Net Effect

The old and very young, the injured and ill, the weak and weary—these are what natural-selection claims on its daily rounds. The slowest zebra, the most conspicuous mouse, the butterfly with a bum wing.

But the hunting and fishing done by humans sometimes work in the opposite way, by killing the biggest or healthiest of a species. Remember, the hunters were after Bambi’s big, bad dad, not little, vulnerable Bambi.

This kind of selection can have powerful consequences for animal populations. For instance, several species of sea turtles around the world suffer from a disease called Fibropapillomatosis, which causes large, unsightly tumors that grow out of the animals’ eye sockets, armpits, genitals, and carapace. Some places, like the Caribbean archipelago of Turks and Caicos, still allow the legal harvest of sea turtles, and research shows that fishermen there almost always throw tumor-ridden turtles back into the sea.

After all, if you had the choice between one steak and another with a warty-looking thing hanging off the side of it, which would you pick? (By the way, there are no studies suggesting that eating turtles with Fibropapillomatosis is dangerous to humans. But there’s also no research saying its safe.)

“By taking healthy animals but throwing the diseased animals back, this may effectively increase the prevalence of the disease in the local population,” says Thomas Stringell, a biologist at the University of Exeter.

The lead author of a paper published last month in the journal Frontiers of Marine Science, Stringell and his team investigated the relationship between turtle tumors and the turtle-fishing industry on Turks and Caicos. And he’s the first to say that when it comes to fisheries management, the implications for theses findings are tricky.

FP tumors may not only impair the animals’ ability to see, swim, and forage but also mess with organ function inside the carapace. All of which adds up to a disease that may further hinder worldwide green turtle populations from bouncing back. So, if fishers are going to haul a turtle in, it might be better if they took the more sickly ones.

That said, green turtles (Chelonia mydas) are also endangered. Surveys of nesting sites reveal that the global population—as measured by the number of nesting females—has fallen between 48 percent to 67 percent over the last three generations. So it would be easy to make a conservation argument that fishermen should leave all of these seafaring reptiles alone, no matter what.

Fibropapillomatosis is also not a death sentence.

“They have tumors, but [the tumors] are not yet affecting their ability to forage,” says marine biologist and coauthor Annette Broderick. Many of the turtles they caught with Fibropapillomatosis, she said, were doing fairly well.

Likewise, a 26-year study of green turtles in Hawaii found that even the animals with advanced forms of the disease were sometimes able to kick it. Stringell says this suggests that culling the sick turtles (or convincing fishermen to eat them specifically) isn’t likely to help the species either.

What is proven to help in the recovery of sea turtle populations is setting up protections for the breeders. You see, for long-lived species such as green turtles, it can take somewhere between 20 and 50 years for the animals to reach reproductive maturity. And that makes adults disproportionately important. “Adults are the powerhouses of population maintenance,” says Stringell.

That’s why he and his team have been working with the Turks and Caicos government to stop the harvest of bigger turtles. In July 2014, their research prompted the government to mandate that fisherman can no longer catch turtles larger than 17 inches long. (Stringell and company originally pushed for a limit of about 12 inches but had to compromise to get buy-in from the locals. For more on that story, check out this piece by onEarth contributor Ben Goldfarb.)

As for the tumors, there’s still a lot we don’t know or understand. For instance, there’s a possibility that they are contagious. Turtles that hang out near the coast are also far more likely to develop the growths than those living in the open ocean. The exact reasons for this will require more research, but there seems to be a correlation between presence of the disease and poor environmental quality of the habitats it’s found in. Watersheds with high nitrogen content and invasive microalgae have also been implicated.

Fortunately, older green turtles—i.e., the breeders—spend most of their time in the open sea and are thus usually free of the disease. Juveniles spend much of their early lives close to shore, however, and are hardest hit by Fibropapillomatosis. And herein lies the problem—it’s these younger, smaller turtles that can be most sustainably harvested, but if more and more of the reptiles turn up covered in disgusting knobs, fishermen will likely flout the new regulations and catch the bigger, disease-free breeders.

Because these animals can live 80 years or more in the wild, it’ll be some time before Stringell and Broderick can assess if or how well the new size restrictions are helping to boost green turtle numbers. Until then, turtle tumors go on the list of just one more way our food preferences alter the world around us.