• Northern or Steller sea lion, Eumetopias jubatus
• California sea lion, Zalophus californianus

Nomenclature

Occasionally this species is referred to as Eumetopias jubata. The Northern Sea Lion is also commonly named the Steller Sea Lion.

Appearance

The Northern sea lion is the largest of the sea lions. Males may be upto 310 cm in length and weigh around 900 kg. Females are much smaller, 200 cm (291 according to Loughlin and Nelson, 1987) and they can weigh upto 300 kg. The Northern sea lion is usually yellowish brown in color. The males have distinctive manes. Pups are dark brown during the first year.

Distribution

In the breeding season, the males form territories on rocky, semi-exposed areas and beaches. The range of this species is from the Sea of Japan at 43°N, north to the Pacific rim at 66°N and then south the North American Pacific coast to San Miguel Island at 34°N. Some migration seems to occur. On the Oregon coast, Northern sea lions and California sea lions live together in caves.

Feeding

The diet consists of mainly non-commercial fish species and cephalopods. Also the commercially exploited pollock can be part of the diet (Loughlin and Nelson, 1987).

Population dynamics and life history

The age at maturity is for females 4-5 years, for males 7-9 years (9-13 years according to Loughlin and Nelson, 1987). The pregnancy rate is 85-86%. Gestation lasts one year, including a delay of implantation of about 3 months. Longevity is estimated to be about 23 years.Pup mortality ranges, depending on the situation in the rookery, from 10 to 100%. York (1994) estimated the survival rates for animals of age 3-4 at 0.93, with slowly declining survival rates at increased age (0.841 at age 12-13). York estimated the fecundity (average number of female offspring produced by each female) of animal of 6 years and older at 0.315 for the Alaskan population.

Trophic relations

There might be some competition with Zalophus. Natural predators include sharks and the killer whale.

Interactions

Northern sea lions get killed in nets in the pollock fisheries in Alaska. In 1982 between 958 and 1436 drowned in the nets of fishermen. Due to changes in fishing techniques and gear, this number was reduced to 237-355 in 1984 (Loughlin and Nelson, 1987). Mate (1982) estimated an annual incidental kill in fisheries of 2500 in the US fisheries and 12500 in other fisheries. In the southern part of the East Pacific range, the sea lions seem to suffer from pollution.

Population size

Mate and Gentry (1979) mention the following data for Northern sea lion population:

• San Miguel: < 100
• Año Nuevo: ±2000
• Oregon: 2000
• Washington: 600
• British Columbia: 5000
• Kerouard and Scott Islands: > 200,000
• Hokkaido: 10,000- 13,000 (this is a non-breeding population)
• Soviet Union: 20,000-50,000.

Exploitation

In the US this species is protected. A few can be taken for public display purposes. Small numbers are taken for consumption in Alaska and Japan.

Threats to the population

Currently the population as a whole is not in danger. If the pollution in California continues to increase this might become a threat to the local population in the near future. However, the Alaskan population has been declining since 1975 at a rate of 5% per year (York, 1994). The most likely explanation for this is an increase in juvenile mortality and/or a decrease in fecundity (reproduction).

References

Loughlin, T.R., Nelson, Jr., R. (1987)

Incidental mortality of Northern Sea Lions in Shelikof Strait, Alaska. Mar. Mamm. Sci. 2(1): 14-33

Mate, B.R. (1982)

History and present status of the Northern (Steller) sea lion, Eumetopias jubatus. in: Mammals in the Seas, Volume IV: small cetaceans, seals, sirenians and otters. FAO Fisheries Series, No. 5, vol. IV, pp. 311-317

Mate, B., Gentry, R.L. (1979)

Northern (Steller) Sea Lion. in: Mammals in the Seas, volume II: pinniped species summaries and report on sirenians. FAO Fisheries Series No. 5, Vol II, pp. 1-4

Schusterman, R.J. (1981)

Steller Sea Lion, Eumetopias jubatus (Schreber, 1776). in: Ridgway, S.H. and Harrison, R.J. (eds.): Handbook of Marine Mammals, vol. 1: The walrus, sea lions, fur seals and sea otter, pp. 119-141, Academic Press Inc., Ltd, London

Trites, A.W. and Larkin, P.A. (1996)

Changes in abundance of Steller sea lions (Eumetopias jubatus) in Alaska from 1956 to 1992: how many were there? Aquatic Mammals 22(3): 153-166

