For harbor porpoises, the ocean is a 24-hour buffet

Harbor porpoises are the world’s smallest cetaceans. The marine mammals, which look something like a small, beakless dolphin, live in colder waters of the Northern Hemisphere and tend to stick closer to shore — a trait that led to their name. Because small bodies would lose heat quickly in cold water, scientists have thought that harbor porpoises must eat a lot, consuming as much as 10 percent of their body weight daily, to stay warm and well fed.

Now scientists have figured out just how good harbor porpoises are at finding a meal. These animals can go after hundreds of tiny fish each hour, and they are very successful hunters.

Between September 2012 and August 2014, Danuta Maria Wisniewska of Aarhus University in Denmark and colleagues tagged five harbor porpoises that had been caught as bycatch by fishermen off the Danish coast. The tags, attached via suction cup 5 centimeters behind a porpoise’s blowhole, recorded the animal’s movements and the clicking noises it made as it hunted prey. After about 24 hours, the tag detached and was retrieved.

“Tagged porpoises foraged nearly continuously, targeting small prey with remarkably high capture success rates,” the researchers report May 26 in Current Biology.

The tags revealed that the porpoises encountered up to 200 fish during the day and 50 to 550 after dusk, when the animals tended to hunt in deeper waters. The porpoises mostly targeted fish just 3 centimeters to 10 centimeters long, and were successful at catching them more than 90 percent of the time.

Harbor porpoises have features that help them manage this continuous munching: They have small stomachs that can accommodate only 1.9 kilograms of food maximum at a time, and their digestive tracts are short and move meals through the body in just 140 minutes. (Humans require an average of 40 hours.)  

But this need to feed continuously to support a high metabolism has harbor porpoises “operating on an energetic knife-edge,” the researchers note. Because they have to eat and eat and eat to survive, it doesn’t take much to disturb their routine and put them on the path to starvation. “Individual porpoises have been reported to starve to death in less than a week,” the team notes.

Anthropogenic disturbances and changes to the marine ecosystem could be especially dangerous to this species, one in which starvation rates have been rising.