A. Longevity.
1. Harbor seals may live a maximum of about 25 to 30 years. Males tend to have a shorter lifespan, possibly due to the stress of fighting during breeding seasons.
2. Pup mortality is about 21% in the first year; not unusually high compared to most species of animals in the wild. Pups may starve, be abandoned, wash away from pupping areas during high seas, or become ill or injured.
B. Aging studies.
As a harbor seal ages, it periodically produces growth layer groups of dental material. Age can be estimated by examining a sliced section of a tooth and counting these layers.
Researchers can estimate a harbor seal's age by examining a sliced section of tooth and
counting the growth layer groups.
C. Predators.
Harbor seal adults and pups can be preyed upon by killer whales, sharks, polar bears, Steller sea lions, walruses, coyotes, and eagles.
D. Human impact.
1. In the 1900s, fur traders hunted harbor seal pups for the fine coats they have when they are less then four weeks old.
2. Harbor seals were hunted by salmon fishermen because of fishing competition. Hunting was so extensive that many harbor seal populations abandoned traditional haul-out areas.
3. Marine debris is a threat to harbor seals. They can become entangled in nylon fishing nets or plastic packaging materials, causing severe injury or drowning. Harbor seals also ingest plastic debris, which can cause starvation (bulky plastic in the stomach sends a "full" signal to the brain, so normal eating is inhibited). Ingested plastic can also cause obstructions in the digestive tract.
4. Indigenous Arctic peoples legally hunt harbor seals for food, clothing, and other raw materials. For centuries, hunting harbor seals has been an important part of their culture and traditions.
E. Disease and parasitism.
1. Harbor seals are susceptible to a number of diseases and parasites.
a. Harbor seals may suffer from viral and bacterial infections.
b. Harbor seals are host to a variety of parasites, which may attack the heart, lungs, blood vessels, stomach, intestines, nasal cavities, and skin.
2. In 1988, a devastating epizootic (animal equivalent of an epidemic) wiped out more than 18,000 harbor seals throughout northern Europe. The epizootic was caused by a previously undiscovered virus resembling canine distemper. The virus was named phocine distemper virus (PDV). The harbor seals died from the PDV virus itself as well as from secondary viral and bacterial agents.
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