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Steller Sea Lion (Eumetopias Jubatus)

Biology

General

The earliest accounts of Steller sea lions were provided by Scammon (1874), Allen (1880), Elliot (1882)and Rowley (1929)--but because they were not commercially harvested and specimens were unavailable, little was learned about their general biology until the 1960s. More recent overviews have been provided by Schusterman (1981), Loughlin et al. 1987, Hoover (1988) and Loughlin (1998; 1999). The Steller sea lion is currently one of the most intensively studied marine mammals in the world (see NMFS 1992; Strick 1993; Hunter and Trites 2001).

In British Columbia, the first study of Steller sea lions was conducted by Newcombe and Newcombe (1914) and Newcombe et al. (1918). Although the species was subjected to major control programs during the first half of the 20th century, little research was done until G. Pike and colleagues examined specimens from commercial harvests during the 1960s (Pike and Maxwell 1958; Pike 1961; Spalding 1964a, b). Behavioral studies were initiated during the 1970s at rookeries and haulouts by H.D. Fisher and his students (Harestad and Fisher 1975; Brenton 1977; Edie 1977; Harestad 1977; Fisher 1981). Assessments of the abundance, distribution and status of Steller sea lions in BC were conducted during the 1980s (Bigg 1984, 1985, 1988) and 1990s (Olesiuk 2003; Olesiuk et al. 2003).


Life Cycle

Only sexually mature Steller sea lions return to rookeries (along with a few dependent young with their mothers). Bulls are the first to arrive in early May to compete with other mature males and establish territories (Gisiner 1985). Pregnant females begin arriving on rookeries during the latter half of May and give birth to a single pup within a few days of their arrival (Gentry 1970). Mothers will stay on shore with their pups for about 1 week before leaving on regular feeding trips that average 1 day and are followed by 1 day on shore (Higgins 1984; Merrick 1987; Hood and Ono 1997; Milette and Trites 2003). Copulations usually occur prior to the first feeding trip.

Pups are precocious--they have open eyes and can crawl at birth. They begin to enter tide pools and inter-tidal areas at about 2 weeks of age, and swim in the open ocean starting at about 4 weeks of age when mothers begin moving their pups from the rookeries to nearby haulouts (Sandegren 1970; Gentry 1974). By the end of August, few animals remain on the rookeries.

Year-round haulouts are used by immature animals, non-pregnant adults and females nursing pups from previous summers that do not return to the rookeries. Some bulls also use summer haulouts and establish territories, and occasionally breed with mature females (Trites, unpubl. data). Outside of the summer breeding season, Steller sea lions use year-round haulouts as well as winter-haulouts that may be considerable distances from their rookeries. Females with dependent young may stay at a single haulout or may move their pups to any number of haulouts. Average length of feeding trips by lactating females in winter is about 2 days, followed by 1 day on shore (Trites and Porter 2002). Haulouts are not restricted to any single age or sex class during the non-breeding season. However, sea lions without dependent young may spend extended time at sea between visits to shore.


Reproduction

Steller sea lions have a polygynous mating system that appears to be synchronized throughout the entire range (Bigg 1985). Males may begin producing sperm by 3-7 years of age (Calkins and Pitcher 1982), but only those holding territories are known to mate. Most territorial males are 9-13 years old (Thorsteinson and Lensink 1962) and may hold a territory for several years in succession (range 1-7 years) (Gisiner 1985). The ratio of cows to territorial bulls is generally about 10-15:1 (Pike and Maxwell 1958; Merrick 1987). Successful males will usually maintain their territory for an average of 40 days (20-68 days) without feeding (Gentry 1970). The advantages of larger body size in acquiring and defending territories, and in providing energy and possibly water reserves during tenure, probably accounts for the sexual dimorphism in body size in Steller sea lions(Fisher 1958; Repenning 1976).

Females ovulate first at about 3-6 years of age. Following fertilization, embryonic development is suspended for about 3 months until implantation occurs in September or October (delayed implantation), resulting in a gestation period of about 8-9 months (Vania and Klinkhart 1967; Calkins and Pitcher 1982). The majority of mature females conceive each year, but the rate of reproductive failure and abortion appears to be high. Pitcher et al. (1998) reported that 97% of females sampled in the Gulf of Alaska were pregnant during early gestation, but that pregnancy rates declined to 67% and 55% during late gestation in the 1970s and 1980s respectively. Pregnancy rates have not been estimated for Steller sea lions in BC.

