Life History
By Teddi Triphon
Young harbor seal (1)
Most male harbor seals are sexually mature at 5-6 years of age, while females reach sexual maturity earlier at 2-5 years
(4). Adult males aggregate in potential breeding areas and compete intrasexually by performing aquatic displays,
underwater vocalizations, and fighting (3). Males lose up to 25% of their body weight during the breeding season due to
their activity to attract mates (4). Mating season is in the warmer months (4). Unlike other seal species, mating takes
place mostly in water (4). Females are ready to breed about 6 weeks after they give birth (4). A harbor seal’s gestation
period is around 9 to 11 months, and usually only 1 pup is born each year, and it is 70-100 cm in length and weighing 812kg (4). This shows harbor seals, as most mammals, are a K-selected species. Harbor seal can crawl and swim almost as
soon as they are born, which is beneficial to pups born in intertidal areas (2).
Abiotic Factors
Pacific Harbor seals are found along the Pacific coast of North
America. Their distribution ranges from the Pribilof Islands to Baja
California, Mexico (3). They rely on shallow areas where sandbars,
rocks and beaches are uncovered during low tides or otherwise
available for the seals to haul out (2). When site availability is low,
the process of pupping is more difficult to carry out (2) El Niño
events can decrease food availability which affects harbor seal
populations (4).
Trophic Relationships
The harbor seal diet varies seasonally and regionally, as harbor seals are
opportunistic feeders and different prey are more available during certain
times (2). They usually consume benthic and schooling prey (4). Common
prey species include herring, flounder, perch, octopus, squid, and shrimp (4).
Adult harbor seals in water are hunted by orcas, large sharks, and polar
bears. Smaller sharks, foxes, and birds of prey will also hunt pups (4).
Human Impacts on the population
Humans have hunted harbor seals for their skins, oil, and meat (4). Human
trash also affects harbor seals as they become tangled in nylon fishing nets
or plastic packaging materials. Harbor seals also ingest plastic debris, which
Biotic Factors affecting Harbor Seal Populations
can obstruct the digestive system or cause starvation (4). Harbor seals were
The abundance of prey is a density dependent factor on harbor seal
hunted by fishermen who viewed the seals as competitors for fish. Lastly,
populations, and prey availability is commonly cited as the primary
harbor seals are threatened by environmental contaminants such as
factor in observed changes in harbor seal distribution and abundance (3). pesticides, industrial runoffs, and oil, which harm their immune systems and
Disease also regulates populations. Diseases such as brucellosis,
reduce reproductive abilities (4).
pneumonia, an epizootic (an outbreak of disease affecting many animals
of one kind at the same time of phocine distemper virus, and an
influenza A virus of avian origin have all been documented and have
resulted in the deaths of hundreds of harbor seals (4). The effects of
these diseases are density dependent as well, and easily spread when
populations are high (4).
Literature and Resources cited:
1.
2.
3.
4.
Nutrient Cycling
Harbor seals are affected indirectly by upwelling, as upwelling is needed to
provide enough nutrients to the prey of the harbor seal. If there is
insufficient nutrient cycling through upwelling, harbor seals will face an
inadequacy of prey. Also, when harbor seals die, they are decomposed and
the mass they acquired throughout their lifetime is returned to the ocean.
1. Wey, Peter. Young Harbor Seal. Photograph. RedBubble. Web. http://www.redbubble.com/people/peterwey/works/2715988-young-harbor-seal. (Accessed
11/10/11).
2. Grigg, Emma Kathryn. Environmental Predictors of Habitat Use Patterns of Pacific Harbor Seals (Phoca Vitulina Richardii) in an Urbanized Estuary. Ann Arbor:
ProQuest LLC, 2008.
3. Terhune, J.M. 1985. Scanning behavior of harbor seals on haul-out sites. Journal of Mammalogy 66:392-395.
4. MarineBio.org - Marine Biology, Ocean Life Conservation, Sea Creatures, Biodiversity, Oceans Research "Harbor Seal, Phoca Vitulina at MarineBio.org."
http://marinebio.org/species.asp?id=158. (Accessed 11/16/11)