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In-depth: Hawaiian monk seals
A struggle to survive: Environmental threats endanger monk seals
by Gretchen Weber
Although some skeletal structures of these shy marine animals haven't changed for 15 million years, their fragile
"Because of their geographic restrictions, Hawaiian monk seals are extremely vulnerable to human impact," says marine biologist and Ocean Adventures diver Holly Lohuis. "Harbor seals, for example, have the whole coast, but unlike any other seals, monk seals are limited to such a small area - the Hawaiian Islands - and this makes them more vulnerable."
Whereas other species of seals swim hundreds of miles to feed or mate, Hawaiian monk seals do not leave their island chain home. The NWHI have been referred to as "the extinction capital of the world" because even small changes in the environment have significance for the species here. Because the islands are so isolated, there is no buffer. There is no place for animals to go when conditions change, such as when the food supply decreases or the number of
Bud Antonelis, chief of the Protected Species Investigation of the Pacific Islands Fisheries Science Center, National Oceanic and Atmospheric Administration (NOAA), disputes this theory, citing the extraordinary challenges that monk seals have faced in their fragile habitat throughout the years.
"Given the history of insults to the habitat of the Hawaiian monk seals, I find it hard to say that this is a case of natural selection," says Antonelis. "There's nothing natural about monk seals being harvested by sealers and nothing natural about the activities associated with World War II throughout the Northwestern Hawaiian Islands. Also, there's nothing natural about the huge amount of dredging that was done in the past so that boats could come in and service the encampments at some of the
There is no question that human disturbances impact the seals themselves and their habitats. Some effects are obvious, such as when seals get tangled in fishing gear and other debris left behind by humans. In recent years, up to 25 seals in the NWHI have been entangled in marine trash. Although some of these trapped seals are rescued, others are already dead when found, and more are never discovered at all.
"Hundreds of tons of
But other effects of human disturbances are more difficult to measure. For example, scientists know that human activity near the beaches that seals use for pupping will cause mother seals to avoid these favored spots and instead choose locations that are less safe for themselves and their pups. This choice results in a decline in pup survival rates, which in turn results in fewer adult seals giving birth a generation later. Of course, the actual number of seals lost in this way is difficult to estimate.
Whereas the direct impact of humans plays a role in the decline of these mammals, other factors, perhaps indirectly linked to humans, also play a role, such as increased shark attacks, a decreased food supply, disease and climate change as well as the seals' own harmful behavior, such as mothers weaning their pups too soon and aggressive males severely harming juveniles in efforts to mate.
On some islands, up to 50 percent of monk seal breeding beaches simply do not exist anymore. The NOAA's Antonelis says that this could be a result of climate change - rising seas, pattern changes in currents and the slowing of coral reef growth, for example. The effects of a shrinking habitat are significant because as these pupping beaches disappear, the seals must cluster together, often on beaches that are more exposed, which makes the seals more vulnerable to shark attacks.
According to researchers in the field, less than 200 pups are born in the NWHI every year. Of those, about 90 percent survive the six week nursing period, during which they can grow from 30 pounds to 200 pounds. But when the mother's milk is gone, the pup is "weaned" and left to fend for itself. This is when the survival rate of the pups varies dramatically from island to island and from year to year. In the 1980s, the first-year survival rate was 80 percent to 90 percent, but in recent years, this rate has dropped to as low as 30 percent on some islands, such as on French Frigate Shoals. The low survival rate is especially significant given that female monk seals do not bear young until they are 6 or 7 years old, that they have only one pup per year and that most do not pup every year.
Because research shows that many juveniles die from emaciation, scientists believe that climate changes may have reduced the monk seal food supply at some islands. A reduced food supply means increased competition, and juveniles often do not have the skills to compete with other creatures. Because of new technology, including "critter cams," which are attached to creatures, scientists can now observe the underwater behavior of these seals. Recently, researchers have been able to witness young monk seals competing for food with other species. Antonelis has seen footage of sharks and other large fish gathered around a monk seal, waiting for it to overturn a rock so they can seize whatever
As mammals and as predators, monk seals are a valuable indicator species whose health reflects the health of the entire marine environment in which they live. Therefore, says Antonelis, if people are concerned about monk seals, their concern needs to center on the health of the complete habitat - not just the creatures themselves.
"We must take a holistic approach that says, 'OK, there's a reason these seals aren't doing so well, and if we cherish them and want to preserve them, we need to look at the entire
Currently, the NOAA Fisheries Service is conducting various projects designed to help the monk seal population recover, including research into seal foraging behavior, resource availability, reproductive biology and disease assessments as well as public education campaigns designed to build awareness about conservation and the fragile ecosystems of the NWHI. There are also ongoing efforts to disentangle seals and remove trash, nets and other man-made materials from seal haul-out sites. In fact, since 1996, NOAA, other federal agencies, nongovernmental organizations and the Coast Guard have removed more than 492 metric tons of marine debris from the NWHI. For more information, go to http://www.nmfs.hawaii.edu/psd/mmrp/.Sources:
Monachus Guardian.org: Information on Mediterranean, Hawaiian and Caribbean
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