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.
Record numbers of California sea lion pups have been starving and stranding on beaches by the thousands in recent years, and now new research finds that a decline in their mothers’ food quality is behind the disturbing trend.
High calorie sardines and anchovies are now harder for sea lion moms to find, causing them to eat more rockfish and market squid that can be great for people on diets, but aren’t as hearty a meal for hungry sea lions. The findings are published in the journal Royal Society Open Science.
“For human consumption, highly oily fish may actually be less desirable to consumers,” lead author Sam McClatchie told Discovery News. “In contrast, for predators with high energy demands, such as nursing female sea lions, eating fish with higher energy density due to higher content of calories and fats provides a more effective way to meet their nutritional demands.”
Human demand for anchovies, in particular, has been on the rise due to the savory fish’s popularity in Caesar salad dressing and in “junk foods” like pizza. The real junk food for breeding female sea lions, minus the high calories, turns out to be the other types of fish that are now more prevalent in their feeding areas off central California.
McClatchie, an oceanographer at NOAA Fisheries Service’s Southwest Fisheries Science Center, and his team studied both the sea lion pup strandings and population trends for fish that adult sea lions eat. Data for the latter came from surveys that are conducted each summer off the coast of California.
A perfect storm now appears to be in place that is hurting the sea lions. Due to conservation efforts, numbers of these marine mammals increased from about 50,000 40 years ago to around 340,000 now. No one knows what the historic populations were like, given that Native Americans also frequently hunted sea lions and there are no detailed sea lion population estimates prior to the 1970s.
At a time when the sea lion population appears to be approaching what the researchers call its current “carrying capacity” for the region, sardine and anchovy numbers plummeted while market squid and rockfish abundance increased.
Several researchers, such as marine biologist Malin Pinsky of Rutgers, have attributed this and prior dramatic fish population plunges to overfishing by humans. He and his team note that such collapses started to occur more frequently in sardines and anchovies after the advent of efficient fishing vessels and techniques following World War II. Anchovies and sardines are important to the pet food and fish oil industries, in addition to their other mentioned common uses.
“Overfishing is a problem throughout the world and across all species, including slow-growing fish like sharks, many of which are in serious trouble,” said Pinsky. “But it turns out that fishery collapses are three times more likely in the opposite kinds of species — those that grow quickly.”
McClatchie and his team, however, believe that the problem is “environmental,” and not because of overfishing. This distinction is important, as federal regulators are planning to do an official stock assessment of anchovies in the fall and will consider updating the 25,000-ton rule that now limits catches.
“Sardine and anchovy populations both show large inter-annual variability that is environmentally driven and prior to any fishing,” McClatchie said.
Joshua Lindsay of the National Marine Fisheries Service told Discovery News that populations of anchovies, sardines and other small fish “are linked to prevailing environmental conditions. NMFS has been working to better understand these environmental processes driving fish populations as well as the diet linkages between forage fish species and higher order predators to enhance the ecosystem science used in our fisheries management.”
Marc Mangel, a professor at both the University of California at Santa Cruz and the University of Bergen, says the study “will help refocus the discussion about the causes of sea lion declines. More importantly, in my opinion, it is a terrific example of how we can use marine mammals and birds as sentinels or samplers of the environment.”
In the meantime, the short-term outlook for sea lions is worrisome, particularly for rescue centers that have been stretched to their limits. McClatchie and his colleagues say that they “expect repeated years with malnourished and starving sea lion pups,” but can’t predict when that will end.