Harbor Seals: The Diminutive Pinniped Living Large

Generally, harbor seals forage at night and return to the shore during the day to rest. photo MBNMS

Harbor seals are one of the most ubiquitous marine mammals, occurring in coastal areas around the world. Although often seen resting on shore (e.g., on mudflats, isolated sandy beaches, and offshore rocks), these creatures are not easily approached and spend most of their time underwater, so much of their life has remained a mystery. Below some of the results of harbor seal studies conducted by researchers from Moss Landing Marine Laboratories (MLML) are described.

In the eastern North Pacific, harbor seals occur from Baja through Alaska, with about 30,000 off California. At birth they are about eighty centimeters in length and ten kilograms in mass, and they are weaned within three to six weeks. The largest individual we have captured in Elkhorn Slough was a 145-kilogram male. Harbor seals have their pups and mate at different times, depending on latitude: they pup earliest in Baja and progressively later northward. Harbor seals in Puget Sound, however, pup one month later than individuals at the same latitude on the Washington coast. Meg Lamont (MLML 1995) determined that the individuals in Puget Sound were genetically different than those on the coast and probably more closely related to harbor seals to the north. This probably was caused by past glacial periods when harbor seals were forced southward, and a small group became reproductively isolated in southern Puget Sound.

After the pups are weaned they disperse widely, probably trying to find suitable habitat. Michelle Lander (MLML 1998) examined the survival and dispersal of wild and rehabilitated harbor seals pups. She found that, although there were some blood values that differed between wild and rehabilitated pups, once released, dispersal, diving behavior, and survival were relatively similar between the groups. Stori Oates (MLML current) also has found juveniles dispersing great distances. One of the factors that probably determines choice of location is presence of food resources.

In their early years, harbor seals eat small fishes and crustaceans, but as they mature they begin eating larger fishes and cephalopods (e.g., squids). Mike Torok (MLML 1994) determined that in San Francisco Bay, harbor seals were eating mostly staghorn sculpin, plainfin midshipman, and white croaker, but the most common prey was yellowfin goby—an introduced species from Asia. It is possible that harbor seals are helping contain the numbers of this introduced species. In Monterey Bay, Dion Oxman (MLML 1995) and Steve Trumble (MLML 1995) found that harbor seals ate primarily octopus, market squid, spotted cusk-eel, flatfishes, white croaker, and juvenile rockfishes, generally five to twenty centimeters in length.

Harbor seals are susceptible to human influences. photo MBNMS

In Elkhorn Slough we have radio-tagged individuals and monitored their movements and dive behaviors. Generally, harbor seals exit the Slough at dusk and forage in specific locations in the north bay until daylight, when they return to the Slough to rest. Harbor seals tagged off Monterey also foraged in the northern portion of the bay at night, indicating this part of the bay may have more food resources for harbor seals. Tomo Eguchi (MLML 1998) attached time-depth recorders to harbor seals and found that they could dive deeper than 500 meters and remain submerged in excess of twenty minutes, although generally dives averaged less than 100 meters deep and eight minutes long. Some harbor seals fed in Monterey Canyon.

The reproductive behaviors of harbor seals are poorly known because mating occurs underwater. Teri Nicholson (MLML 2001) identified more than 300 individual harbor seals off Monterey and used this to examine social structure. Older, larger males made long-duration, low-frequency calls (like a roar) underwater that attracted younger males. During frequent interactions underwater these males apparently established a hierarchy, and possibly the calls helped with recognizing higher ranking males in the society. Playback of calls from young males will elicit a response from older males apparently holding underwater territories (Sean Hayes, University of California Santa Cruz student). This indicated that mature male harbor seals may establish underwater territories, use vocalizations as a way of advertising their presence, and presumably enhance their ability to mate with females.

Because harbor seals are coastal, they are susceptible to human influences (e.g., pollutants, disturbance, hunting). Rob Suryan (MLML 1995) found that, in Puget Sound, they were disturbed on 71 percent of days, often by powerboats that approached within 130 to 150 meters of seals resting on shore.

Disturbance often leads to short-term effects of animals vacating an area, but pollutants can have a long-term effect. Although Doreen Moser (MLML 1996) did not find that harbor seals in Elkhorn Slough had elevated levels of DDE, PCB, or heavy metals, she did find that seals with red pelage had hair shafts with irregular cuticles, possibly allowing deposition of iron. We suspect that the degradation of the hair may be from ingestion of selenium, which can alter the composition of hair. Dianne Kopec and I have found that some harbor seals in San Francisco Bay had levels of PCBs, mercury, cadmium, and selenium that exceeded levels of toxicity.

Although harbor seals are fairly small pinnipeds, they live an amazing life—becoming independent from their mothers within a month of birth, diving deep into Monterey Bay for food, roaring underwater to attract mates or establish territories, traveling thirty miles each night to feeding areas—all the while coping with increased levels of human disturbance and pollutants. Maybe they are not as small as we thought.

Jim Harvey
Moss Landing Marine Laboratories