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Harbor Seal
Harbor Seal
Common Name: Harbor Seal
Scientific Name: Phoca vitulina
Description
Harbor seals are mammals,
that is they are hairy, warm-blooded, air-breathing animals
which suckle their young. They weigh about 24 pounds (11 kg)
at birth and gain weight rapidly during a month-long suckling
period, perhaps doubling their weight. Average weight for
adults is about 180 pounds (82 kg); males are somewhat larger
than females. They are covered with short, stiff, bristle-like
hair. Coloration varies, but two basic patterns occur: a dark
background with light rings, or light colored sides and belly
with dark blotches or spots. Harbor seals molt annually,
usually in late summer.
Harbor seals are well adapted to life in
the sea. They are able to dive to depths exceeding 600 feet
(183 m) and can remain submerged for over 20 minutes. Oxygen-conserving
adaptations that allow such dives include reduced peripheral
circulation, reduced heart rate, and high levels of myoglobin
(muscle oxygen binder). Harbor seals are
graceful and efficient swimmers as they use their hind flippers
for propulsion and foreflippers as rudders. Movement on land,
however, is slow and laborious.
The harbor seal (Phoca vitulina),
a widespread species in both the north Atlantic and Pacific
oceans, is found in Alaska along the coast from British
Columbia north to Kuskokwim Bay and west throughout the Aleutian
Islands. Harbor seals are often called “hair”
seals by coastal residents of southern Alaska. Most harbor
seals are associated closely with coastal waters, although
occasional observations up to 50 miles (81 km) offshore have
been made. One radio-tagged animal crossed 45 miles (72 km)
of open ocean between two islands in the Gulf of Alaska, and
another moved over 50 miles (81 km) from Prince William Sound
to Middleton Island. Harbor seals haul out
of the water periodically to rest, give birth, and nurse their
pups. Reefs, sand and gravel beaches, sand and mud
bars, and glacial and sea ice are commonly used for hauling
sites. Harbor seals are sometimes found in rivers and lakes,
usually on a seasonal basis (present in summer, absent in
winter). At Iliamna Lake seals are present year-round and
are probably resident. Births of harbor seal pups
are not restricted to a few major rookeries (as is the case
for many species of pinnipeds) but occur at many hauling sites.
Harbor seals do not appear
to make long annual migrations like some species of marine
mammals. However, considerable local movements occur. Tagging
studies have shown that juveniles move up to 150 miles (242
km) from their birth places. A radio-tagged adult was discovered
120 miles (193 km) from its tagging site. As more seals are
being satellite-tagged, much more information is becoming
available about winter and summer movements.
Harbor Seal Facts
Winged Foot- The harbor
seal is the only pinniped that breeds along Puget
Sound. Pinnipeds (seals, seal lions, and walruses)
spend part of their lives in the water but depend on land
to give birth and raise young. The term "pinniped"
comes from the latin word "pinna" meaning winged and "ped"
meaning foot.
Small & Spotted- Adults are mottled tan
or blue-grey with dark spots, measuring between four and
seven feet long, weighing 250 to 300 pounds. To distinguish
harbor seals from other pinnipeds along Puget Sound (California
sea lions, Northern sea lions, and elephant seals)
look for the harbor seal's small size, earless head, and
spots.
Deep Diver- The harbor seal
can plunge 300 feet and stay underwater up to 28 minutes.
It can swim a fast 15 knots.
White Before Birth- Most harbor
seal pups shed a white wooly coat before they are
born. Sometimes pups are born in the water. Pups can often
swim after birth, when the tide returns.
How to Identify Harbor Seals
 The harbor seal, northern fur seal,
California sea lion, Steller sea lion and northern
elephant seal are the five species of pinnipeds a
visitor might see at Point Reyes. Harbor seals
can usually be distinguished from elephant seals
by size alone. The harbor seal is a rather small marine
mammal, only getting up to 1.5-1.8 m (five-six feet)
in length and 115 kg (250 pounds) in weight, whereas elephant
seals are much larger. Bull elephant seals average about 1,360
to 2,500 kg (3,000 to 5,500 pounds), while females range in
weight from 360 to 545 kg (800 to 1200 pounds). However, yearling
and weaned pups weigh about 135 kg (300 pounds), so a young
elephant seal could be mistaken for an adult harbor seal,
if one attempts to identify the seal by its size. What color
is the seal? While young elephant seals are a uniform gray,
harbor seals are typically silver, white or gray, with black
spots, although some harbor seals also are black or brown
with white spots. Sometimes a harbor seal may have a reddish
colored head or body, which is due to iron oxide deposits
on the hair shafts.
Harbor seals and elephant
seals are in the Family Phocidae (the earless seals)
so unlike sea lions and fur seals,
they do not have external ear flaps on the head, just a small
hole where their ear is. Harbor seals and elephant
seals also are unable to rotate their pelvis, and so they
drag their body inchworm fashion around on land, on beaches,
or other nearshore substrates that have a low slope. Sea
lions, in contrast, can rotate their pelvis forward
and walk on all four limbs, enabling them to use steep, rocky
shoreline habitat unavailable to harbor seals.
