Marine-foraging river otters in the Salish Sea

Overview

North American river otters (Lontra canadensis) are semi-aquatic mammals that range over most major river drainages, large lakes, and coastal marshes of North America. On the west coast, from Northern California (Bouley et al., 2015) to Alaska (Bowyer et al., 2003), river otters forage in and rely on healthy subtidal and intertidal zones where they feed on a variety of marine fish and invertebrates. The subspecies L. c. pacifica is found throughout the Salish Sea’s rivers, estuaries, coastlines, and coastal islands and plays an important role in the nearshore marine ecosystem (Larivière & Walton, 1998). The public often confuses marine foraging river otters with sea otters (Enhydra lutra), the, because if it is in the ocean, it must be a “sea” otter. This is especially true in Canada, Alaska, Washington, and California, where both river otters and sea otters occur coastally. River otters do use and rely on marine resources, but taxonomically, they are not considered marine mammals like sea otters.

Status & trends

Unregulated trapping and water pollution reduced river otter populations over 75% of their historical range by the early 1900s, but ecosystem recovery efforts and river otter reintroductions restored otter populations to much of this range (Erb et al., 2018; Roberts, et al., 2020). Now river otters can be found in most historical locations in the United States, where they may be approaching their maximum potential distribution (Erb et al., 2018). This recovery is a true conservation success story (Roberts et al., 2020). It is important to recognize that in addition to translocating over 4,100 river otters from stable populations to recovery areas, improvements in water quality related to the US Federal Clean Water Act and better protection of riparian areas, some attributable to the US Federal Farm Bill, were also essential components of river otter population recovery (Bricker et al., 2022). In contrast to the United States, there has not been a detailed evaluation of the distribution, conservation status, and management of river otters in Canada, although they are reported to occupy all provinces and territories except Prince Edward Island (Bricker et al., 2022).

Using harvest data from 2006-2009, river otter population trends in Washington State were reported as stable, but not expanding, with an estimated harvest of approximately 250 animals a year (Bricker et al., 2022). River otters are classified as furbearers in forty U.S. states, including Washington, requiring a trapping license and open season (Sandoval, 2025). In addition to these traditional sources of population information, community-science programs provide another means of documenting river otter occurrence and distribution. In Washington, the Woodland Park Zoo’s Carnivore Spotter allows community members to report observations of river otters and other carnivores. At a broader geographic scale, the California-based nonprofit River Otter Ecology Project supports the largest community-science program dedicated specifically to river otters, through which participants across North America can contribute sightings to its online Otter Spotter database. The global community-science platform iNaturalist provides an additional source of river otter observations, further contributing to our understanding of the species’ distribution (see links at end). Because these datasets are influenced by variation in observer effort and community participation, an absence of reported sightings does not necessarily indicate that river otters are absent from an area, only that they have not been observed and/or reported.

In Canada, the river otter is federally considered secure and does not warrant candidate status under the Committee  on the Status of Endangered Wildlife in Canada (COSEWIC). The province of British Columbia lists it as yellow, which means it is not designated endangered or threatened (B.C. Conservation Data Centre, 1997).

As reviewed by the International Union for Conservation of Nature (IUCN), North American river otters have a Red List Status of Least Concern (IUCN, 2020). They are included in Appendix II of the Convention on International Trade of Endangered Species (CITES Identification Manual Reference Number A-112.004.023.001), not because of concern for species status, but to assure that legal trade does not contribute to illegal trade of similar “look-alike” species of otters with a higher level of conservation concern (IUCN, 2020). 

Biology

General characteristics

Members of the Mustelidae family, river otters have an elongate body with a long tail that can be one-third the length of the animal. Sexual dimorphism exists with males being larger than females. Average length and weight for 10 males and 10 females captured and released in the San Juan Islands, Washington as part of a pathogen exposure study provided the following average weights:

Beyond these general physical characteristics, many of the river otter’s adaptations reflect the sensory and physiological demands of moving between terrestrial and aquatic environments.

