We rely on roving ocean creatures to fetch us all kinds of data we couldn't get otherwise. Carrying cameras or GPS units or sensors glued to their bodies, marine animals collect data for human scientists about the health of ocean ecosystems or how their own species migrate. Yet lugging our equipment through the sea may be harder for these creatures than we realize. By tagging them, we might be slowing down or even harming the same species we're trying to preserve.
When scientists tag birds, the authors of a new paper in the journal Methods in Ecology and Evolution explain, they follow the "five percent rule": any transmitters they attach to a bird must be less than five percent of its body mass. This ensures the animal can still take off and fly without trouble. But underwater, heaviness doesn't matter as much. Everything gets a boost from buoyancy. What matters more, the authors say, is drag.
To study how drag from tags affects marine animals, NOAA scientist T. Todd Jones and his coauthors put turtles into a wind tunnel. They chose turtles because they're popular—more than 50 published studies per year involve sticking some kind of device to a turtle. Some sea turtle species migrate across entire oceans, so they may carry these devices for hundreds or thousands of miles. And most are endangered.
Instead of propping up live endangered animals inside their wind tunnel, the researchers built fiberglass models. These were casts of 11 types of turtle bodies, minus the front flippers, made from frozen or stuffed carcasses. (The carcasses themselves were too heavy to mount in the wind tunnel.) The researchers also compared a fiberglass cast to a real turtle carcass in water, to make sure the shell materials themselves didn't cause different amounts of drag.
Turtles, of course, don't fly. "Air and water are both fluids," Jones explains; by matching the Reynolds number—a measurement representing turbulence—of the wind to that of water, the scientists could simulate a swimming turtle. Wind speeds of 8 to 20 meters per second corresponded to turtle swimming speeds of 0.5 to 1.3 meters per second.
Attaching seven different kinds of tags to their fake turtles, the researchers saw that most devices increased drag on adult turtles by 5% or less. With a young turtle and a bulky tag, though, it's possible to double the normal drag on the animal. The authors provide charts that other scientists can use to estimate how much drag they're adding to a turtle, based on the animal's size and species as well as the size and shape of their equipment.
It's nearly impossible to tell how tagging equipment affects animals in real life, thanks to a Schrödinger's-turtle paradox: we can't follow the long-term activities of ocean animals that aren't tagged in some way, so we can only compare tagged animals to other tagged animals. However, Jones worries that putting a lot of extra drag—or even a little extra drag—on an animal like a sea turtle could be harmful. These species may burn through every last bit of their energy as they make long-distance migrations, so any extra burden could hurt their odds of surviving and reproducing. (The added brake on their swimming speed might also make them more vulnerable to predators.)
"By following our guidelines, researchers can minimize the drag effects to their study organism," Jones says. This should help keep animals as safe as possible, and keep scientists' results in line with natural conditions for the animals. "However," Jones adds, "sometimes the guidelines will suggest that a certain tag simply should not be used on a particular animal."
Since sea turtles sometimes carry barnacles on their shells, which also add drag, Jones recommends that researchers take the time to pry off a few barnacles while they're gluing on equipment. That way they can make up for some of the extra burden on the turtle, and perhaps alleviate their own guilt about replacing one pest with another. T. Todd Jones, Kyle S. Van Houtan, Brian L. Bostrom, Peter Ostafichuk, JonMikkelsen, EmreTezcan, Michael Carey, Brittany Imlach, & Jeffrey A. Seminoff (2013). Calculating the ecological impacts of animal-borne instruments on aquatic organisms. Methods in Ecology and Evolution DOI: 10.1111/2041-210X.12109 Images: top, USGS/photo by Kristen Hart; middle, T. Todd Jones.