Harbor seals also differ from sea lions in their smaller size
and lighter color. When in water, harbor seals propel themselves
with their hind flippers in a sculling motion, and steer with
their front flippers, whereas sea lions and
fur seals propel themselves with their fore-flippers,
like wings.
Life History
The young pups are able to swim almost immediately
after birth. They normally remain with their mothers about
one month, after which they are weaned and separate from their
mother. At that time over half their body weight may consist
of fat, providing them a head start on self-sufficiency. Sexual
maturity occurs at between 3 and 7 years. Mature females mate
shortly after the weaning of their pups; however, the embryo
does not implant in the uterus until about 11 weeks later,
a trait called delayed implantation. Active
fetal development is about 8½ months.
The sex ratio of harbor seals at birth is approximately equal and remains so until about 5 years of age. Thereafter mortality rates for males are higher, and females become relatively more abundant. Maximum ages estimated from annual rings in their teeth are 26 years for a male and 32 years for a female.
Diet
Commonly eaten prey include walleye,
pollock, Pacific cod, capelin, eulachon, Pacific herring,
salmon, octopus, and squid.
Harbor seals are within
the Order Carnivora, which means that they primarily eat meat.
The meat they eat is fish in the nearshore waters of the park,
such as herring, anchovies, sardines, hake, flounder,
sole, salmon and sculpin. They also eat invertebrates
such as octopus and squid and even crabs. The harbor seal
is considered an “apex predator” because
it feeds towards the top of the food chain. Apex predators
are often used as an indication of the condition of their
ecosystem because they can’t do well unless all of the organisms
within their habitat are doing well. Point Reyes has been
using the harbor seal as one of the indicators of the condition
of the area’s marine systems. The population has grown and
stabilized over the past decade and females give birth to
pups around every year. During El Nino years, though, female
seals often skip giving birth and the population counts onshore
are lower, likely because seals are spending more time in
the water looking for food.
What Eats Harbor Seals?
White sharks are the primary
marine predator of harbor seals, but occasionally other large
sharks and killer whales eat them. Terrestrial
predators such as coyotes and bobcats can also occasionally
prey on harbor seals resting onshore, particularly pups that
are very young.
Rarely, male elephant seals
have been documented killing harbor seals in California
at harbor seal colonies such as Jenner. This elephant
seal behavior is very unusual, and the male seals do
not appear to interact or haul out with other elephant seals.
Instead, they haul out at harbor seal colonies.
Hauling Out
Harbor seals (and sea lions) haul
out (come out of the water) almost daily to rest
and to warm up. They cannot maintain their body temperature
if they stay in cold water all the time because of their smaller
size and thinner blubber layer. Northern elephant
seals lose less heat than harbor seals
because are much larger and have a thicker blubber layer that
allows them to stay at sea for months at a time before coming
onshore to rest and give birth.
All pinnipeds give birth on land, and that
is one fact that distinguishes them from cetaceans, another
group of marine mammals. Harbor seals give
birth between March and June on tidal sandbars, rocky
reefs and pocket beaches. They can give birth on areas
which are inundated at high tide because harbor seal pups,
unlike most pinniped species, can swim at birth. During the
pupping season, mother seals will spend more time onshore
nursing pups and resting, for an average of around 10-12 hours
per day. The mother harbor seal stays with the pup almost
continuously and rarely leaves the pup alone onshore. Mothers
can take their pups with them when they go swimming and feeding
because pups are adept swimmers.
A mother caresses and nuzzles its baby pup constantly, and for four to six weeks nurses it with her rich milk. The 48% fat content of milk makes the pup gain weight rapidly, and by around 30 days they are weaned. Pups weigh around 11 kg (25 lbs) at birth but when they are weaned they may weigh as much as 22 kg (50 lbs).
During the breeding season, male seals hold territories in the waters adjacent to where females haul out on shore, called maritory. Females are receptive to mating around when the pups are weaned and mating occurs in the water. Male seals will protect their maritory from other males and engage in stylized fighting during the breeding season.
Shortly after the pups are weaned, the seals begin their annual molt of their sea worn fur. The fur sheds much like a dog and the seals turn a luminous color with new fur. The molt period begins around mid-June and extends through July. During this time, seals will spend more time resting onshore because it is energetically taxing. Also, studies have shown that hair follicles grow faster in onshore than in the water. Seals can stay onshore resting for an average of 12 hours per day during the molt compared to around 7 hours per day during fall-winter months.
Abundance and Trends
Harbor seals are a difficult
species to census because they can be accurately counted only
when they are hauled out. They haul out in
thousands of locations in Alaska, and even if seals at all
sites could be counted, the proportion of the total population
hauled out at any given time is unknown. The total Alaska
harbor seal population probably ranges between 200,000
and 300,000 animals. Since implementation of the Marine Mammal
Protection Act in 1972, hunting has been restricted to Alaska
Natives. In some areas, harbor seals are an important part
of the subsistence economy. The annual harbor seal harvest
is about 2,500 to 4,000 animals.
The number of harbor seals
has declined in several areas of the Gulf of Alaska and Prince
William Sound since the mid 1970s. At Tugidak Island near
Kodiak, numbers have declined 90 percent from approximately
11,000 seals to 1,000. The reasons are unknown
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