The lactation period is extremely long for a pinniped species. A few pups have stayed with their mothers for 3 years, although most are believed to wean sometime prior to their first birthday (Calkins and Pitcher 1982; Trites et al. 2001). In some cases, females on rookeries may nurse both a newborn and a yearling.


Survival

Besides humans, the main predators of Steller sea lions are killer whales (Morton 1990; Baird and Dill 1995; Ford et al. 1998), which may selectively prey on pups and juveniles (Barrett-Lennard et al. 1995). Large sharks may also prey on Steller sea lions in the southern part of their range (Stroud 1978; Ainley et al. 1981). Bears have been observed and may prey upon pups on rookeries in Russia (T. Loughlin, National Marine Mammal Laboratory, Seattle, WA, pers. comm.).

Mortality of pups during the first month of life (< 1 month) generally appears to be high and influenced by factors such as storms (Pike and Maxwell 1958; Orr and Poulter 1967). The principle cause of death for pups is drowning--not because they are not able to swim, but because they are not able to get back out of the water (Orr and Poulter 1967; Edie 1977).  Being bitten, tossed or trampled by older animals takes a toll on pups, as does being abandoned or separated from their mothers (Orr and Poulter 1967; Gentry 1970; Sandegren 1970; Sandegren 1976).

Juvenile mortality is difficult to estimate due to potential sampling biases, but appears to be fairly high for both sexes. Calkins and Pitcher (1982) and York (1994) estimated that about 48% of females and 26% of males survived to 3 years of age. Mortality rates are significantly lower for adults (~10-15% per year for females, and ~13-25% for males). Higher mortality rates for males results in a progressively skewed sex ratio favouring females. The oldest animals aged from the wild were about 18 years for males and 30 years for females (Calkins and Pitcher 1982). However, longevity (defined at the 99th percentile of known aged individuals) is about 14 years for males and 22 years for females (Trites and Pauly 1998). It should be noted that the only life tables available were derived from specimens collected in the Gulf of Alaska just prior to major population declines (see Population Sizes and Trends), and life history and population parameters (e.g., life expectancy, generation time) may vary with status of populations. In marine mammals, density dependence is generally thought to be expressed primarily in the parameters that affect reproductive rates, especially of younger animals (i.e., age at first reproduction, fecundity rates, and juvenile survival) (Eberhardt 1985; Fowler 1987).

Life tables for Steller sea lions (Calkins and Pitcher 1982; Trites and Larkin 1992; York 1994) indicate that the average age of sexually mature females (generation time) is about 10 years, and that the number of mature individuals (males and females) capable of reproduction is about 40% of the total population (all ages, including pups).


Physiology

Food requirements vary with the type and quality of prey(Perez 1994; Rosen and Trites 1999, 2000b, c). Captive sea lions fed a mixed diet of various fishes consume an average 10-12 kg per day for full-grown females and 20 kg per day for full-grown males (Kastelein et al. 1990; Perez et al. 1990). However, bioenergetic models predict that daily food requirements for Steller sea lions in the wild (which are more active, reproduce and tend to consume a lower quality diet) are closer to 15-20 kg for mature females, and 30-35 kg for mature males (Winship et al. 2002). For females, daily energy requirements are about 14% of body weight for a 1 year old and 7% for a mature individual. Sea lions that consume higher proportions of low fat fishes such as gadids require significantly more prey than those that consume fattier fishes such as herring (Trites and Donnelly 2003; Winship and Trites 2003).

Steller sea lions are capable of diving to depths of at least 310 m (Andrews 1999) and staying submerged for over 8 minutes (Swain and Calkins 1997), with most dives in the range of 15-50 m and lasting 1.5-2.5 min (Merrick and Loughlin 1997; Swain and Calkins 1997; Loughlin et al. 1998; Andrews 1999; Swain 1999). Diving capabilities are developed during the first year of life. Pups aged less than one month dive to a maximum depth of 10 m, but this increases to nearly 100 m by 5 months of age, and to over 200 m by 10 months of age (Merrick and Loughlin 1997; Rehberg et al. 2001).