York, A.R. (1994)

The population dynamics of northern sea lions, 1975-1985. Mar. Mamm. Sci. 10(1):38-51

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Nomenclature

Three subspecies are recognized: Zalophus californianus californianus (Lesson, 1828), Zalophus californianus wollebaeki (Sivertsen, 1953) and Zalophus californianus japonicus (Peters, 1866), each living in a clearly separate range. According to Rice (1998), the differences between these types justifies classification as separate species: Zalophus californianus, Zalophus wollebaeki and Zalophus japonicus

Appearance

Males measure on average 220 cm and weigh 275 kg. Females are much smaller: 180 cm and 91 kg. Pups are about 75 cm long and weigh 5-6 kg. The coat is dark greyish brown to black. The males have small manes and an enlarged sagittal crest.

Distribution

Males form territories in breeding time on rocks and sand or gravel beaches. Each subspecies has its own range. Zalophus californianus californianus ranges from the West coast of Mexico at 19°N to British Colombia at 51°N. Zalophus californianus wollebaeki lives exclusively on the Galapagos Islands. Zalophus californianus japonicus is located in the Sea of Japan, but is now probably extinct.

Feeding

Not much is known about the feeding habits of the California sea lion. Apparently it is an opportunistic feeder. Its diet includes several fish species, such as anchovy, whiting and herring, as well as squid.

Population dynamics and life history

Despite its popularity as a zoo animal, very little is known about its vital parameters in the wild. The age of maturity, the pregnancy rate and mortality rate are unknown. Gestation lasts about 50 weeks and lactation 5 to 12 months. The longevity is estimated to be around 17 years.

Trophic relations

The California sea lion competes with Eumetopias for habitat and food. Natural predators include sharks and killer whales.

Interactions

There is competition with commercial fisheries for species such as anchovy and herring. Sea lions are known to damage fishing gear and steal or destroy fish in the nets. As a result a lot of California sea lions drown in nets. DeMaster et al (1985) reports that in 1980 at least 1571 sea lions were killed, mainly in gill nets. Recently, a number of California sea lions were involved in a conflict with the steeleye salmon fisheries in Washington. The animal that was considered the main culprit was captured and taken to Sea World. (A license to shoot him was already being considered). Pollution, especially with DDT and PCB, can potentially create problems for this species. Mate (1982) point to the possibility of exchanges of diseases between sea lions and man.

Population size

Mate (1979) reports a total population for Zalophus californianus californianus of around 50,000, which could be increasing. Mate (1982) estimates the total population at 75,000 to 100,000 and calls it a conservative estimate. The Galapagos sea lion, Zalophus californianus wollabaeki, numbers about 20,000. The japanese sea lion, Zalophus californianus japonicus, is probably extinct.

Exploitation

The California sea lion is a protected species throughout its range. The recent successes in captive breeding have made live capture for public display purposes unnecessary.

Threats to the population

Over-exploitation of food fish species, such as herring and anchovy, could have negative effects on the population. Also the increase of environmental toxins can have negative effects.

References

DeMaster, D., Miller, D., Henderson, J.R., Coe, J.M. (1985)

Conflicts between marine mammals and fisheries off the coast of California. in: Beddington, J.R., Beverton, R.J.H. and Lavigne, D.M. (eds.): Marine Mammals and Fisheries, pp. 111-118, George Allen & Unwin (Publishers) Ltd, London

Mate, B. (1979)

California Sea Lion. in: Mammals in the Seas, volume II: pinniped species summaries and report on sirenians. FAO Fisheries Series No. 5, Vol II, pp. 5-8

Mate, B.R. (1982)

History and present status of the California sea lion, Zalophus californianus. in: Mammals in the Seas, Volume IV: small cetaceans, seals, sirenians and otters. FAO Fisheries Series, No. 5, vol. IV, pp. 303-309

Odell, D.K. (1981)

Calfornia Sea Lion, Zalophus californianus (Lesson, 1828). in: Ridgway, S.H. and Harrison, R.J. (eds.): Handbook of Marine Mammals, vol. 1: The walrus, sea lions, fur seals and sea otter, pp. 67-97, Academic Press Inc., Ltd, London

Rice, D.W. (1998)

Marine mammals of the world - Systematics and distribution
Society of Marine Mammalogy Special Publication Number 4, 231 pp.

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