Animals

Harbor Porpoises Rarely Stop Eating

This means that even a moderate level of disturbance in the busy shallow waters that the marine mammals share with humans could put them in serious jeopardy.

<p>Photo: Harbor porpoise. Credit: Erik Christensen</p>

Harbor porpoises must hunt and eat almost constantly in order to satisfy their unique nutritional needs, new research finds.

The discovery, reported in the journal Current Biology, means that even a moderate level of disturbance in the busy shallow waters that the marine mammals share with humans could put the porpoises in serious jeopardy.

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"Our results show that porpoises hunt small fish, typically less than five centimeters (about 2 inches), nearly continuously day and night at ultra-high rates, attempting to capture up to 550 fish per hour, and frequently more than ten per minute with a remarkable success rate of more than 90 percent," co-author Danuta Wisniewska of Aarhus University said in a press release.

"The tiny fish targeted by porpoises are not of interest in commercial fisheries; however, relying on such small prey makes porpoises especially vulnerable to disturbances, because there is no room for compensation," Wisniewska added.

The harbor porpoise is one of the smallest marine mammals. As its name suggests, it stays close to coastal areas or river estuaries. Whale watchers often see these animals in the North Atlantic since the porpoises are right in the territory that tour groups in this region often traverse.

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Animal experts have noted that harbor porpoises are "living in the fast lane." Being smaller than other cetaceans and living in cold northern waters means that the porpoises require a lot of energy to survive, making them prone to starvation.

Curious to learn more about porpoise's lifestyle, Wisniewska and colleagues attached miniature computers developed at the University of St. Andrews to five wild porpoises. The computers recorded the porpoises' echolocation calls and the echoes that came back as those calls bounced off of nearby prey.

By analyzing the sounds, the researchers were able to determine how often porpoises attempted to catch fish. They could also estimate the size of those fish and whether the fish managed to escape.

"This is the first time we have been able to measure simultaneously how a marine mammal hunts and how often it is successful," Wisniewska said. "The trick here was to tap into the echolocation sounds that porpoises use to sense their environment. Porpoises make hundreds of clicks a second as they approach prey, and the echoes coming back give us incredible detail about what the prey are doing."

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The data showed that the porpoises hunt around the clock. The marine mammals are also incredibly successful in landing their prey.

"We were surprised by the efficiency with which these small predators feed," Wisniewska said. "A success rate of over 90 percent, translating into as many as 3,000 fish caught per day, means that porpoises are among the most successful known hunters."

There is little room for error, though, given that a porpoise can easily go hungry if its prey, ability to hunt, or other facets of its "fast lane" life are compromised.

The researchers next plan to explore the porpoise's different foraging tactics, such as hunting in various parts of the water column. The scientists will also investigate how noise from ship traffic or other human activities could affect the harbor porpoise's ability to hunt.

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Introduction

Originally designed to live on land, marine mammals are a diverse, charismatic group of animals that include more than 120 species. The animals share key characteristics of land mammals. They have hair, breathe air, give birth to live young, which feed off mother's milk when young. They have warm bodies and usually thick blubber to keep their body temperatures high. The bottlenose dolphin is probably the most widely recognized marine mammal, easily spotted just offshore from beaches around the world. Small groups of 20 or less can live in close proximity to shorelines, but groups living more offshore can reach several hundred. Bottlenose dolphin calves stay with their mothers for up to six years, learning how to hunt and become good dolphin citizens. Full-grown dolphins reach eight to 12 feet in length and can weigh up to 1,430 pounds. The bottlenose dolphin is protected in U.S. waters.

Humpback Whale

What makes them "marine" depends on the animal. They either live mostly in the sea or, like polar bears, depend on the ocean for food. The largest in the group are whales -- including humpback whales. These massive animals reach up to 50 feet in length and weigh up to 79,000 pounds. To maintain their weight, the animals feed on tons of krill and fish. They neared extinction due to whaling, but have recovered somewhat since a 1966 moratorium on whaling was introduced.

Polar Bear

While polar bears live mostly on land or ice, they are excellent swimmers and have been known to swim up to 45 miles a day. The massive animals, weighing up to 1,500 pounds, hunt mostly seals. In recent years, biologists have observed that the bears are swimming now more than ever as melting stretches the distances between Arctic ice flows. Because they depend on sea ice to hunt seals, the polar bear is considered threatened as global warming melts and thins ice in this region.

Sea Otter

This member of the weasel family is also the smallest marine mammal, with females weighing about 60 pounds and males weighing up to 90 pounds. They may be small, but they're also clever. They're the only marine mammals known to use tools. They use stones to break open clams and store food they gather in the folds of their armpits! Another feature that sets them apart is their lack of blubber. These marine mammals depend mostly on their fur to stay warm. That feature makes them particularly vulnerable to oil spills, which can compromise their fur's insulating effect.

Sea Walrus

Immediately recognizable by its long tusks and whiskers, the sea walrus is a hefty, flippered member of the Odobenidae family and is, in fact, the last living member of this group. Since both the males and females have big tusks and not much for teeth, the animals feed by sucking up shellfish from the ocean floor. So, just what are those tusks for? The longer they are (they grow to be up to four feet long in males), the higher an animal is ranked in the group. Males attack each other with their tusks to establish dominance. The ivory appendages are also handy for poking holes in the winter ice and for helping the animals pull themselves out of the water.

Manatee

Manatees, also known as sea cows, are gentle herbivores that live in marshy areas in tropical and subtropical waters. The average adult manatee can weigh up 1,200 pounds and is around 10 feet long. Because of their slow metabolism, these animals can only survive in warm waters. Due to the unusually long, cold winter this year in part of the southeastern United States, populations of manatees throughout Florida were devastated. During the day, manatees usually like to stay close to the surface. At night, manatees will often sleep about three to 10 feet below sea level. This is why these gentle animals are so often accidentally injured, maimed or killed by passing boats.

Harbor Seal

Found up and down the North American coastlines, these marine mammals spend half of their lives swimming. Although they can reach up to six feet in length and weigh around 180 pounds, when on land and in plain sight harbor seals may not be easy to spot. Their spotted brown or tan fur allows harbor seals to blend in with sand and rocks. Unlike their very vocal relatives -- sea lions and elephant seals -- harbor seals are quiet creatures that make little noise. They like to hang out on beaches, sand bars and rocks during low tide to bask in the sun and sleep, but they never go far from the water. At the slightest sign of danger, they will quickly slip back under the waves. These expert swimmers have been known to plunge to depths of more than 1,600 feet and stay underwater up to 28 minutes.