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.
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.
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.
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.
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.
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.
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.
The deaths of over 100 melon-headed whales, which stranded on the shores of a lagoon in northwest Madagascar in 2008, were likely primarily triggered by a form of sonar being deployed by an ExxonMobil survey vessel, according to a scientific review panel.
This is the first known marine mammal mass stranding event to be closely associated with what are known as high-frequency mapping sonar systems; but it is merely the latest in a long line of incidents in which industrial noise in the ocean has been implicated in deaths and injuries to marine mammals, and specifically cetaceans.
One area of particular focus is the use of active sonar by the United States and other navies. In 2004, the Scientific Committee of the International Whaling Commission concluded that “there is now compelling evidence implicating military sonar has a direct impact on beaked whales.” In 2001, the U.S. Navy acknowledged that its active sonar played a role in the stranding deaths of 14 beaked whales, two minke whales, and a dolphin in the Bahamas in 2000. Necropsies of the beaked whales revealed that the animals had suffered acoustic trauma resulting in hemorrhaging around the brain, in the inner ears, and in the acoustic fats located in the head that are involved in sound transmission.
Then there is the use of air guns to conduct seismic surveys for oil and gas deposits in the sea bed. Because of concerns that the intense sounds from these air guns can either cause physical damage to cetaceans or cause them to flee, the U.S. Marine Mammal Protection Act requires visual observers to examine the area for marine mammals for a period of at least 30 minutes. Assuming the coast is clear, the survey must ramp up slowly by firing first one seismic gun and then others over a period of between 20 and 40 minutes. However, if a whale or other marine mammal appears within an exclusion zone of 500 meters from the center of the seismic array, the operation must shut down, and visual examination must resume for 30 minutes.
The presence of seismic survey vessels was initially considered a likely reason for the 2008 Madagascar incident, not least because melon-headed whales are an open-ocean species that had never before (and have never since) been recorded in the island’s shallow tidal Loza Lagoon estuarine system; it seemed reasonable to infer that something had startled them into entering the area. Experts with the International Fund for Animal Welfare and the Wildlife Conservation Society attempted to rescue the whales, but with minimal success, largely because of the large distance (over 69 km/43 miles) from the point where they were becoming stranded to the open ocean.
Following the incident, the International Whaling Commission facilitated a review of the evidence and an Independent Scientific Review Panel (ISRP) of five experts was invited to examine that evidence. The ISRP has just published its findings, and concluded that air gun blasts from seismic survey vessels in the area were not to blame, as they in fact took place several days after the incident.
However, a vessel using a different type of surveying technique — a high-power 12 kHz, multi-beam echosounder system (MBES) — was, says the report, “moving in a directed manner down the shelf-break the day before the event, to an area approx. 65 km offshore from the first known stranding location. The ISRP deemed this MBES use to be the most plausible and likely behavioral trigger for the animals initially entering the lagoon system.”
Most attention has focused on the likely impacts of mid-frequency and low-frequency sound sources, because of their greater propagation through water; it is possible, the ISRP noted, that high-frequency sounds like the MBES also affect some cetaceans more than previously realized but that, under normal circumstances, those cetaceans swim away. It was to the considerable misfortune of those melon-headed whales five years ago that they were in the wrong place at the wrong time, and that when they turned tail and fled, they swam into an inhospitable area from which there was no escape.
“Implications go well beyond the hydrocarbon industry, as these sonar systems are widely used aboard military and research vessels for generating more precise bathymetry (underwater mapping),” Howard Rosenbaum, director of the Ocean Giants program for the Wildlife Conservation Society, said in a statement. “We now hope that these results will be used by industry, regulatory authorities, and others to minimize risks and to better protect marine life, especially marine mammal species that are particularly sensitive to increasing ocean noise from human activities.”
IMAGE: A pair of melon-headed whales breach the surface off Reunion in the Indian Ocean. (Wolfgang Kaehler/Corbis)