Today we are talking about another of our local marine mammals: the northern fur seal! Though a bit rare here in the Salish Sea, this species has a wide range along the west coast of North America and the western Pacific. Spending most of their time at sea, these guys can be found as far south as Japan and Southern California in the Pacific Ocean in the winter and migrating as far north as the Bering Sea in Alaska in the spring to rookeries. This species is the dominant fur seal found in the Northern Hemisphere; the Guadalupe fur seal slightly overlaps its range with the Northern in southern California.
The northern fur seal is the largest of the fur seals and, as is the case with many fur seals, there is stark sexual dimorphism in this species. Weighing in at 600lbs and 7 feet long, the males are over four times heavier than their female counterparts (120lbs, 5 feet long). There is also a slight coloration difference between the two sexes (despite all being born black). While the males have a darker brown or black coat, the females exhibit a wider range of coloration, having a dark grey or brown on their backs and lighter gray or cream coloration on their throat and underside. The coat providing these colorations is thick and dense (hence the name fur seal) covering most of the body and ending at the wrist of the flippers. The flippers are also very long – their hind flippers longest of all sea lions and fur seals at a quarter of the animal’s total body length due to cartilaginous extensions on the toes! The fore flippers are also extremely strong, supporting their bulk and enabling them to run on land. Compared to the rest of their body, this fur seal has a small head and very short snout.
Diet & Behavior
Northern fur seals live a double life. Well, at least a socially opposite one! Most of the time they are solitary, living about 80% of their time alone in the open ocean. But when it comes time to breed they congregate in large colonies for a few months to give birth, nurse and mate, before heading back out to sea (even the most solitary animals need some time together!).
They spend the winter and spring in the Bering Sea and North Pacific Ocean, diving in the open ocean to around 200ft on average and a maximum of 600ft. Being a generalist or opportunistic forager, their diet will vary with geographic area and time of year, but mainly consists of midwater shelf fish (including Walleye Pollock, Pacific sand lance, Pacific herring, Northern smoothtongue, Atka mackerel, and Pacific salmon) and squid species. Like other marine mammal species, they may concentrate around oceanographic features like eddies and frontal boundaries, where prey are often found in high abundance. They are mainly nocturnal, foraging at night.
Starting in late May the males return first to the breeding sites (most in the Bering Sea) to stake out their territory, which they will aggressively defend. This is a tough time for males. Their ability to mate with females is dependent on their body size, fighting ability, and size and location of the territory (it turns out females don’t choose individual males, but specific breeding areas) along with the skills for interacting with females. Breeding males (usually 10 years and older) are very aggressive on land and will push and bite other males when defending their territories (and females within them), sometimes to the death! On top of this, they are fasting for 46 days on average (they can’t leave their territory to feed, or they would lose it), losing 20-32% of their body mass. Size really does matter for these males, in their ability to fight, and retain strength as they lose weight. It is no wonder that their breeding time is short – an average of only 2 breeding seasons, and that most of the mating is accomplished by only a few males each season.
Males have another option other than keeping a territory. Those males hang out on the fringes of the colony, or in haulouts with juvenile males adjacent to the breeding area, waiting for an opportunity. When the breeding males leave in August, these males can move into a territory while females are still there. The one downside is that most of the females have mated at this point, but there is always a chance that they may be able to sneak in a mating!
Pregnant females will arrive on the breeding sites in late June and will give birth on average 1.3 day after arrival – talk about perfect timing! Females give birth at 5-6 years of age, until 23 years, but are in their prime at 8-13 years. They also don’t waste any time, and will mate within an average of 5.3 days of giving birth – no rest for the weary! They, like many pinniped species, have embryonic diapause (also called delayed implantation) where the fertilized egg does not implant in the uterus until after lactation or weaning. This ensures the right timing for a 9 month gestation and their return to the breeding site next year. Pups are nursed for 5-6 days, then the mothers head out to feed for 3.5-9.8 days, and this cycle continues for the 4 month nursing period. The pups will leave the breeding site before their mothers to feed independently for the first time. Females don’t always get along and may also be aggressive to other females, though it is quite mild, mainly using open mouth threats and not physical interaction like males do.
The colonies are breeding males with females, surrounded by idle males without females on the fringe, and non-territorial and juvenile males on haulouts (teenager hang out!) outside or adjacent to the breeding area.
Currently northern fur seals are listed by the IUCN as “Vulnerable”, and is protected throughout its range by the MMPA. Specifically, the Pribilof Island/Eastern Pacific stock is listed as “depleted” in 1988 under the MMPA due to the massive decline in numbers of northern fur seals previously discussed in the Introduction. The latest NOAA population estimates of this particular stock from 2017 were recorded at just over 620,000 fur seals.
Although commercial harvests of northern fur seals have ceased, subsistence harvests still occur and is managed by NOAA Fisheries and the tribal governments of St. Paul and St. George.
- Marine Entanglement: Northern fur seals are, unfortunately, quite prone to entanglement in active, lost, or abandoned fishing gear. They are particularly susceptible to entanglement in drift nets, trawling gear, monofilament nets, and other marine debris where they either become entangled and swim off with the gear attached, or get stuck in the gear and cannot move. Northern fur seals are also exposed to significant levels of marine debris during their winter and spring migrations, where they forage in the transition zone and eddies (which will also swirl in various pieces of debris). When bogged down by debris that attaches to them, fur seals experience impaired foraging capabilities, fatigue, and even eventual death by continuing to swim for miles and miles. Severe trauma or injury from the entanglement may also lead directly to their death.
- Changes in Prey Availability: Due to the competition with commercial fisheries and the changes in prey distribution due to climate change, northern fur seals are particularly at risk from reductions in prey availability. Even seasonal prey fluctuations can lead to significant nutritional stress, which makes sense when you think about finding prey in the open ocean – it takes a lot of swimming to find it somewhere else if it’s not where you expect it to be!
- Contaminants: We’ve talked about this as a risk for other marine mammals, and northern fur seals are no different. As top predators, they too will be at a higher risk of contaminants accumulating in their tissues, especially fat. If the animal is under nutritional stress, it metabolizes the fat and gets dosed with these contaminants. Sources of contamination include oil spills, industrial run-off, micro-plastics, and vessel discharge.
- Predation: Killer whales, sharks, and even Steller sea lions will predate on northern fur seals, with the females being particularly at risk (due to their smaller size).
- Other Threats: Climate change, oil and gas exploration (which will lead to more pollution and risk of oil spills due to the rough sea conditions), habitat destruction, human disturbance, and illegal killing are all other threats that the northern fur seal must contend with.
OK now we’ve gone through all the tough stuff, let’s get to some fun facts!
- Europeans first named the northern fur seal “sea bears” (likely due to their thick pelt and point noses); even their Latin name Callorhinus ursinus literally means “bear-like”. Genetic evidence also suggests that northern fur seals have evolved from the same land-ancestor that gave rise to both bears and dogs.
- Northern fur seals spend a long time in the open ocean, with some pups spending up to 22 months at sea at a time!
- When out in the open ocean, fur seals sleep in what’s called a “jug-handle” position. They will flip onto their backs, extend their nose, both hind flippers and one or both front flippers also above the surface…when in this pose, they literally look like the curved handle of a jug! We don’t really know why they do this but it’s thought it may be to help thermoregulation (in this case, to help conserve body heat) in the cold waters.
- Females can recognize their pups by call! Even after 4 years apart, females can recognize their offspring by specific calls which, on a packed beach of fur seals, is pretty handy!