In the biological treasure that is made up by the Northwestern Hawaiian Islands, the atoll of French Frigate Shoals stands head and shoulders above the rest of the chain. Although the atoll has just 74 acres of land, divided among a dozen or more tiny islets, it boasts some 104,000 acres of coral reef, 20-plus species of coral, more than 400 species of mollusks, and a whopping 700 species of fish. Ninety percent of the threatened Hawaiian green sea turtles nest at French Frigate Shoals, which is also home to 200,000 seabirds, representing 16 separate species.
And when it comes to the endangered Hawaiian monk seal (Monachus schauinslandi), French Frigate Shoals is Seal Central. Approximately a third of all seals in the Northwestern Hawaiian Islands use the atoll.
The largest island in the atoll is Tern. When President Theodore Roosevelt established the Hawaiian Islands National Wildlife Refuge in 1909, land on Tern Island was just 11 acres. After the United States entered World War II, the Navy expanded Tern to 34 acres by placing 660,000 cubic yards of coral fill, dredged from the atoll, behind steel plates that form a huge seawall. After the Navy left, the U.S. Coast Guard established a LORAN station on the island, which was in use until 1979.
As human activity declined, the seal population rose. By 1986, the mean beach count of seals (excluding pups) was 284, six to eight times what it had been three decades earlier. But after reaching a peak in the late 1980s, the seal population at French Frigate Shoals has declined. The annual number of births dropped from a high of 127 in 1988 to 72 in 1995. Most recently, births have increased, but survival of pups has been as low as 10 percent, according to Bud Antonelis, monk seal expert with the National Marine Fisheries Service in Honolulu. This low rate means that the normal age distribution of populations has become inverted. In other words, instead of having an age distribution chart that looks somewhat like a pyramid, with the younger ages having the greatest numbers, that for French Frigate Shoals is top-heavy. In the near future, as older females die or otherwise leave the reproductive population, there will not be the same number of younger females to replace them. As a result, says Antonelis, “we’re predicting a dramatic decline” in the population at French Frigate Shoals.
What has caused this collapse?
While scientists haven’t pinned down a single culprit, they have managed to compile a long list of suspects. They include:
Sharks
In the last three years, shark attacks on pups have increased dramatically at French Frigate Shoals – 17 in 1997, 16 in 1998, and 25 in 1999. The fact that the predation is increasing has led members of the Hawaiian Monk Seal Recovery Team to speculate that the shark behavior may be learned and could even expand in the future. “The HMSRT is very concerned that this source of pup mortality will appreciably increase the probability that the [French Frigate Shoals] population of monk seals will either fail to recover or become extirpated,” it wrote in its recent report to the National Marine Fisheries Service. It recommended that NMFS, “if at all possible,” start removing sharks before or at the very least during the 2000 pupping season. (The peak season for pupping is between February and June, although births are known to have occurred at all times of the year.)
Mobbing
Seals mate at sea, making their coupling a phenomenon that has been little observed by humans. As Timothy Ragen and David Lavigne write, “A small number of observations indicate that the male mounts the female’s back by grasping her sides with his foreflippers and biting her back.” (“The Hawaiian Monk Seal: Biology of an Endangered Species,” in John R. Twiss Jr. and Randall R. Reeves, Conservation and Management of Marine Mammals, Smithsonian Institution Press, 1999.)
“The reproductive behavior of male seals,” they go on to say, “has become a serious concern in recent years because of a phenomenon called ‘mobbing.’ Mobbing occurs when a number of males gather and repeatedly attempt to mount and mate with a single seal. The mobbed seal, which may be an adult female or an immature animal of either sex, is often severely injured or killed.”
The dying or injured seals can attract sharks, thus creating a kind of spiraling mortality among the seals. As the females die off, the competition among males for mates increases, with disproportionately more males than females in the population. This, in turn, seems to exacerbate the problem of mobbing.
While mobbing and shark predation might account for some deaths among the young seals, “the number of recorded injuries cannot account for the decline,” they write. “And, like shark predation, mobbing behavior does not explain the poor condition of juveniles.”
Lack of Food
For most of the last two decades, pups born at French Frigate Shoals have been skinnier on average than pups born elsewhere in the archipelago, while females at the atoll have reached reproductive maturity a year or more behind their counterparts elsewhere. Scientists interpret both these phenomena as indicators of relatively poor nutrition.
Not much is known about seals’ foraging and feeding habits. Some satellite tags have recorded seals from French Frigate Shoals diving to great depths and foraging in fields of gold and black coral at sites 20 to 40 miles from the atoll. Not all seals engage in this behavior, but those that do are thought to forage for deep-water eels that live among the corals.