Movements/Dispersal

Steller sea lions generally return to breed on their natal rookery, although there may be some exchange between neighbouring rookeries (Calkins and Pitcher 1982, 1996). At least one animal branded as a pup on Forrester Island in southeast Alaska was subsequently seen 400 km away with a newborn pup on the Cape St. James rookery (Raum-Suryan and Pitcher 2000). In some cases, rookeries are augmented by breeding females from other rookeries, as evident from the rapid expansion of several new rookeries monitored in southeast Alaska since the 1980s (Calkins et al. 1999; Pitcher et al. 2003).

Telemetry and branding studies have shown that animals are highly mobile, and may travel hundreds of kilometres and utilize numerous haulout sites over the course of a few weeks or months (Merrick and Loughlin 1997; Loughlin et al. 1998, 2003; Raum-Suryan et al. 2002). An individual tagged as a pup at Sugarloaf Island in the Gulf of Alaska was resighted 4 years later in Douglas Channel on the central BC coast, a near-shore distance of roughly 1700 km (Loughlin, pers. comm.; Olesiuk, unpublished data). Conversely, animals tagged as pups on Cape St. James have been observed as subadults on Cape St. Elias in Prince William Sound, Alaska, a near-shore distance of about 1500 km (Calkins 1981; Fisher 1981). Numbers of animals using year-round haulout sites is fairly constant throughout the year, but numbers on winter sites declines during the May-August breeding season as animals move to rookeries (Bigg 1985).

Although considered non-migratory, there are well-defined seasonal movements in certain parts of their range. Following the breeding season, both Steller and California male sea lions have been observed to migrate north along the Oregon coast (Mate 1975), coinciding with a sharp increase in the number of animals wintering off southern Vancouver Island (Bigg 1985).

Prior to weaning, dependent young (ages 0-3 yrs) appear to stay relatively close to haulouts while their mothers forage at sea (Trites and Porter 2002). Once weaned, young males appear to disperse more widely than females and have been seen many hundreds of kilometres from their natal rookeries (Raum-Suryan et al. 2002). However, both males and females appear to return to their rookeries of birth as they mature sexually.

Animals on rookeries tend to haul out more during daylight hours, with peak numbers occurring on land between 10:00 and 18:00 (Withrow 1982; Higgins 1984; Milette 1999). No apparent diurnal haulout pattern has been recorded during winter when daylight is significantly reduced (Porter 1997). A number of environmental factors correlate with and may affect haul out behaviour. These include sea state, air temperature, wind speed and direction, fog and cloud cover, barometric pressure, swell height and tide level (Withrow 1982; de Blois 1986; Kastelein and Weltz 1990; Porter 1997; Calkins et al. 1999). Such factors are likely more important on haulout sites, which tend to be more exposed and offer less protection, than on rookeries.

At sea, Steller sea lions are commonly seen as individuals or in groups of several animals (Bonnell et al. 1983). However, animals feeding on small schooling fishes appear to feed co-operatively in groups of up to 100 animals that dive and surface in synchrony (Fiscus and Baines 1966; Loughlin et al. 1983; Olesiuk, pers. obs.; Loughlin and DeLong 1983). Foraging appears to occur primarily at night based on satellite telemetry (Loughlin et al. 1998; Loughlin et al. 2003) and diurnal haul out patterns (Withrow 1982; Higgins 1984; Milette 1999). Incidental takes during fishing operations are also most prevalent from 20:00 to 05:00 (Loughlin and Nelson 1986).

Foraging is more localized during the breeding season. In contrast, seasonal movements of animals during the non-breeding season (September-May) are much wider and are likely related to distribution of forage fish. Major wintering areas of sea lions off southern Vancouver Island shift in relation to changes in distribution of pre-spawning herring. Sea lions also congregate in estuaries in autumn when salmon are spawning and at the mouth of the Fraser River in spring when eulachon are running (Bigg 1985; Bigg et al. 1990, Olesiuk unpub. data). Foraging trips of satellite-tracked adult females in Alaska have averaged about 17 km during summer, compared to 153 km during winter (Merrick and Loughlin 1997). Off the coast of California, Steller sea lions were concentrated within 1-13 km (mean 7.0 km) of rookeries during summer and were seen less frequently compared to autumn when they were up to 7-59 km offshore (mean 28.2 km) (Bonnell et al. 1983). Foraging ranges of immature non-breeding animals appear intermediate to the summer and winter foraging ranges of adults (Merrick and Loughlin 1997).


