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Recovery Strategy for the Sea Otter

1.4.1 Habitat and biological needs

Habitat

Sea otters forage primarily on invertebrates, which they obtain by diving to the sea floor. The seaward extent of their habitat is, therefore, limited by their diving ability. Most foraging dives are in depths of less than 40m, thus, sea otters seldom range beyond 1-2 km of shore, unless shallows extend further offshore (Riedman and Estes 1990; Bodkin et al. 2004). In coastal BC, sea otters generally occur along stretches of exposed coastline characterized by complex rocky shorelines with small islets and offshore rocky reefs. Specific kelp beds are often used habitually as rafting sites by groups of otters, as well as by individuals (Loughlin 1977; Jameson 1989). Kelp beds are also used for foraging and are important, but not required, habitat components. Soft-bottom communities that support clam species are also very important foraging habitat for otters (Kvitek et al. 1992; Kvitek et al. 1993). Habitat is unlikely to be limiting in BC at this time, as much of the coast remains unoccupied by sea otters.

Habitat quality, and thus the density of otters, seems to be indicated by substrate characteristics. Areas with irregular rocky substrate appear to support more otters than areas with little relief. Certainly this is true in California (Riedman and Estes 1990; Laidre et al. 2001), although in parts of Prince William Sound sea otter densities are high in some soft sediment habitats that support an abundance of clams (J. Bodkin pers. comm. 2003). In general, rocky substrate probably supports a greater variety of invertebrate prey species (Riedman and Estes 1990).

Weather and sea conditions may influence use of habitat, but these are little more than anecdotal observations in coastal BC. During periods of calm weather, sea otters tend to occur near offshore reefs, but they may aggregate inshore during inclement weather (Morris et al. 1981; Watson 1993).

Foraging

Sea otters forage along the bottom as well as in kelp beds. Most foraging takes place in subtidal areas, although otters forage in the intertidal zone at high tide (Estes 1980; VanBlaricom 1988: J. Watson pers. comm. 2002) and actually leave the water to feed on mussels exposed at low tide (Harrold and Hardin 1986). The depth at which sea otters forage may vary geographically and depends on prey availability. In California, sea otters typically forage in depths less than 25 m and rarely exceed 40 m, whereas in parts of Alaska, sea otters may forage in deeper waters (Riedman and Estes 1990).

Sea otters capture their prey with their forelimbs, often storing prey in the loose flaps of skin under the forelimb. Dives to obtain prey can range from 50 seconds to more than three minutes (reviewed in Riedman and Estes 1990). Prey is consumed at the surface. Sea otters will use rocks or other hard objects to open hard-shelled prey and are among only a few animals known to use tools.

Diet

Sea otters eat a wide variety of prey species; diet varies geographically, by duration of residency and by individual.  In recently re-occupied rocky habitats where sea urchins are abundant, sea urchins are consumed preferentially, probably because of ease of capture. As the abundance of preferred prey is reduced, the diet of the sea otter population in an area diversifies to include a larger array of invertebrates, including various bivalves, snails, chitons, crabs, sea stars and even fish in some areas  (Estes et al. 1981). In soft sediment habitats, where clams occur, sea otters excavate their prey. Clams are an important part of the sea otter diet in Southeast Alaska and in BC (Kvitek et al. 1992). Evidence of sea otters excavating butter clams, horse clams and geoducks in BC (Keple 2000; J. Osborne pers. comm.2003; L. Nichol pers. comm. 2002; UHA geoduck surveys 2002) suggests that these species are an important part of the diet. Fish are important prey in some parts of the Aleutian, Commander and Kuril Islands (Estes and VanBlaricom 1985; Watt et al. 2000). Even within a population, sea otters display a great deal of individual prey preference. These preferences can persist for long periods of time and appear to be transmitted from mother to offspring through learning during the period of mother-young association (Estes et al. 1981; Estes et al. 2003)).

Social Organization

 Sea otters segregate by gender with males and females occupying spatially-distinct areas. However, individual adult males establish and occupy breeding territories in female areas (Garshelis et al. 1984; Jameson 1989; Riedman and Estes 1990; Watson 1993). Male rafts occur in the range of established populations and occur at the periphery of the range of expanding populations (Jameson 1989; Watson 1993). During the peak breeding season, male rafts are composed largely of sub-adult males, because adult males have established territories closer to female raft areas. Territorial males re-join the male rafts, although some males maintain territories year-round (Garshelis et al. 1984; Jameson 1989).

