Ribbon Seal (marine mammals)

 

I. Diagnostic Characteristics and Taxonomy

he ribbon seal is an endemic species of the North Pacific. Although it used to be grouped together with the Greenland seal into the same genus, Pagophora (Naumov and Smirnov, 1936), it is now considered to belong to the monotypic genus Histriophoca (Scheffer, 1958; Geptner et al. 1976).

The ribbon seal’s coloring is bright and unusual (Fig. 1). There are four light stripes against a black or brown background: one stripe goes round the neck, another encircles the body at the bottom of the flippers and two more symmetrically underline the base of the pectoral flippers. During the first year the color is fairly dull and becomes brighter only when the seal is 2 years old. The skull of the ribbon seal is short, the cranium braincase and cheekbones are wide, and the face is short and narrow. The teeth are small; their number varies from 32 to 36 (Geptner et al, 1976).

II. Anatomical and Physiological Features

The ribbon seal differs from the rest of the seals by its respiratory anatomy. There are no lobes in the lungs; the trachea in its middle part consists of semirings at the top and a membrane at the bottom. Through a narrow fissure the membrane is connected with a thin-walled air sac that is quite well developed in a mature male. Females have smaller air sacs, whereas immature animals have nondeveloped ones. The air sac also functions as a hydrostatic organ used in phonation (Sokolov et al, 1968).

The ribbon seal has several features that show the degree of its adaptation to deep diving and fast swimming, with well-developed internal organs among them. Their weight as a proportion of body weight is high: heart on average, 8%o, lungs 17%c. diaphragm 7%c, liver 22%o, and kidneys 2.7%o. These indices are higher than those of other seals (Fedoseev, 2000).

The ribbon seal is physiologically well adapted to a pelagic lifestyle and deep diving (up to 600 m): it has the highest number (3.9-4.7 million) and volume (50-72%) of erythrocytes and the highest blood hemoglobin contents (18-26 g%) among all seals (Sokolov, 1966).

III. Distribution and Population Size

The ribbon seal inhabits that area between the southern part of the Chukchi Sea and Japan and Korea. There are three populations of this particular species: two in the Okhotsk Sea and one in the Bering Sea (Fig. 2). A part of the latter in spring and summer migrates to the southern region of the Chuckchi Sea. Single ribbon seals have been seen close to the California coastline (Morro Bay). The range of the migration is little known.

In 1961 the population size of the ribbon seal in the Bering Sea was estimated at 115,000 to 120,000. By 1969, because of sealing, the population was reduced to 60,000 to 70,000. After sealing had been limited to 2000 to 3000 animals a year (vs 9500), the population size by 1987 reached 120,000 to 140,000. In the Okhotsk Sea, according to data collected for many years, the population size fluctuated between 200,000 (1968-1974) and 630,000 (1988-1990). For many vears the average number was 370,000 with the central northwestern population being 320,000 and the southern 50,000 (Fedoseev, 2000).

IV. Ecology

The ribbon seal belongs to the ice seals, whose life is closely connected with ice. However, if the ice is thicker than 10-15 cm, the ribbon seal is unable to make holes in it. This is why it dwells in areas with stable white ice broken into huge chunks and avoids areas of solid ice fields. Broken ice usually appears in regions of an abrupt continental shelf, where due to high water circulation this form of ice dominates; moreover, it constantly gets moved around. In such areas there are a lot of fish and squid, which are the main foods for ribbon seals. In the Bering Sea, these areas coincide with the sea ice edge. In the Okhotsk Sea, where the climate is more severe and ice covers up to 80-90% of the sea surface, reproductive rookeries are located deep in the ice, whereas in the southern part of the sea they can also be found where there is a deep sea bed with cyclic water turnover that facilitates spacing the ice out.

Male ribbon seal.

Figure 1 Male ribbon seal.

The period of mating and pup rearing is adjusted to spring ice breakup. When it shifts together with the beginning of ice breakup, this results in the reproductive isolation of different populations. It is in the southern part of the Okhotsk Sea where ribbon seals start delivering the pups the earliest, i.e., in the second part of March. In the northwestern part of the Okhotsk Sea and in the Bering Sea it takes place in April, with the peak in the middle of the month.

There are four reproductive ecotypes related to ice formation (Fig. 2). What is special about the reproductive ecotypes is that the variety of breeding conditions defined by different types of ice, its thickness, shape, the hummocks, the amount of snow on it, its location, and the speed of its decomposition determine the time of rearing and pup growth, which is reinforced by natural selection. Due to the existence of ecotypes, the populations of the ribbon seal are adapted to bodi short-term (within 1 year) and long-term (several years long) modifications of ice conditions: it has a wide range of features adapted to breeding in various conditions and can keep population size relatively stable.

