General characteristics: Ribbon seals are of intermediate size compared to other ice-associated seals in Alaska waters. They are smaller than bearded seals and larger than ringed seals. Average length (nose to tail, not including hind flippers) is about 58 inches (145 cm) and average weight is 154 pounds (70 kg), about the same as harbor and spotted seals. At birth, pups are about 34 inches (86 cm), and they weigh about 23 pounds (10.5 kg). Hair color and pattern changes from birth to 4 or 5 years of age. Newborn ribbon seals have a coat of soft, white hair (called lanugo) which provides insulation until the thick layer of blubber develops. This white coat is normally completely shed by 5 weeks of age. The first-year coat does not show the distinctive coloration of adults, and, until they molt at age 1, ribbon seals are blue-black on the upper flanks and back and silver-gray on the lower flanks and belly. By age 4, the color pattern is basically one of four distinct, light-colored ribbons on a dark background—one encircling the neck or head, one encircling the posterior trunk on or behind the level of the navel, and one on each side of the body broadly encircling the foreflippers or “shoulders.” Width of the ribbons is quite variable, and on some seals they fuse together. The pattern tends to be similar on males and females. The background color in males is very dark, almost black. In females, the background is a lighter brown, resulting in less obvious contrast with the white bands.

Ribbon seals have very large eyes which seem uniformly black. The whiskers are somewhat flattened in cross section. Teeth are weakly rooted and relatively small compared to those of harbor seals. Normally there are 34 teeth composed of three upper and two lower incisors, one upper and lower canine, four upper and lower postcanines, and one upper and lower molar on each side.

The trachea or windpipe is unique, as the anterior part is broadly connected to an air sac which is present in males and females, though it is considerably more developed in males and sometimes absent in females. The air sac overlies the ribs on the right side of the body. The function of this interesting structure, which is not present in other seals, is unknown. It may be involved in production of underwater sounds.

 

Biology: Female ribbon seals become sexually mature between 2 and 4 years of age; males at 3 to 6 years. Females give birth anytime from early April to about mid-May, though the peak period is between April 5 and 15. Pups are born on ice floes. Their only protection from the elements is provided by snow drifts, ice ridges, and their mother. The nursing period lasts from 3 to 4 weeks, during which time a pup's weight more than doubles. Mating occurs about the time pups are weaned. Most adult females become pregnant every year. Embryonic development is delayed for two and one-half to three months (delayed implantation). After weaning, pups spend a great deal of time on the ice, apparently slowly achieving proficiency at diving and feeding.

During this period pups lose a considerable amount of weight, from approximately 60 to 66 pounds (27 to 30 kg) to an average of about 48 pounds (22 kg). The thick fat (blubber) reserves accumulated during the nursing period are no doubt very important to the survival of pups during this time.

 

Food habits: Ribbon seals eat a variety of different foods, but their main prey is fish. Studies by Soviets and Americans have shown that in regions where walleye pollock are present they usually comprise the major single prey item. Interestingly, the centers of ribbon seal abundance usually coincide with regions where pollock are also abundant. Other important food items in the Bering Sea, depending on the region in which the seals occur, are eelpouts, capelin, Greenland halibut, pricklebacks, Arctic cod, and saffron cod. Herring and sandlance are also probably important on occasion. Foods other than fishes include cephalopods (primarily squids), shrimps, mysids, and crabs. Little is known about the foods ingested by ribbon seals during summer to late winter. It is presumed that they prey mainly on the same items that make up their diet during spring and early summer.

 

Seasonal movements: These seals are concentrated mainly in the ice front of the Bering Sea pack during late winter and spring. It is here, during the pupping, nursing, and molting periods, that they are most commonly seen. As sea ice recedes northward and melts in May and June, ribbon seals use the shrinking ice remnants, mainly in the central Bering Sea. When that ice disappears, they become pelagic, living in the open sea. The center of abundance probably remains in the Bering Sea during the open water period, though it is known that they occur from the Pacific Ocean side of the Aleutian Islands to the summer-autumn ice edge in the northern Chukchi Sea. The population has increased since the 1960s, and more sightings outside of the Bering Sea are being made.

 

Behavior: Ribbon seals are surprisingly tolerant of boats and humans. They frequently rest on the ice at considerable distance from water and for considerable periods without lifting their heads to look around. Pups are left unattended by their mothers for long periods of time. These traits suggest that ribbon seals mainly occupy regions relatively free from predation by foxes, polar bears, and humans. They tend to be solitary, though many will occur loosely scattered throughout favorable areas. Their movement over the ice is different from that of the other northern seals. Instead of usual wiggling or inch-worm forward movement, ribbon seals slide or slither across the ice with head and neck held low, the body pulled forward by alternate extension of the powerful foreflippers, and the pelvis and hindflippers moved from side to side.

 

Population size and hunting: Because of the mostly pelagic distribution of ribbon seals, relatively few are taken by coastal-based hunters in most years. Occasionally, during infrequent years of minimal ice cover in the Bering sea, these seals are taken in significant numbers by residents of St. Lawrence and Diomede islands. Usually fewer than 100 per year are taken by Alaska residents.

In the late 1950s, prior to intensive commercial hunting by Soviet sealers, the Bering Sea stock of ribbon seals was estimated to have been about 120,000. About 13,000 ribbon seals were taken annually from 1961 to 1967, resulting in a significant decline. Quotas were reduced to 3,000 in 1969, and that quota remains in effect. Since the late 1960s, the population has increased dramatically and is now thought to be once again approaching pre-exploitation level. No studies have been conducted on ribbon seals in Alaska since the late 1970s.