Movements and Home Range

 Sea otters are non-migratory and show great site fidelity, although seasonal movements and occasional long distance movements of individuals may occur (Garshelis 1983; Jameson 1989). Sea otters occupy relatively small overlapping home ranges varying in size from a few to tens of kilometres of coastline (Loughlin 1980; Garshelis et al. 1984; Jameson 1989). In California, adult male territories average 40 ha. Female home ranges are larger, but on an annual basis adult males may use a much larger area (Jameson 1989).  In California, adult males on an annual basis used over 80 kilometres of coastline (Ribic 1982; Jameson 1989). Population range expansion typically occurs when males move en masse from the periphery of the occupied range into previously unoccupied habitat. Females gradually occupy the areas vacated by males (Loughlin 1980; Garshelis et al. 1984; Wendell et al. 1986; Jameson 1989). In this way population growth and range expansion are linked.

Reproductionand Maternal Care

 Female sea otters reach sexual maturity at two to five years (Bodkin et al. 1993). Males reproduce between five and six years of age, although sexual maturity in males may be attained earlier (Riedman and Estes 1990). By five years of age, all females have given birth (Bodkin et al. 1993; Jameson and Johnson 1993). Sea otters remain reproductive until death. Females have a higher survival rate than males (Siniff and Ralls 1991) and live 15 to 20 years, whereas males live only 10 to 15 years (Riedman and Estes 1990).

Mating occurs year-round, although peak pupping is noted in some populations, including coastal BC. Pupping appears to peak in March and April in BC (Watson 1993). Gestation, including a period of delayed implantation, lasts six to eight months (Riedman et al. 1994). Sea otters are polygynous; males form pair bonds consecutively with several females throughout the year. Female sea otters produce a single pup per year (Siniff and Ralls 1991; Bodkin et al. 1993; Riedman et al. 1994). Gestation is followed by birth in the water or on land; twins are rare (Kenyon 1969; Jameson 1983; Jameson and Bodkin 1986; Jameson and Johnson 1993; Riedman et al. 1994).

At birth, a sea otter pup weighs 1.4 to 2.3 kg (Riedman and Estes 1990). Pups remain dependent on their mothers for the first six to eight months after which they are weaned (Payne and Jameson 1984; Jameson and Johnson 1993; Riedman et al. 1994). Throughout the period of pup dependency, care is provided entirely by the female. During the first month, the pup depends exclusively on its mother’s milk, by four months it feeds almost exclusively on prey provided by the mother, and at five months a pup can dive, capture and break open prey, and groom itself. Pre-weaning mortality can be high; 60 to 78% in areas where populations are nearing equilibrium with resources, but as low as 15% in growing populations (Siniff and Ralls 1991; Bodkin et al. 1993; Jameson and Johnson 1993; Monson et al. 2000a).

1.4.2 Ecological role

The sea otter is a nearshore species feeding primarily on benthic invertebrates, which it obtains by diving to the sea floor. The sea otter is recognized as a ‘keystone species’ contributing significantly to the structure and function of nearshore benthic communities and upon the life history of their invertebrate prey (Estes and Palmisano 1974; Estes et al. 2005). These interactions are ecologically important, and have significant implications for many invertebrate fisheries.

The keystone species concept was presented by Paine (1969) to describe the role sea stars, Pisaster ochraceous, play in structuring rocky intertidal communities. A keystone species is one that has an effect on community structure that is greater than would be expected based on its abundance (Power et al. 1996). The sea otter is a prime example of such a species. Research over the past several decades has demonstrated the sea otter’s keystone role, particularly in rocky subtidal habitats (Estes and Palmisano 1974; Estes and Duggins 1995) and the effect in soft sediment habitats as well (Kvitek and Oliver 1992). Sea otter predation reduces the abundance and size of invertebrate prey species, which in turn has important consequences for nearshore community structure (Estes et al. 1989).

The extirpation of sea otters from much of their range likely had widespread effects on nearshore community structure (Estes and Duggins 1990). This may have affected ecological processes and had evolutionary effects on many species of otter prey (Estes et al. 1989, Watson 2000, Estes et al. 2005). Sea otters regulate the abundance and size of their prey. By preying on herbivores such as sea urchins, sea otters reduce grazing pressure and increase algal abundance. Consequently when sea otters are removed from a system, it can become deforested by urchin grazing (Estes and Palmisano 1974). In the absence of sea otter predation, sea otter prey species likely became larger and more abundant because sea otters are energetically constrained to feed on large prey items (Estes et al. 1989). Thus, in areas where otters forage, prey species tend to be both less abundant and smaller, and in many cases occur in crevices and under rocks, which offer a physical refuge from foraging otters (Hines and Pearse 1982, Fanshawe et al. 2003). Furthermore, in areas with sea otters, herbivorous invertebrates may switch from active grazing to feeding passively on drift algae, which becomes abundant as kelp increases (Harrold and Reed 1985).

