Marine plastic debris often emits a smell that many seabirds associate with food, according to new research that helps explain why the birds are attracted to plastic waste and eat it.
Resolving the problem, outlined in the journal Science Advances, should be a conservation priority because the consequences of seabirds consuming plastic can be debilitating at best, and deadly at worst.
"Malnutrition sometimes leading to starvation, gut blockage and occasionally perforation" are just some of the health issues that can occur, said lead author Matthew Savoca, who performed the study as a graduate student in the lab of UC Davis professor Gabrielle Nevitt.
"Chemical toxicity from things in or on the plastics have been shown to bioaccumulate up the food chain," he said. Certain dolphins, for example, are known to consume seabirds. If their prey consumed plastic, then the chemicals would wind up in the dolphin, too.
The new research built upon prior studies conducted by Nevitt, who in the 1990s and 2000s identified compounds that seabirds were attracted to when foraging. One of these is a sulfur compound called dimethyl sulfide, or DMS. It's released by marine algae, and serves as a call to birds.
Savoca provided this analogy: "If you're hungry and hear a dinner bell, you don't come running because you want to eat the dinner bell, you come running because the dinner bell indicates when and where you'll have a high likelihood of finding food."
He and his team discovered that plastic debris in oceans and some other water bodies tends to accumulate algae, which coats the garbage. There the algae produce, concentrate and release DMS.
DMS is also released when algae are eaten by animals like krill, which is one of the seabirds' favorite meals. Algae plastic doesn't give off the odor of food — rather the odor smells like food being eaten, which attracts the birds.
Surprisingly, the idea that seabirds even have a sense of smell is a fairly recent one, going back to about the mid 20th century. Now we know that they not only can sniff out odors, but also that many — such as petrels and albatrosses — possess exceptional senses of smell.
"Seabirds need a good sense of smell because they forage over vast expanses of open ocean on food sources that are very patchy in nature," Savoca said. "In other words, they are looking for a needle in a haystack."
Plastic can give off all, or most, of the cues that such birds have evolved to use in detecting food. Even the color and reflectiveness of plastic could further mimic these cues, making the garbage all the more attractive to hungry seabirds. Some fish species, including whale sharks and sea turtles also use DMS as a foraging signal, making them vulnerable to ingesting plastics.
Solving the problem won't be easy. The researchers believe that monitoring and conservation efforts should focus more intently on the birds and marine life threatened by the debris. They also hope that plastic might be manufactured differently, so algae will grow on it less readily.