How to count a sea turtle

Earlier this month, two North Carolina groups representing local fishermen filed a notice asking the federal government to assess the status of sea turtle populations, saying that the fishermen will sue if the government doesn’t address the issue within 60 days.

The fishermen are seeing more turtles in the water, and they’re beginning to wonder if the measures they have to take in the name of sea turtle conservation are necessary. They see that the rules for their businesses can be a burden and say there should also be restrictions for boating and recreational fishing. And they question whether the methods used for counting turtles are giving an accurate picture of the species’ populations.

But it’s not the absolute numbers that count when it comes to sea turtle conservation — the overall trend is what matters, says Bryan Wallace, a turtle researcher at Duke University and Stratus Consulting who serves in the International Union for Conservation of Nature’s Marine Turtle Specialist Group. That’s the group responsible for collecting information about and assessing the status of the world’s seven marine turtle species for the IUCN’s Red List that describes the conservation status of the world’s species. The turtle species are all currently classified as vulnerable or worse on the list.

For sea turtles, the most common population data collected is the animals’ nesting activities. When female marine turtles reach adulthood and breed, they come onto land to deposit their eggs in nests on beaches. Even if the female adults can’t be counted directly, their nests can be counted. Counting animals in the world’s vast oceans isn’t easy. That sea turtles have this terrestrial phase of life actually makes them easier to study than other marine creatures, Wallace says.

The gold standard for population studies, Wallace says, uses a technique called “mark and recapture.” A number of animals are caught, marked in some manner and then released into the wild. Some time later, another group of animals is captured. Based on the number of marked animals in that second group, scientists can calculate the size of the population. Marking and tracking individual animals also helps scientists learn more about life histories, which they can then use to make generalizations across populations and species.

“We get really good representative information by studying nesting females,” Wallace says, “but we’ve known for a long time in the turtle community that we need to know a lot more about what goes on in the water.” So there are efforts to study turtles in their marine environment. Scientists recently discovered, for instance, where young loggerhead turtles go after they hatch on Florida beaches.

The North Carolina fishermen’s perception that there are more turtles isn’t wrong, Wallace says. “They know the water and marine biology better than marine biologists.” But even if scientists were able to count every turtle in the sea and that turned out to be a huge number, that “doesn’t mean that we should back off on regulations,” he says. Sea turtles live for decades, and even as adults they may not reproduce every year. Population growth can be slow, which makes rapid recovery from a decline difficult.

Where there’s potential for human impacts on turtles — such as getting caught in fishing nets, losing eggs to people’s meals, losing habitat to coastal development or climate change — their future is dependent on conservation efforts, Wallace says. And if you remove those measures, “they will decline.”