The spatial structure of the populations, their morphological characteristics, and the maturation of animals in different populations are determined by the proportion of the ecotypes in different regions. Ribbon seals that breed on the ice above the deep sea bed in the southern part of the Okhotsk Sea are most specific—both the conditions of reproduction and the foraging situation are different from those in shelf zones and abrupt continental shelf zones in the northwest and in the Bering Sea.

V. Forage

Thirty-five species inhabiting the Okhotsk and the Bering Sea constitute the ribbon seal’s food. During the first year it feeds mostly on euphausiids. At the age of 1-2 it feeds mostly on shrimp, whereas adult animals eat cephalopods and fish. In the Okhotsk Sea, adult animals eat mostly Alaskan pollack (65%), whereas in the Bering Sea they mostly feed on squid and octopus (67%). The ribbon seal’s daily consumption is 8-10 kg, including invertebrates and fish; the annual consumption of the whole ribbon seal population is more than 300,000 tons in die Bering Sea and about 1,160,000 tons in the Okhotsk Sea (Fedoseev, 2000).

VI. Life Cycle and Behavior

The length of a newborn seal’s body is from 73 to 98 cm and it weighs 6-10 kg; specific features of different populations are taken into account. After the first year of life, the length of animals belonging to different populations becomes 128-134 cm and the weight 40-50 kg. During the period of maturation when the animals are 2-3 years old, the average body size is 145-155 cm and the weight is 55-70 kg, whereas mature animals’ (who are more than 10 years old) body length is 165-175 cm and weight 72-90 kg. Ribbon seals inhabiting the southern part of the Okhotsk Sea are bigger and heavier than those from the northwestern part and from the Bering Sea.

During the breeding season (March-April), animals do not segregate by age and sex; adult animals dwell on the ice to give birth to pups and immature seals are there for molting. In May and June when adult animals start molting, gathering of ribbon seals on the ice increases because of melting and decomposition of the ice. In June, when there is little ice left, one can see a mass of ribbon seals consisting of different populations. The latter should be taken into consideration when a population approach to data analysis is used. Where the ribbon seal dwells when there is no ice left: has been little studied.

Different populations and breeding sites of the ribbon seal in North Pacific seas. Broken line, ice edgesolid areas, breeding sites; horizontal stripes, the Bering Sea population; vertical stripes, the northwestern part of the Okhotsk Sea population; and diagonal stripes, the southern part of the Okhotsk Sea population.

Figure 2 Different populations and breeding sites of the ribbon seal in North Pacific seas. Broken line, ice edgesolid areas, breeding sites; horizontal stripes, the Bering Sea population; vertical stripes, the northwestern part of the Okhotsk Sea population; and diagonal stripes, the southern part of the Okhotsk Sea population.

Due to the pelagic way of life and dwelling far from the sea coast, the ribbon seal has not developed a fear of humans. This seal is very trusting and hardly reacts at all when a ship approaches and people come out on the ice. It makes the ribbon seal a perfect object for ecological tourists who can take pictures of the animals on the ice. In the open water the ribbon seal is hardly noticed at all as it disappears from the sea surface noiselessly: it sticks its black head out vertically and takes it back like a periscope.

VII. Demographic Parameters of Populations

The ribbon seals age is defined by horn covers on the claws and the layers of cement on the tusk apexes; analysis shows that it can reach the age of 30 and beyond.

Analysis of age data and the female contingent in the breeding process showed that the proportion of newborns in populations can be 24-31%, and the proportion of females having pups for the first time (recruits) 4-6% (Fedoseev, 2000). During the period of study (1982-1985) the potential of population growth in the Bering Sea was lower than in the Okhotsk Sea because the range of reproductive ecotypes and the distribution in the Okhotsk Sea were wider than in the Bering Sea (Fedoseev and Volokhov, 1991; Fedoseev, 2000).

VIII. Relations with Humans

Between 1956 and 1992, seals living on ice, ribbon seals among diem, were the subject of commercial hunting. Up until 1969 the hunt was not limited, with the average annual yield being 11,000 animals in die Okhotsk Sea and 9000 in the Bering Sea. Between 1969 and 1992 the annual yield was reduced to 5000 to 6000 animals in the Okhotsk Sea and 3000 to 4000 in die Bering Sea.

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