The relationship between sea otters, sea urchins and kelp was first described in the Aleutian Islands (Estes and Palmisano 1974). Since then, studies in Southeast Alaska (Estes and Duggins 1995), BC (Morris et al. 1981; Breen et al. 1982; Watson 1993), Washington State (Kvitek et al. 1989; Kvitek 1998) and California (Laur et al. 1988) have provided supporting evidence for the generality of this interaction. Although there is little dispute that sea otters have a great impact on invertebrates and that this leads to changes in the abundance of kelp, there are other physical and biological processes that can affect the abundance of kelp and sea urchins (see Foster and Schiel 1988, Konar and Estes 2003). Furthermore the importance of sea otters in regulating community structure must be viewed in a geographical context. For example, in southern California, where alternate predators can control the abundance of sea urchins, sea otters may play a less important role in enhancing kelp abundance (see Steneck et al. 2002 for a review). Likewise in the inner waters of Puget Sound and the Strait of Georgia where sea otters may never have been abundant, factors other than urchin grazing may help to regulate kelp abundance (Carter et al. 2007).

Sea otter predation also has indirect effects on ecological processes and community structure. Kelp forests enhance nearshore productivity, and enter food webs as detritus from drift algae and dissolved organic material. At islands in the Aleutian chain that are dominated by sea otters, kelp-derived carbon accounted for more than half the carbon in food webs. In these habitats, nearshore productivity, measured as growth of invertebrates, is two to five times higher than in areas where sea otters and kelp are absent (Duggins et al. 1989). Kelp also enhances the structure of the water column by creating a complex three-dimensional habitat that supports a large variety of invertebrate and fish (Bodkin 1988; Ebeling and Laur 1988; Laur et al. 1988; Duggins et al. 1990; Carr 1991). Nearshore fish have been shown to be more abundant in areas with kelp beds than in urchin barrens, or in areas without kelp. Furthermore, stands of kelp dampen tidal currents and wave height and influence dispersal, settlement rates and recruitment of benthic invertebrates and rockfish that live within them (Duggins et al. 1990; Carr 1991). Fertilization, larval settlement and recruitment processes may all be affected by the presence of kelp (Reed et al. 2000, Watson 2000).

Sea otters also exert ecological effects on soft bottom communities, although their role in these communities is less well understood. Sea otter predation on clams can reduce the abundance andsize of these species. Clams probably form an important part of the sea otter diet in coastal BC. In Southeast Alaska, clams are the major food resource of sea otters (Kvitek and Oliver 1992). As well as influencing these species through direct predation, sea otters may exert secondary community level effects, although perhaps not to the same extent as in rocky habitats (Kvitek et al. 1992). Nonetheless, by disturbing the sea floor and adding shell litter (hard substrate), sea otter predation may support settlement and recruitment of various species that require hard substrate (Kvitek et al. 1992; Kvitek et al. 1993).

Sea otters feed on both clams and mussels in the intertidal zone. Predation on mussels creates gaps in mussel beds that allow other species to attach (VanBlaricom 1988). Clam predation in intertidal areas may also have secondary consequences for birds and other mammals that feed on intertidal species, although these have not been well studied (Bodkin et al. 2001).

1.4.3 Limiting factors

The sea otter is a density-dependent species and population growth is thought to be regulated by resource availability. The abundance of prey affects juvenile survival, whereas female reproductive rates (0.83 to 0.94) remain relatively constant regardless of whether the population is growing or stable and at equilibrium (Siniff and Ralls 1991; Bodkin et al. 1993; Jameson and Johnson 1993; Monson et al. 2000). As the number of sea otters in an area increases and food becomes limiting, otter density in the area is maintained at equilibrium through mortality and emigration (Estes 1990). Pre-weaning survival ranges from 22- 40% in populations near equilibrium to 85% in growing populations. Survival post weaning to one year of age tends also to be lower in populations near equilibrium (Monson et al. 2000a). Otters ≥ two years of age generally have high rates of survival, approaching 90% regardless of population status (Monson et al. 2000a).

Predation

Pup carcasses found at eagle nests suggest eagles may be a source of pup mortality in BC (Watson et al. 1997). In the Aleutian Islands, sea otter pups comprise five to 20% (by frequency) of the eagle diet during the sea otter pupping season (Anthony et al. 1998). Killer whales are not thought to be a significant source of mortality in BC, although there is one anecdotal account of killer whales pursuing sea otters in Kyuquot Sound (Watson et al. 1997). In contrast, killer whale predation may be significant in western Alaska, where dramatic declines in the sea otter population are underway. Estes et al. (1998) hypothesize that because of dramatic declines in seal and sea lion populations in response to a large-scale ecosystem shift, mammal-eating killer whales have switched to preying on sea otters and are the cause of the observed decline in the sea otter population. White shark predation is a significant cause of mortality in the southern sea otter population and has increased through time, particularly during the current and recent period of the southern sea otter population decline (Estes et al. 2003). The decline in western Alaska suggests that a better understanding of sources of predation in the Canadian sea otter population may be warranted.