Recently, tests of seal blubber suggest that lobsters may play a role in the seals’ diet far more important than had been thought. The abundance of lobsters has crashed in the last two decades, so much so that when divers sought to obtain lobsters as part of a survey of contaminants at French Frigate Shoals, they were able to find just one animal in three days of diving.
Commercial overfishing and decadal shifts in the ocean environment have been among the reasons suggested for the decline in lobsters. But, write Ragen and Lavigne, “regardless of the reasons for decreased prey availability, the lack of food for monk seals at French Frigate Shoals poses a serious threat to the persistence of the Hawaiian monk seals. Even if these changes reflect natural processes, the relatively high mortality of juveniles due to starvation represents an important loss of reproductive potential that will significantly impede recovery of this subpopulation and of the entire species.”
Marine Debris
When it comes to marine debris, the Northwestern Hawaiian Islands act as a sort of filter in the Northern Pacific Ocean. All kinds of discarded or lost fishing gear and trash are carried by currents to the islets and atolls, which trap and hold the detritus. The debris presents a lethal threat not just to seals – especially playful pups – but also to seabirds, turtles, and other marine life, including corals.
For several years, NMFS has worked with a number of other governmental agencies (federal, state, and county of Honolulu), nongovernmental organizations (Center for Marine Conservation, the Hawai’i Wildlife Society, the University of Hawai’i), and private corporations to organize clean-ups of the Northwestern Islands. For this, the NMFS-led Reef Cleanup Team received the National Performance Review Silver Hammer Award from Vice President Al Gore last spring. Each spring, the clean-up crews remove tons of debris from the area, but, as one fishery scientist explained recently, using a term borrowed from biologists, the “recruitment rate for debris is phenomenal. In other words, no sooner is the trash hauled out than it is replaced by still more. In 1998, six tons of debris were collected from French Frigate Shoals in just six days. And the clean-up crews estimated that they had left behind more than 70,000 discarded net segments at French Frigate Shoals and Pearl and Hermes Reef.
At least four seals are known to have perished after becoming entangled in marine debris. Probably many more have died as a result of entanglement without leaving a trace. Researchers have cut some seals from nets that, in time, would have surely killed them. At Lisianski, especially, removal of debris is credited with improved survival rates of that monk seal population.
Toxic Dumps
Five years after the Coast Guard took down its LORAN station and left French Frigate Shoals, the seawall propping up Tern Island began to show signs of serious deterioration. As the seawall failed, the island began to erode rapidly. Wildlife – including monk seals – would become trapped in the area behind the wall, with at least two seals known to have died as a result. In addition, toxic wastes that had been buried for years suddenly surfaced. Among the worst of these are lead and polychlorinated biphenyls (PCBs).
Studies by the Fish and Wildlife Service indicate that contaminants have already entered the food chain at Tern Island. That lone lobster found by divers was found to have lead levels in its tissue at 11 parts per million. Lead levels in the sediment were 10 to 44 ppm. In an area used as a landfill by the Navy, PCBs were found at 2,300 ppm, while lead was measured at 2,600 ppm.
With these toxins bioaccumulating, it was only to be expected that the seals themselves would have levels of contaminants in their body fat. This was confirmed by the Fish and Wildlife Service, which measured PCBs at 6 parts per million in monk seal blubber and at 9 ppm in seal blood.
The Monk Seal Recovery Team described the finding of PCB residues in reef fish and seals “both alarming and disconcerting.”
“Residue values of up to 42 ppm of total PCB in moray eels represent very high values in one reef predator and are certain indication of point-source input, probably from leaking transformers in the Tern Island landfill,” the team wrote in its report to NMFS of January 2000. They went on to note that the PCB measurements in monk seal blubber and blood, “are, however, a source of confusion and concern … in that they indicate levels in blood some 50 percent higher than in blubber, a situation not commonly seen in pollutant dynamics within mammals.” In other words, the team members were puzzled by a finding of higher PCB levels in blood than in fats, which usually contain the highest concentrations of oil-soluble contaminants such as PCBs. The team recommended that the PCB residue data be reviewed by an independent expert before continuing with studies on the possible impact these pollutants might have on the seals’ immune system or reproductive health, both of which can be affected by PCBs.
Meanwhile, the Fish and Wildlife Service faces the challenge of maintaining the integrity of Tern Island. The cost of repairing the seawall has been estimated at $15 million. The U.S. Coast Guard has already removed tons of debris from the area and is scheduled in 2000 and 2001 to remove what remains of the landfill.
— Patricia Tummons
Volume 10, Number 10 April 2000