Harbor Seal Facts and Figures
Harbor seals are fascinating creatures
with a number of
unique adaptations to accommodate their cold aquatic
habitat. The seals seen at Rome Point are almost exclusively
Western Atlantic Harbor Seals (Phoca vitulina concolor). On some
occasions several Grey seals might be spotted; much more rare are
sightings of harp seals or hooded seals. Juvenile harp seals
and harbor seals are indistinguishable at a distance (to me at
least) so some mis-identification is possible.
Population and Distribution - The best estimate is about 60,000 to 70,000 seals ranging from eastern Canada down to New Jersey. There are reports of individual seals being spotted as far south as Florida. The population is thought to be increasing steadily, so harbor seals are not a threatened species at this time. Harbor seals in the United States were protected under the Federal Marine Mammal Protection Act in 1972; the Act prohibits the taking or harassment of marine mammals in US territorial waters. This has allowed the seal population to recover dramatically from levels estimated as low as 5000 seals in the 1960's. The increasing seal population and greatly improved water quality in Narragansett Bay resulted in harbor seals returning to the bay in the 1990's. The bay-wide population of wintering harbor seals is likely between 350 to 500 seals, at 15 to 20 regular haul-out sites.
Feeding - Harbor seals prey primarily upon fish such as menhaden, alewives, Atlantic herring, sea bass, and flatfish. Opportunistic feeders, they may also take mollusks, squid, or crustaceans. A steady supply of fish which have a high oil content is a critical component of the seal's diet; adult harbor seals will take 8 to 12 pounds of fish per day when forage fish are plentiful. Seal pups are nourished by their mothers fat-rich milk for about 4 weeks after their birth.
Size and Weight - Adult male harbor seals range in size from 4 to 6 feet in length and weigh 200 to 300 pounds. Adult female harbor seals are slightly smaller, ranging in size from 4 to 5.5 feet in length and weighing in 200 to 250 pounds. Seal pups are about 2 to 3 feet long at birth, weighing 15 to 25 lbs.
Swimming and Diving - The estimated maximum speed at which harbor seals can swim is 15 mph, with possible acceleration to as much as 20 mph in short bursts. It is believed that they are capable of diving to depths as deep as 2000 feet. There are documented cases of harbor seals remaining under water without surfacing for 15 minutes; other reports of seals staying down for as long as 25 minutes are possibly true but less well-substantiated. More typically, they will cruise underwater 2 to 3 minutes between breaths. Seals exhale when they dive to reduce their buoyancy and their heart rate slows considerably to reduce oxygen consumption while the seal is submerged.
Reproduction and Life Cycle - Male harbor seals sexually mature at 5-6 years and females become mature at 2-5 years. Courtship and mating occurs mostly underwater, although in the spring we sometimes get to observe frisky courtship behavior on the surface of Narragansett Bay. Female harbor give birth to a single pup around May; the pups of the Rome Point female seals are born in the seals' summer habitats. The precocious pups are born on land but are able to swim and dive within an hour or two of their birth. The pups are cared for by their mother and will nurse for 3 to 6 weeks; after weaning the pups tend to leave their birth site to explore their ocean realm.
Female harbor seals generally live to 25-30 years of age, with some venerable individuals hanging in there for up to 35 years. Male harbor seals can live for 20-25 years; the stress of competing during the mating season is a possible explanation for the male seals shorter lifespan.
Predators and Mortality - Western Atlantic harbor seals have few natural predators, especially with the recent decline in population of the large shark species. There have been increasing reports from Cape Cod in recent years of observers witnessing shark attacks on seals, these would be mostly Grey seals in the waters around the Cape. Coyotes, foxes, or large birds of prey have been known to take seal pups, and males can be mortally wounded when engaged in competitive battles for breeding dominance. In Rhode Island waters, there is negligible natural predation of harbor seals.
Fishing-related mortality of harbor seals is significant, with entanglement in gill nets being the primary cause. As offshore fisheries have declined aquaculture has become more prominent; seals are sometimes killed in interactions with aquaculture installations and Canada is allowing aquaculture operators to shoot specific seals that have become habituated to raiding salmon rearing pens. One estimate places the fishing and aquaculture-related death rate for western Atlantic harbor seals at approximately 1000 seals per year.
There have been several disease-related unusual mortality events affecting Western Atlantic Harbor seals in the past 10 years. In 2011, a form of avian flu afflicted the seals; this illness affected mostly young seals or other seals with diminished immune system capacity. In 2018, an outbreak of phocine distemper virus resulted in over 1200 seal strandings and deaths in the waters from Maine to Delaware; a similar event previously occurred in 2006. Distemper outbreaks affect seals of all ages, but seals that survive the illness develop antibodies that serve to increase their resistance to future infection.