Nutrition and Interspecific Interactions

Over 50 species of fish and invertebrates have been identified in the diets of Steller sea lions (Wilke and Kenyon 1952; Pike 1958; Spalding 1964b; Pitcher 1981; Sinclair and Zeppelin 2002). Regionally, diet appears to vary according to which prey are locally and seasonally most abundant or accessible. Preferred prey appear to be small or medium-sized schooling fishes, which in British Columbia include species such as herring, hake, sandlance, salmon, dogfish, eulachon and sardines (Pike 1958; Spalding 1964b; Olesiuk and Bigg 1988, Trites and Olesiuk, unpubl. data). Bottom fish, such as rockfish, flounder and skate, can also be important dietary items (Trites and Olesiuk, unpubl. data). In addition to fish, squid and octopus are sometimes consumed, but their importance was probably exaggerated in earlier studies because cephalopod beaks may accumulate in stomachs over extended periods (Bigg and Fawcett 1985). Crabs, mussels, clams and other invertebrates are occasionally recovered in stomachs and scats, but these may represent secondary prey that had been consumed by the prey species eaten by sea lions. Steller sea lions have also been observed to prey on gulls (O'Daniel and Schneeweis 1992) and other pinnipeds, including neonate fur seals (Gentry and Johnson 1981) and harbour seals (Pitcher and Fay 1982, E. Mathews, University of Alaska, Juneau AK, pers. comm.). Predation on other pinnipeds seems quite uncommon, but may be locally significant.

A shift in diets from fatty fishes (i.e., herring) to low-fat fishes (i.e., walleye pollock) has been implicated in the decline of Steller sea lions in the Gulf of Alaska and Aleutian Islands (Alverson 1992; Alaska Sea Grant 1993; DeMaster and Atkinson 2002; Trites and Donnelly 2003). Large-scale shifts in climatic and oceanic conditions can affect the dynamics of marine organisms (Benson and Trites 2002), as can selective- or over-fishing (Pauly et al. 1998), both of which could affect the quantity or quality of Steller sea lion prey. Controlled-feeding studies have shown that sea lions, particularly young animals, consuming large amounts of low-fat prey such as pollock may be unable to maintain body mass (Rosen and Trites 2000c; Azana 2002). Thus, interactions between climate, fisheries and prey may significantly influence the nutritional status and ultimately the survival of Steller sea lions.


Adaptability

Observations over the past century suggest that it is unlikely that a rookery will be recolonized if all the breeding animals are killed or forced to leave. Instead it appears that year-round haulouts can become rookeries if sufficient numbers of pregnant females successfully give birth there. This has been observed at several sites in southeast Alaska and one in the Gulf of Alaska, but nowhere else (Calkins et al. 1999). Given the rigidness and traditional nature of breeding sites, along with the sensitivity of sea lions to disturbance (Lewis 1987; Porter 1997), it seems unlikely that new rookeries could be established with human intervention. Steller sea lions have been successfully born and/or raised in captivity (e.g., Hardervijk Dolphinarium Holland, Vancouver Aquarium Marine Science Centre), but it is unclear whether such individuals could survive on their own if released in the wild. Recolonization of BC, if ever needed, would likely only occur through immigration from rookeries in Oregon or southeast Alaska.

Steller sea lions can tolerate a wide range of air and water temperatures. They consume wide groups of prey, ranging from bottom fish to midwater schooling species. Thus they should be reasonably adaptable to periodic changes in the quality and quantity of prey available. However, Steller sea lions tend to continue frequenting sites for many years after prey appear to have shifted distribution (Olesiuk, pers. obs.). Thus, changes in prey abundance associated with different oceanic regimes may influence the carrying capacity of Steller sea lions.