Skip booklet index and go to page content

Steller Sea Lion (Eumetopias Jubatus)

Limiting Factors and Threats

There are two broad categories of factors limiting Steller sea lions. The first can be grouped as anthropogenic threats such as shooting, incidental take in fishing gear, entanglement in debris, catastrophic accidents, environmental contaminants, and displacement or degradation of their habitat. The second category of threats includes naturally fluctuating prey populations, predation by killer whales, and disease.

For most of the 20th century, the main factor limiting Steller sea lions was undoubtedly killing by humans. Nowhere was the intentional destruction of Steller sea lions more intense than in British Columbia. Although relatively small numbers of kills still occur for predator control at salmon farms and spawn-on-kelp operations, and unknown numbers are taken incidentally in fisheries, harvested by natives for subsistence, or killed illegally, the recent recovery of populations implies that current levels of kills are within sustainable limits.

Aboriginal hunting of Steller sea lions does occasionally occur, but harvest levels are unknown. Use of sea lions by First Nations people appears to have declined during the 1800s and sea lion meat has not been a mainstay of their diet since the early 1900s (Duff 1977; Bigg 1985). In Alaska, household surveys indicate that about 350 Steller sea lions have been taken annually in recent years, but mostly in the northern part of their range. Less than 1% of the harvest originates from southeast Alaska (Wolfe 1997; Wolfe and Hutchinson-Scarbrough 1999; Loughlin and York 2000), an area which is probably more indicative of native harvesting levels in BC.

Steller sea lions are also killed incidentally in various fisheries, particularly drift gillnet fisheries for salmon, but there are presently few programs in BC to monitor bycatch levels. Animals can get trapped in trawl nets or entangled in drift and gill nets, and ultimately drown. Annual deaths in US waters are estimated at about 30 animals per year (Loughlin and York 2000; Angliss et al. 2001). Steller sea lions occasionally take fish from troll gear, and it is not uncommon to see animals that are hooked in the stomach with salmon flashers dangling from their mouths. Illegal and undocumented killing undoubtedly occurs in BC as many fishermen continue to consider sea lions to be a nuisance and perceive them as having a negative impact on fish stocks.

Predator control at fish farms in BC constitutes the largest known source of fishery-related mortality for Steller sea lions in the North Pacific (Angliss et al. 2001; Jamieson and Olesiuk 2001). Most of the 90 or so salmon farms currently operating in British Columbia waters possess permits to shoot pinnipeds. Quarterly reports filed by licence holders indicate that a total of 316 Steller sea lions and another 21 sea lions not identified to species were killed from 1990-2000. Numbers of Steller sea lions killed annually were low (averaging less than 10) up to the mid-1990s, but escalated and peaked at an estimated 91 in 1999 (Jamieson and Olesiuk 2001). This increasing trend, and the expected expansion of fish farms in BC and the associated killing of sea lions that might come with it, is cause for concern. Another 13 Steller sea lions were killed under special permit during 1983-85 for research to examine seasonal changes in body condition and composition of males (Olesiuk and Bigg 1987).

Another anthropogenic factor that may limit Steller sea lion populations is displacement from or degradation of essential habitat. Repeated disturbances of breeding or haulout sites by aircraft, boats, pedestrians, construction, and fishing activities can lead to animals temporarily leaving haulouts and rookeries (Sandegren 1970; Calkins and Curatolo 1980; Johnson et al. 1989; Brown 1997) and eventually to permanent abandon­ment (Pike and Maxwell 1958; Kenyon 1962). Nevertheless, Steller sea lions at winter feeding sites often habituate to such disturbances, and some haulout sites are located in high traffic areas close to major urban centres such as Vancouver and Victoria (Bigg 1985; Olesiuk, unpub. data).

Environmental contaminants such as heavy metals, organochlorines (e.g., DDT, dioxins and furans) and poly­chorinated biphenyls (PCBs) bioaccumulate through marine food chains. High levels in marine mammal tissues have been implicated with reproductive impairment (Addison 1989), premature births (DeLong et al. 1973; Gilmartin et al. 1976; Martin et al. 1976), birth defects (Arndt 1973), skeletal deformities (Bergman et al. 1992), suppression of the immune response (de Swart et al. 1994; Ross et al. 1995; Ross et al. 1996) and disruption of endocrine function (Brouwer et al. 1989). Nursing pups tend to be particularly susceptible since high doses of fat-soluble contaminants may be transferred through their mothers’ milk. Such contaminants are now ubiquitous in wildlife (Risebrough 1978), making it difficult to establish cause-and-effect relationships. Moreover, deleterious effects may only be manifested during periods of nutritional stress, when fat reserves are used and contaminants are mobilized, making it difficult to separate contaminant-related effects from other stresses. As has been shown in many other animals, contaminant concentrations in Steller sea lions (predominantly organochlorines) accumulate with age. The highest concentrations are in old males, while females transfer most of their loads to their pups during lactation (Lee et al. 1996). Contaminant levels have not been examined in Steller sea lions in BC. New chemicals are likely being introduced to the marine environment, which have not yet even been identified or can be detected, and the potential toxic effects of these are obviously unknown.