Visual and tactile discrimination

River otter eyes are well suited to their amphibious lifestyle. A transparent nictitating membrane protects the eye underwater, while accommodation of the lens allows otters to compensate for the different refractive properties of air and water. Yet even these adaptations have limits. Comparatively little is known about color discrimination in North American river otters. The available experimental evidence indicates that they can use color as a visual cue, although color is only one of several features used to discriminate among objects (DeLong et al., 2019). More recent work suggests that other visual features, including line shape and symmetry, are also salient to river otters (Wegman et al., 2026).

This is consistent with an animal that routinely forages under conditions where suspended particles, changing light levels, and water depth can rapidly degrade visual information. Like other carnivores, otters possess two classes of cone photoreceptors, short-wavelength-sensitive (S) and long-wavelength-sensitive (L) cones, supporting dichromatic color vision (Griebel & Peichl, 2003). But vision is only part of the sensory system river otters bring to prey capture. North American river otters hunt by both sight and touch, relying increasingly on their vibrissae and forefeet to locate prey in shallow or turbid water (Larivière & Walton, 1998). Across the 14 extant species of otters, the relative importance of these tactile structures varies with foraging specialization, from hand-oriented species, such as Asian small-clawed otters and sea otters, to mouth-oriented piscivores, including North American river otters (Timm-Davis et al., 2015; de Ferran et al., 2024).

For North American river otters, the long facial vibrissae are particularly important during underwater foraging. These highly sensitive whiskers allow otters to detect prey and movements in water when visual information is limited (Larivière & Walton, 1998). Experimental evidence from Eurasian otters provides a particularly striking demonstration of their importance: trimming the vibrissae had little effect on prey capture in clear water but substantially reduced hunting efficiency in turbid water (Green, 1977; Griebel & Peichl, 2003). Thus, vision and touch do not operate as competing sensory systems. Rather, vibrissal information becomes increasingly important precisely under the conditions in which vision becomes less reliable.

Together, these visual and tactile adaptations allow river otters to locate and pursue prey in an environment where sensory conditions can change rapidly. But finding prey underwater is only part of the challenge; pursuing it also requires the physiological capacity to remain submerged and active.

Diving

The diving ability of river otters has been studied, specificially in relationship to how exposure to hydrocarbons affects river otter exercise physiology and diving behavior (Ben-David et al., 2000). In that work, Ben-David et al. (2000) calculated the aerobic dive limit (the maximum breath hold possible without an increase in blood lactate concentration during or after a dive) as 51.4 seconds and found that captive river otters usually dove within their aerobic limits, but were capable of performing longer anaerobic dives. In the wild, their maximum recorded diving depth is approximately 18.3 m (60 ft) from two river otters that drowned in a crab pot set at that depth in Fish Bay near Sitka, Alaska in 1949 (Scheffer, 1952).

Reproduction

Males may produce sperm as early as eight months of age and females can breed as early as one year of age (Melquist et al., 2003). River otters are induced ovulators, triggered by  copulation. The copulation to parturition period is approximately 336.5 days with a delayed implantation period of 273.5 days and a true gestation of approximately 63 days (Melquist et al., 2003). Photoperiod is thought to be the trigger for hormonal control of implantation and pups are born in the spring (Melquist et al., 2003). In Alaska, neonates remain in the den for approximately 8 weeks before emerging (Noll, 1988). 

Longevity

River otters can be classified into four age groups (1 yr old, 1–4 yr old, 4–8 yr old, and >8 yr old) by subjective examination of dental wear (Baitchman and Kollias, 2000). They also can be reliably aged to year by microscopic calculation of cementum annuli from an upper canine tooth (Tabor et al., 1977). Within the Salish Sea, the oldest documented river otter is a 13-year-old male that was found dead in the San Juan Islands (Gaydos et al., 2020). 

Sociality and olfaction

Marine-foraging river otters can be solitary but often live in small social groups that include mother and offspring or unrelated adults, with males being more social than females. A study in Alaska showed that female group size did not vary by month, but male group size did and was related to forage fish abundance. Males were found in groups of up to nine individuals with group size being largest between May and October. Social groups were not kin-based, instead social groups facilitated cooperative foraging, which increased hunting efficiency and reduced home range (Blundell et al., 2002; 2004).

