The Hawaiian Monk
Seal
The remaining Hawaiian Monk seal (Monachus
schauinslandi) populations inhabit the small,
northwestern Hawaiian Islands such as Lysan and Lisianski
Islands, French Frigate Shoals, and Pearl and Hermes
Reef. The monk seal is about 3 feet at birth and weighs
approximately 35 pounds while adults approach 7 feet and
can weigh up to nearly 600 pounds. They generally live up
to 20 to 25 years. They are relatively solitary animals
and are extremely sensitive to human disturbance. Monk
seals, being highly opportunistic feeders, have a varied
diet which includes such species as reef fishes,
octopus, and spiny lobsters. Sharks, notably the tiger
and the Galapagos, are likely their only natural predator
and may be an important cause of mortality. The monk seal
is the only pinniped species inhabitating Hawaiian
waters, ancl is one of Hawaii's only two endemic
mammals. Monk seals (Monachus monachus) are
also found in parts of the Mediterranean Sea and along
the northwestern coast of America though they are
critically endangered, now probably numbering less than
500 animals overall. The Caribbean monk seal (Monachus
tropicalis) was hunted into extinction -- the last
reported sighting was 1952.
- Numbering approximately 1,300 animals, the Hawaiian
monk seal is the only endangered marine mammal found
entirely in U.S. waters.
- Despite decades of protection and management effort
population numbers have, for the most part, steadily
declined and the species is now considered by scientists
to be highly endangered -- to the point that a natural
catastrophe, such as a disease epidemic or a mass
biotoxin poisoning event, could push them over the edge
to extinction.
- Within the last 10 years overall population numbers
have declined by approximately 30% because of a dramatic
decrease in birth rates and juvenile survival at the
species largest colony at French Frigate Shoals.
- Though historical documentation is limited, it
appears that the species was already significantly
reduced by the mid- 1800s as a result of commercial
sealing. The species was then virtually ignored until the
first population counts were conducted during the
mid-1950s. Twenty years later the population had dropped
by one-half -- from 3,000 to 1,500 -- with human
disturbance considered as the most likely main
factor.
- Protection measures against such disturbance are
presumed responsible for a very gradual increase in monk
seal numbers from 1975 to 1985. However, numbers began
declining again in 1989 -- and this decline is continuing
at the present time. Numerous factors may be involved
though much information is still lacking:
- Entanglement in 'ghost' fishing gear and other
debris: On the basis of surveys it has been
estimated that some 38,000 net fragments are ensnared
in coral reefs surrounding French Frigate Shoals
alone. In 1997, limited surveys found 16 seals
entangled in nets throughout their range.
- Reduced genetic variability because of depleted
numbers: The much-reduced numbers of Hawaiian monk
seals after sealing may have caused a 'population
bottleneck' thereby resulting in low genetic
variability and local inbreeding. This in turn may
impact reproductive performance or increase
vulnerability to disease or changing
environmental conditions.
- Death by "mobbing": In a surprising
increase of aggressive behavior, some adult male seals
have been killing or mortally-wounding adult females
and juveniles of both sexes.
- Reduced food availability: High numbers of
undersized young seals have been noticed at French
Frigate Shoals suggesting nutritional stress
from lack of food. Potential causes of food reduction
could include commercial fisheries, a climatic regime
shift, or a combination of the two.
- Poisoning from ciguatoxins (natural
poisons produced by certain coral reef-related
phytoplankton): In 1978 at least 50 seals died in a
mass mortality event that may have been caused by
ingesting ciguatoxin-contaminated food. However, the
current significance of ciguatoxin poisoning, if any,
is not known.
- Disease: As with all pinniped species, a
wide range of pathologies have been noted including
parasitism, emaciation, and infectious disease.
Recently, 9 of 12 pups captured for rehabilitation
developed serious eye problems though the prevalence
of this condition in the free-ranging population is
not known. Further, the degree to which disease is
influencing recent population trends is also
unknown.
- Despite much research, the cause of the current
Hawaiian monk seal decline remains poorly understood.
This is not surprising given the great complexities of
ecosystem dynamics and the difficulties associated with
studying a species that is highly sensitive to
disturbance and which spends much of its life in the
ocean.
- Though robust animal populations can withstand major
natural mortality events (e.g., from disease, food
shortage, etc.), the same occurrences in depleted
populations such as the Hawaiian monk seal can be
disastrous. Indeed, approximately 60% of the isolated
west African population of Mediterranean monk seals died
recently in a mass mortality event (likely caused by
algal poisons), thus further pushing the species as a
whole towards extinction.
- While such possibilities continue to remain a threat
for the Hawaiian monk seal, the species also faces
potential, and human-related, problems. The major of
these include: marine debris entanglement and fisheries
related food shortage; disturbance from proposed
fisheries-related activities (e.g., for black coral);
disturbance from a U.S. Navy program to construct and
operate missile launch sites in the northwestern Hawaiian
Islands; and, a crumbling seawall which threatens the
continued viability of the monk seal population at Tern
Island.
- Because of the rapidly increasing number of
threatened and endangered species and the costs required
to effectively conduct research and implement protection
programs, it has been argued that overall government
funding for such may now not be adequate. This clearly
seems to be the case for the Hawaiian monk seal. Research
into their foraging ecology and important programs
involving seal translocation and the removal of net
debris have been restricted, and the seawall at Tern
Island continues to deteriorate.
Kretanann, M B. et al. 1997. Low genetic variabiliy in
the Hawaiian monk seal. Conservation Biology II (2):
482-490
Marine Mammal Commission. 1998. Hawaiian monk seal, pp.
45-53 in Annual Report to Congress. Marine Mammal
Commission, Bethesda, MD.
|