Sea lions can also be impacted by catastrophic accidents such as chemical and oil spills (St. Aubin 1990), although the impact on populations has rarely been established. The main threat is likely through contact with heavy oil accumulations when the source of the spill is near critical habitats such as rookeries and haulout sites, and to a lesser degree from absorption through skin, incidental ingestion of oil directly or through feeding, exposure to vapours, and partial fouling of pelage from fresh oil (Smith and Geraci 1975; Engelhardt et al. 1977; Englehardt 1987; St. Aubin 1990). Sea lions are insulated by a subcutaneous layer of blubber, so oiled fur does not interfere with thermoregulation (Kooyman et al. 1976). In other species, heavy fouling in thick oil can impede swimming and result in drowning (Geraci and St. Aubin 1980), but light contamination and light-viscosity oil usually wears off within several days (Geraci and Smith 1976). During their detailed investigations in Alaska, Calkins and Pitcher (1982) reported seeing sea lions with tar lodged in their throats or around their lips, jaw and neck. Interestingly, during the Exxon Valdez oil spill (EVOS) in Prince William Sound, oil did not persist on the coats of Steller sea lions as long as it did on harbour seals (Calkins et al. 1994a). Nevertheless, sea lions were observed in the vicinity of the oil spill and metabolites in the blood showed they had been exposed to hydrocarbons. Premature births were more common and pup production was somewhat lower in the year following the spill, but limited data prior to EVOS and the ongoing population decline in the area made it difficult to assess the statistical significance of the impact (Calkins et al. 1994b; Loughlin et al. 1996). Several Steller sea lions with small patches of oiled fur were observed during the Nestucca spill that spread along the west coast of Vancouver Island in 1988 (Harding and Englar 1989), but rumours of large numbers of completely fouled animals were almost certainly California sea lions, which have a black pelage and share the same haulout sites (Olesiuk, unpublished data). Because Steller sea lion populations are widely dispersed along the entire BC coast, the potential threat of oil and chemical spills is one of local depletion, particularly at rookeries during the breeding season, as opposed to impacting the entire population. Nevertheless, considering that over 70% of pup production in BC occurs on the Scott Islands, an oil spill in that area during the pupping season could have a significant impact.

The increasing prevalence of synthetic debris (net fragments, plastic bags and packing bands, etc.) is a growing problem worldwide and has been implicated in the declines of other species of pinnipeds (Fowler and Merrell 1986; Fowler 1988). Debris such as net fragments and packing bands can get caught around necks, eventually leading to abrasion or cutting deeply into tissue as animals grow. In Steller sea lions, entanglement begins to occur at 2 to 4 years of age (entanglement of pups and yearlings has not been observed), and entanglement rates have been estimated at about 0.07% for adults, with packing bands and net debris being the most common material (Calkins 1985; Mate 1985; Loughlin et al. 1986; Stewart and Yochem 1987; Fowler 1988). However, as Fowler (1988) has noted, much of the debris found at sea or washed ashore may be too large for an animal to transport, so the observed rate of entanglement at haulouts could represent a small fraction of numbers actually being entangled and drowned at sea. Although entanglement can lead to what is surely a slow and excruciatingly painful death for individual animals, it does not appear to pose a threat to the overall viability of Steller sea lion populations.

Environmental factors may also play a role in limiting Steller sea lion populations, either directly or indirectly through changes in their prey or by increasing their susceptibility to disease. Storms can lead to pups being washed from rookeries (Edie 1977), and El Niño events have led to abnormally high incidences of mortality in California (Allen et al. 1999). With the focus on climate change, researchers are beginning to appreciate that the environment fluctuates, and are noting evidence of decadal-scale oscillations that affect the biota of the North Pacific (Benson and Trites 2002).

Environmental shifts and fishing can both affect the abundance and availability of prey (e.g., Alverson 1992; Benson and Trites 2002), which in turn can affect both foraging behaviour and the population dynamics of pinnipeds (e.g., Trillmich and Ono 1991; Boyd et al. 1994), and ultimately determine the population levels that can be supported (carrying capacity) (Trites et al. 1997). Steller sea lions consume many of the same prey resources sought by other predators, including humans (McAlister and Perez 1976; Kajimura and Loughlin 1988; Fritz et al. 1995; Wada 1998; Trites et al. 1999b), but our understanding of the effects of marine mammal predation in marine ecosystems is inadequate to assess these interactions (Bowen 1997; Trites 1997; Trites et al. 1999a). One hypothesis suggests the decline of the western population of Steller sea lions was driven by a change in diet, which reduced body growth, birth rates and ultimately survival (Calkins and Goodwin 1988; Calkins et al. 1998; Pitcher et al. 1998; see review by Trites and Donnelly 2003). However, debate continues over the relative influence of natural fluctuations in environmental conditions, regime shifts, and anthropogenic effects that may be the result of global warming, whaling and commercial fisheries (Pascual and Adkinson 1994; Fritz and Ferrero 1998; Trites et al. 1999b; Rosen and Trites 2000a; Shima et al. 2000; Benson and Trites 2002).

Natural predators may also play a role in limiting populations, particularly when populations are at low numbers. It is generally thought that abundance of predators near the top of the food chain, such as Steller sea lions, is regulated mainly by bottom-up processes controlled by the availability of prey (Trillmich and Ono 1991; Boyd et al. 1994; Trites et al. 1997). However, it has recently been hypothesized that some populations may be limited by top-down processes, such as predation by killer whales (Estes et al. 1998). Although detailed data on killer whale predation rates are lacking, models show that predation by killer whales could be a significant source of mortality holding depressed populations of Steller sea lions in a predator pit (Barrett-Lennard et al. unpubl. data). The top-down hypothesis will require additional data on predation rates before it can be scientifically assessed.

Finally, disease may also play a role in limiting pinniped populations, especially at high densities (Harwood and Hall 1990; Lavigne and Schmitz 1990). Steller sea lions are host to a number of diseases including Leptospira interrogans, caliciviruses, Chlamydia psittaci, Brucella sp, morbilliviruses, influenza A, Toxoplasma gondii, phocid herpesviruses, canine parvovirus and canine adenoviruses 1 and 2 (see review by Burek et al. 2003). However, screening for diseases among Steller sea lions has not been conducted in BC.