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NATURAL HISTORY

RIBBON SEAL } Histriophoca fasciata
FAMILY: Phocidae

DESCRIPTION: Ribbon seals are medium-sized pinnipeds, reaching an average length of 1.50 to 1.75 meters and an average weight of 55 kilograms, with males being larger than females. They have a broad head, big eyes, a long, flexible neck, and a relatively slender, streamlined body. Their fur has a distinctive banding pattern characterized by four white or yellow stripes of varied width against a darker background. Newborn seals have a long, woolly white coat that is molted at weaning.

HABITAT: During the late winter, spring, and early summer, ribbon seals are completely dependent on the presence of sea ice for survival, using it for resting, giving birth, nursing pups, and molting. It also affords them isolation from terrestrial predators and disturbance, space to distribute themselves more widely, proximity to food sources, passive transport to feeding areas, sanitation provided by increased space, and shelter.

RANGE: The ribbon seal is endemic to the western and central North Pacific Ocean, with its total distributional range including the Sea of Okhotsk, northern Sea of Japan, Bering Sea, eastern Chukchi Sea, and western Beaufort Sea. Within in its entire range, the species appears to be most abundant in the seasonally ice-covered Bering and Okhotsk seas.

MIGRATION: There is very little information on the migratory patterns of the ribbon seal. Seals from the Bering Sea may either remain there throughout the year or migrate to the Chukchi Sea during the summer. Seals from the Sea of Okhotsk may summer in the Bering Sea.

BREEDING: Ribbon seals are thought to be polygamous, mating shortly after pup weaning. Males are in breeding condition from March to mid-June, and females that had successful pregnancies mate during late April and early May, while those that did not give birth the previous year may mate outside that period. Gestation takes about nine months, after which females give birth on the ice to a single pup. They nurse their pups for three to four weeks.

LIFE CYCLE: Female ribbon seals reach sexual maturity at four to five years, while males do so at five to six years. The maximum lifespan of ribbon seals may approach 30 years, but they likely live about 20 years on average. Demographically, the species exhibits delayed maturity, low reproductive rates, high adult survival, and high longevity associated with a “slow” life-history strategy, resulting in population growth rates that are sensitive to changes in adult survival, as well as slow recovery from population declines.

FEEDING: Studies indicate that the ribbon seal’s diet shifts with age and varies regionally. During the first year of life, seals primarily eat planktonic invertebrates; immature seals eat mostly shrimp and other crustaceans; and adults consume mainly cephalopods and fish.

THREATS: Currently, the ribbon seal is most imperiled by habitat alteration caused by global warming. Other threats include oil and gas development, shipping activity, pollution, hunting, bycatch mortality, and competition with fisheries for prey.

POPULATION TREND: The current population status of ribbon seal populations is unknown because recent censuses have not been conduced. In its most recent 2007 draft stock assessment, the National Marine Fisheries Service reported a global population size of 240,000 individuals, with an estimate of 90,000 to 100,000 ribbon seals in the Bering Sea.

Photo courtesy Flickr Creative Commons/jomilo75