Olfaction is an important part of social behavior among all of the lutrines, as scent marking is one key form of communication. Signaling and scent marking include multiple behaviors for receiving data (sniffing) as well as for sending data (scent marking, defecating and urinating; feet stomping; digging; and allo-grooming and rubbing). These behaviors are influenced by social factors, with differences displayed between otters that were solitary or in a group (Barocas et al., 2021). Sex differences in latrine site use have been identified in marine-foraging otters from Alaska where females irregularly visit numerous latrines, likely to establish the boundaries of their territories, while males use fewer sites with higher intensity (Ben-David et al., 2005). In a small sample of river otters on the   River (Washington), there were no clear spatial differences in latrine use between the sexes, with females and males ranging between latrines as far as 25 and 32 km apart, respectively (Wainstein et al., 2022). The function of latrine use can also vary by solitary versus social males; solitary male otters likely use latrines to facilitate mutual avoidance while social male otters may use latrines for intra-group communication (Ben-David et al., 2005).

Although otters may not be aware of it, or even care, the deposition of feces and urine is not just for social signaling but also moves marine-derived nitrogen into terrestrial plants at latrine sites, which Ben-David et al. (1998) showed had significantly higher values of marine nitrogen than did plants collected from non-latrine sites. The exception to this was alder (Alnus crispa ssp. sinuata), which derives nitrogen from the atmosphere.

Diet

Marine-foraging river otters are generalists, consuming a wide variety of intertidal and subtidal fish and invertebrates as reported by numerous scatological studies from California (Penland and Black, 2009; Cosby and Gunther, 2021), Oregon (Toweill, 1974), Washington (Jones, 2000; Buzzell et al., 2014; Russell, 2015), British Columbia (Stenson et al., 1984; Guertin et al., 2010a), and Alaska (Larsen, 1984; Bowyer et al., 2003). While occasional species of concern like rockfish (Sebastes spp.) and salmon (Onchorrhyncus spp.) have been noted in these diet studies, the majority of fish eaten by marine-foraging river otters in the Salish Sea tend to be common, slow-moving intertidal and subtidal fishes like those in the gunnel (Pholidae), sculpin (Cottidae), and prickleback (Stichaeidae) families (Buzzell et al., 2014; Guertin et al., 2010a, Island, 2026). Otter prey composition varies by season (Jones, 2000), year (Bowyer et al., 2003) and by otter social organization (Blundell et al., 2002). Work conducted in Alaska showed that solitary animals consumed slower-moving intertidal and subtidal fishes, while cooperatively foraging males consumed a higher proportion of fat-rich schooling fish species such as Pacific herring (Clupea pallasi) and Pacific sandlance (Ammodytes personatus) (Blundell et al., 2002; Ben-David et al., 2005).

Marine-foraging otters also will prey on numerous species of birds including ducks and gulls (Toweill, 1974; Hayward et al., 1975; Verbeek and Morgan, 1978; Larsen, 1984; Speich and Pitman, 1984; Jones, 2000; Penland and Black, 2009; Cosby and Gunther, 2021, Island 2026). In one report, otters were noted killing more gulls than they could consume (Verbeek and Morgan, 1978). Although data are sparse, it is not unreasonable to believe that river otters also will prey on non-native rodents. After a survey of 14 islands in the San Juan Islands, Washington failed to find evidence of invasive mice (Mus musculus) and rats (Rattus spp.), the investigators hypothesized that the almost ubiquitous presence of river otters could have been at least partially responsible (Kurle et al., 2013).

River otters near Makah Bay, Washington have been shown to eat invasive European green crab (Carcinus maenas), but with a low occurrence in their scat (0.7-5.2%) suggesting that they are not an important prey item (Buzzell, 2021).This does not necessarily mean that river otters are not capable of consuming more green crabs and could just be a reflection on the density of green crabs at the time of the study relative to other prey items. It does suggest that river otters could play a role in controlling green crab invasion, which warrents further study.