Population and Distribution - The best estimate is about 60,000 to 70,000 seals ranging from eastern Canada down to New Jersey. There are reports of individual seals being spotted as far south as Florida. The population is thought to be increasing steadily, so harbor seals are not a threatened species at this time. Harbor seals in the United States were protected under the Federal Marine Mammal Protection Act in 1972; the Act prohibits the taking or harassment of marine mammals in US territorial waters. This has allowed the seal population to recover dramatically from levels estimated as low as 5000 seals in the 1960's. The increasing seal population and greatly improved water quality in Narragansett Bay resulted in harbor seals returning to the bay in the 1990's. The bay-wide population of wintering harbor seals is likely between 350 to 500 seals, at 15 to 20 regular haul-out sites.
Feeding - Harbor seals prey primarily upon fish such as menhaden, alewives, Atlantic herring, sea bass, and flatfish. Opportunistic feeders, they may also take mollusks, squid, or crustaceans. A steady supply of fish which have a high oil content is a critical component of the seal's diet; adult harbor seals will take 8 to 12 pounds of fish per day when forage fish are plentiful. Seal pups are nourished by their mothers fat-rich milk for about 4 weeks after their birth.
Size and Weight - Adult male harbor seals range in size from 4 to 6 feet in length and weigh 200 to 300 pounds. Adult female harbor seals are slightly smaller, ranging in size from 4 to 5.5 feet in length and weighing in 200 to 250 pounds. Seal pups are about 2 to 3 feet long at birth, weighing 15 to 25 lbs.
Swimming and Diving - The estimated maximum speed at which harbor seals can swim is 15 mph, with possible acceleration to as much as 20 mph in short bursts. It is believed that they are capable of diving to depths as deep as 2000 feet. There are documented cases of harbor seals remaining under water without surfacing for 15 minutes; other reports of seals staying down for as long as 25 minutes are possibly true but less well-substantiated. More typically, they will cruise underwater 2 to 3 minutes between breaths. Seals exhale when they dive to reduce their buoyancy and their heart rate slows considerably to reduce oxygen consumption while the seal is submerged.
Reproduction and Life Cycle - Male harbor seals sexually mature at 5-6 years and females become mature at 2-5 years. Courtship and mating occurs mostly underwater, although in the spring we sometimes get to observe frisky courtship behavior on the surface of Narragansett Bay. Female harbor give birth to a single pup around May; the pups of the Rome Point female seals are born in the seals' summer habitats. The precocious pups are born on land but are able to swim and dive within an hour or two of their birth. The pups are cared for by their mother and will nurse for 3 to 6 weeks; after weaning the pups tend to leave their birth site to explore their ocean realm.
Female harbor seals generally live to 25-30 years of age, with some venerable individuals hanging in there for up to 35 years. Male harbor seals can live for 20-25 years; the stress of competing during the mating season is a possible explanation for the male seals shorter lifespan.
Predators and Mortality - Western Atlantic harbor seals have few natural predators, especially with the recent decline in population of the large shark species. There have been increasing reports from Cape Cod in recent years of observers witnessing shark attacks on seals, these would be mostly Grey seals in the waters around the Cape. Coyotes, foxes, or large birds of prey have been known to take seal pups, and males can be mortally wounded when engaged in competitive battles for breeding dominance. In Rhode Island waters, there is negligible natural predation of harbor seals.
Fishing-related mortality of harbor seals is significant, with entanglement in gill nets being the primary cause. As offshore fisheries have declined aquaculture has become more prominent; seals are sometimes killed in interactions with aquaculture installations and Canada is allowing aquaculture operators to shoot specific seals that have become habituated to raiding salmon rearing pens. One estimate places the fishing and aquaculture-related death rate for western Atlantic harbor seals at approximately 1000 seals per year.
There have been several disease-related unusual mortality events affecting Western Atlantic Harbor seals in the past 10 years. In 2011, a form of avian flu afflicted the seals; this illness affected mostly young seals or other seals with diminished immune system capacity. In 2018, an outbreak of phocine distemper virus resulted in over 1200 seal strandings and deaths in the waters from Maine to Delaware; a similar event previously occurred in 2006. Distemper outbreaks affect seals of all ages, but seals that survive the illness develop antibodies that serve to increase their resistance to future infection.