Threatened Seals Forage Far From Home, Cameras Show
for National Geographic News
|February 19, 2003|
This story is one of a series looking at National Geographic Crittercam research. Crittercam is a research instrument worn by wild animals and equipped with a video camera and other information-gathering equipment. (Get the basics on underwater and terrestrial Crittercams.)
For more on this story, tune in to the Crittercam: Monk Seals episode on the National Geographic Channel in the U.S. Watch video previews online.
Only about 1,300 monk seals are believed to be surviving in the relatively remote waters of the Northwestern Hawaiian islands. To find out more about what is causing the precipitous decline in their numbers, researchers are using National Geographic's Crittercam technologycameras attached to seals to find out how these large marine mammals find their food.
Before he began researching Hawaiian monk seals, Frank Parrish never thought much of the odd creatures. To the fishery biologist, who grew up in Hawaii, the seals were little more than lazy brown logs languishing on the beach.
That's before he saw footage of them underwater shot with National Geographic's Crittercam.
"Riding on the back of the seals, [via Crittercam's perspective], my whole perspective of them changed," said Parrish, who works for the National Oceanic and Atmospheric Administration (NOAA), a U.S. government agency. "They were no longer just logs lounging in the sun. They became guides taking us through the submarine landscape. It was fantastic."
The Crittercam study immediately offered new insights into the foraging behavior of the highly endangered monk seals. Researchers had long focused on the lush coral reefswith plenty of fish around themas the natural feeding ground for the seals. But Crittercam footage showed the seals do not stay on the reef; Instead they venture far off shore for food.
"They go out onto these deep slopes, which appear barren with pretty much just sand and loose rock, and that's where they make their living," Parrish said. "This [knowledge] has changed our mind-set. Now that we know the seals are feeding outside the atolls, we have to take that habitat into consideration in the protection of the seal."
A string of tiny atolls and islands, the Northwestern Hawaiian Islands provide a refuge for 30 species of seabirds, endangered green sea turtles, and the most primitive living pinniped: the Hawaiian monk seal. Pinnipeds are aquatic mammals with all flour limbs modified into flippers.
These seals are dark, two-tone creatures with big eyes and rounded heads that look a little bit like hoods, hence the name "monk seals." An adult can live up to 30 years and grow upwards of 600 pounds (270 kilograms).
Despite their remote and tranquil habitat, the seals are now on the brink of extinction. The population declined rapidly from 1985 to 1993 but has remained relatively stable during the last decade.
Today there are only about 1,300 monk seals left in the Northwestern Hawaiian Islands. Another ominous statistic to consider: In the main breeding colony, at French Frigate Shoals, only two out of ten pups born survive their second birthday.
"The reproductive population is getting older and older, and the survivorship of young seals is not filling the void," Parrish said. "At some point, we may stop having seal births at the Shoals."
Researchers believe that inability to successfully forage is one of the principal causes of monk seal mortality. Aggressive males killing adult females as well as juvenile seals is also a problem. Add to that monk seals falling prey to sharks and getting entangled in marine debris.
A habitat ecologist, Parrish was studying reef fish and lobsters before switching to monk seals eight years ago. At the time, he was less than thrilled to change jobs.
"It was a little intimidating," he said. "Some of these people have 30 years of experience studying monk seals. I hardly felt like someone who should be telling them about their animals."
Working under the supervision of Bud Antonelis, chief of NOAA's Protected Species Division in Honolulu, Parrish has been using 44 Crittercam units to learn where the seals find their food, something satellite tagging and depth recorders had not been able to do.
"The depth of the slopes of these islands can go from 30 meters (98 feet) to 1,000 meters (3,280 feet) in just one kilometer (about 1,100 yards)," Parrish said. "It's one of the most diverse fauna in the world. While some of these other tools were able to tell us if the seals were going to neighboring banks, they really didn't tell us what specific substrate habitats and fish types they were going after. That's when the Crittercam becomes so useful."
Parrish got a counter intuitive finding right off the bat. Instead of going after prey in the coral reefs near the beach, seals would swim way out in the ocean and focus on small fish, digging them out of the sand and flipping rocks over to eat the prey that was hiding underneath.
One possibility for this behavior is that seals find it easier to catch fish on the uniform, flat-bottom seafloor than in the complex coral reefs where the fish have evolved formidable swimming skills to avoid predation.
"Out there, you have this one rock and three fish hiding around it. When the seals flip the rock over, the fish has nowhere to hide," Parrish said. "You can't flip the coral reef over once your prey goes into a hole, so why waste your energy? If you're a veteran seal, its worth the hour's swim to get where you know you're going to have better success."
The researchers also found that seals would dive down to depths where there is no light to feed by on the seafloor. To see what the seals were doing, National Geographic's Remote Imaging Program made a special night-vision Crittercam, which emits a 5-foot (1.5-meter) halo of light in front of the monk seal, barely discernible to the seal.
The National Geographic Channel television documentary made about this work focuses on juvenile monk seals. The question is: Where, when, and on what are the seals foraging?
After putting Crittercams on one-year-old seals, Parrish found that they're swimming out to the same deep habitats as the adults.
"I thought young seals would start out in a tide pool, or something like that," he said. "But in their first year, they're out there competing with the adults and the sharks. It's a big, dangerous ocean for a young seal. The first three years make or break them."
In the 1980s first-year survival of the monk seals reached 90 percent. But in recent years these rates have dropped, and in some places less than 30 percent may survive.
The reasons for the decline in survivorship are still under debate. Significant oceanographic changes may be to blame, as well as disease and competition issues. At some point, Parrish says, the number of seals will ultimately be limited by the carrying capacity of the environment in which they live. The concern is that that this capacity may be lower than what it takes to sustain the population.
"The challenge is for these young seals, especially females, to reach maturity and start producing more seals," Parrish said.
Interactive Extra: A Virtual Crittercam Monk Seal Mission >>
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