from the stained slice of bone to train the robotic drill where and how to drill into the corresponding isotope section under the microscope. For each year in the life of the turtle, the microscope drills out bone powder, working toward the center of the bone to the year the turtle was born. We analyze all of that bone tissue powder and we’re able to tell which bands are offshore or nearshore.
The fisheries and human pressures on turtles in coastal areas are different from those on the high seas. With the new information we’ve gathered on how old these turtles are and how many years they spend in each of these habitats, we can determine how susceptible they are to these different threats.
Do you have plans to test this approach on other species other than green sea turtles?
You can do this research with virtually any species of sea turtle except leatherbacks (they don’t have growth rings in their humerus bones). Green and loggerheads have been the two primary species we have studied, but we are also doing some work with flatbacks (in Australia), hawksbills, and olive ridleys. These are five of the seven global sea turtle species. The only species we don’t work with right now is the Kemp’s ridley.
Leatherbacks are not a hard-shelled species; they’re in another family and entirely different from the hard-shelled turtles in many ways. What is interesting about leatherbacks is that instead of growth rings they have little bones in their eyes called occicles, which presumably grow each year like tree rings. By looking at these occicles we can try to age leatherback sea turtles too. The occicles grow the same way as a fish otolith (ear bone) and a lot of similar work with fish otoliths has already been published.
There are examples of this throughout the animal kingdom. Sea lions, seals, and toothed whales have concentric growth rings in their teeth so there are researchers that have drilled their teeth to get a better idea of changes in isotope levels. The vertebrae of sharks have the same rings and researchers have done similar aging studies in sharks. It’s just a matter of science and technology meeting the questions for different animals.
What did you discover about green sea turtles after doing this study?
Cali Turner Tomaszewicz’s research found that in the Eastern Pacific some green sea turtles have an entirely different life history strategy. They never go into coastal habitats, instead they remain in offshore habitats all the time.
We were expecting that the animals washing up on the shore were coastal animals that got caught up in fishing nets when they left the lagoon. We anticipated based on existing research that these turtles were going to have a nearshore signal in their most recent years. We found totally the opposite. The turtles had an offshore signal and it showed they had never come into coastal waters (and these were big turtles, so it is unusual that they stayed offshore).
We started to figure out that when a green turtle lives in coastal habitats of the eastern Pacific, they are herbivorous, eating sea grass (mostly eelgrass, Zostera marina) or marine algae. Seagrass and algae are a dependable food source, but they are low in energy, so an animal with this diet typically will grow more slowly. For many years, green sea turtles were considered one of the slower growing sea turtle species. But when they are living offshore as adults or large juveniles, they’re eating fish discards, robbing fishing hooks, swimming along the sea floor eating invertebrates, or eating floating invertebrates and jellyfish in the mid-water column. Not only are they living offshore, which we never anticipated; they’re eating nothing but high-energy foods and growing very quickly. These green turtles have the most unusual behavior and life style of any green turtle population we have studied.
What’s the next step in this research? Are there any new technologies you’d like to use to investigate sea turtle age and habitat in further detail?
Trace element or trace metal analysis offers promise for studying sea turtles. Different coastlines and areas in the ocean have different pollutants, and there are hot spots for certain ones. If we sample growth rings and see certain trace elements, then we know there’s a spatial pattern. Trace elements might be able to give us greater specificity for tracking animal movements and tell us that a turtle has traveled inshore at a certain latitude or offshore at a certain latitude.
The holy grail for sea turtle demographic research would be aging a turtle that is alive. Some techniques have been used with live mammals, like DNA methylation. As an animal gets older it has more methyl groups attach to its DNA, so we could look at that and how it changes over time. Telomere length has also been suggested as a way to determine age. I’d like to team up with our geneticist, Peter Dutton, here in the Southwest Fisheries Science Center to combine some of these approaches that might help usages live sea turtles.