Disease

 Various diseases have been documented in sea otters (Thomas and Cole 1996; Reeves 2002; Shrubsole et al. 2005; Gill et al. 2005), but, generally, disease is not thought to be a significant source of mortality in most sea otter populations, excluding California. In California disease explains 40% of beach cast carcasses and contributes to the low rate of population growth compared with other sea otter populations ((Thomas and Cole 1996; Estes et al. 2003).

Genetic Diversity

Genetic diversity can be lost when a population is reduced to a small size and then allowed to increase, a phenomenon known as a bottleneck. The loss of genetic diversity that occurs through inbreeding or because of the limited gene pool in small populations results in lower fecundity, higher rates of juvenile mortality and an overall reduction in population growth rate. Furthermore, loss of diversity reduces a population’s ability to respond to stochastic events. Sea otters in coastal BC have suffered through at least two genetic bottlenecks, the initial global bottleneck  brought about by the species’ near extinction as a result of the maritime fur trade of the 18th and 19th centuries, and a second bottleneck caused by re-introducing a small number of animals to BC.

As a result of the fur trade, the total range-wide population of sea otters was reduced by 1911 to less than 2000 animals, approximately one to two percent of its pre-exploitation size (Kenyon 1969). As a result of this bottleneck, genetic diversity among extant sea otter populations is significantly lower than pre-fur trade sea otters, with a loss in modern sea otters of at least 62% of the alleles and 43% of the heterozygosity, compared to the pre-fur trade population (Larson et al. 2002a).  

Bodkin et al. (1999) demonstrated that mitochondrial DNA (mtDNA) haplotype diversity (a measure of genetic diversity) was inversely correlated with the amount of time remnant and translocated populations spent at their small founding population sizes, and that haplotype diversity was positively correlated with the size of the founding population. Yet with respect to the bottleneck resulting from translocating small numbers of animals, Bodkin et al (1999) could not detect a difference in the genetic diversity of remnant (experienced one bottleneck) and translocated (experienced two bottlenecks) populations in coastal BC, Washington and Southeast Alaska (Bodkin et al. 1999; Larson et al. 2002b). Further loss of genetic diversity has largely been avoided in successfully reintroduced populations that arose from at least 20 to 30 animals, as rapid population growth, aided by a high abundance of food in the reintroduction areas, limited the duration of the bottleneck (Bodkin et al. 1999; Larson et al. 2002b).

In 1989, females with pups were first reported on the central BC coast, more than 235 km away from the reintroduced population on Vancouver Island (BC Parks 1995). The origin of these otters was unknown (Watson et al. 1997), but recent genetic analysis of 18 sea otter samples from the central BC coast in 2003 revealed two mtDNA haplotypes (genetic markers) consistent with otters from Amchitka and Prince William Sound, suggesting otters on the central BC coast are descendents of reintroduced Alaskan otters (DFO unpubl.).  Sea otters in Southeast Alaska and Washington State are of the same origin (Bodkin et al. 1999; Larson et al. 2002b).

Present populations of sea otters are less genetically diverse than pre fur-trade populations (Larson et al. 2002a). This lowered genetic diversity increases the risk of extinction from stochastic events. If a catastrophic oil spill were to occur, and substantially reduce the sea otter population, it is unlikely that recovery would be as rapid (i.e., as occurred following reintroduction) because degradation of the habitat from the spill and the lower abundance of large prey would likely influence growth and recovery of the population (see Bodkin et al. 2002).

Marine Biotoxins

 The toxin responsible for Paralytic Shellfish Poisoning (PSP), produced by certain dinoflagellate species, can accumulate to toxic levels in filter-feeding bivalves. Butter clams, which tend to accumulate the biotoxin PSP, form an important component of the sea otter diet. A large die-off of sea otters in the Kodiak Archipelago in the summer of 1987 was in part attributed to PSP poisoning (DeGange and Vacca 1989). One study has shown that sea otters may be able to detect PSP and avoid clams with lethal concentrations (Kvitek et al. 1991).

Domoic acid, a biotoxin produced by certain diatom species and some marine algae, can accumulate in filter feeding shellfish and be passed through the food chain, thereby affecting not only species that prey on invertebrates, but fish-eating species as well.  First detected on the west coast of North America in 1991, domoic acid has been identified as the cause of several large die-offs of sea birds and sea lions in California. So far, only one case has been confirmed of a sea otter in California dying from domoic acid poisoning.

Although the occurrence of toxic phytoplankton is a natural phenomenon, the problem of harmful algae blooms appears to have increased over the past two decades, and this is the case in the waters around BC (Taylor 1990). Coastal pollution, in particular increased levels of nitrogen and phosphorus abundant in sewage and coastal runoff, is at least partly to blame (Anderson 1994).