Habitat and Density

Marine-foraging river otters preferentially use rugged coastal areas with irregular shorelines associated with short intertidal lengths over coastal areas with long intertidal lengths and limited shoreline vegetation (Melquist et al., 2003). In a study conducted in Prince William Sound, Alaska, Ben-David et al. (1996) showed that river otters preferred habitats with overstory cover and high wave action when compared to sympatric mink (Neovison vison), which . The two species also varied in the timing and location of foraging; mink primarily foraged at low tide in the intertidal area, while river otters showed no preference for tidal status (Ben-David et al., 1996).

A study in the Salish Sea estimated marine-foraging river otter density on southern Vancouver Island, British Columbia at 1 otter per 2.3 km to 0.8 km of shoreline (Guertin et al., 2012).This density likely depends on habiat quality, which may be mitigated by some combination of prey availability, anthropogenic disturbance, and prey contaminant levels (Guertin et al., 2012). 

Threats

Diseases

Because diseases like morbillivirus have the potential to regulate river otter density, investigation into the diseases of free-ranging and captive river otters is part of the International Union for the Conservation of Nature (IUCN) Species Survival Commission's (SSC) primary conservation priority for this species. Diseases detected in river otters inhabiting freshwater have been reviewed (Kimber and Kollias, 2000; Kimber et al., 2000). Less is known about diseases in marine-foraging river otters. In the Salish Sea, Gaydos et al. (2007a) identified antibodies to Toxoplasma gondii and Leptospira interrogans in trapped otters that appeared healthy and suggested higher exposure to these pathogens could be related to marine-foraging river otters living in closer proximity to humans and domestic pets (Gaydos et al., 2007a). Cryptosporidium and Giardia spp. also have been isolated from feces of otters of unknown health status (Gaydos et al., 2007b). A small pathology study of river otters found in the San Juan Islands, Washington, showed a wide range of causes of mortality, including trauma, infections, and neoplasia, as well as infection by parasites like Sarcocystis spp., Coccidia sp., and an intrabronchial metazoan parasite suspected to be Paragonimous kellicotti (Gaydos et al., 2020). The acanthocephalan, Corynosoma strumosum, a widely distributed parasite of pinnipeds, was found in the intestine of a marine-foraging river otter from Vancouver Island, British Columbia, with no associated pathology (Shanebeck et al., 2022). Although pathology was not documented, a study that included marine-foraging otters near California’s San Francisco Bay isolated four species of marine bacteria (Vibrio algynoliticus, V. cholera, V. metschnikovii, and V. parahemolyticus) from dead otters (Bouley et al., 2015). 

Toxicants

Mercury (Ben-David et al., 2001), organochlorine pesticides (Elliott et al., 2008; Guertin et al., 2010b; Wainstein et al., 2022), polychlorinated biphenyls (PCBs - Elliott et al., 2008; Guertin et al., 2010b; Wainstein et al., 2022), polychlorinated dioxins (Elliott et al., 2008), polybrominated diphenyl ethers (PBDEs - Elliott et al., 2008; Guertin et al., 2010b; Nelson et al., 2015; Wainstein et al., 2022), polychlorinated furans (Elliott et al., 2008), and polycyclic aromatic hydrocarbons (PAHs - Wainstein et al., 2022) have been detected in marine-foraging river otters, with PCBs identified in some geographic locations at levels potentially harmful to the reproductive capacity or health of individual animals (Elliott et al., 2008; Guertin et al., 2010b; Wainstein et al., 2022). Despite river otters in some areas being exposed to toxicologically significant levels of PCBs (>9.0 mg/kg lipid weight; Guertin et al., 2010a), movement data suggested that otters with access to uncontaminated adjacent sites may inadvertently mitigate exposure levels (Guertin et al., 2012). In the Duwamish River estuary in Puget Sound, Wainstein et al. (2022) showed that concentrations of PCBs, PBDEs, dichlorodiphenyl-trichloroethane (DDT), and PAHs in otter feces increased directly with urbanization and were best predicted by models that included development zone. 

Trauma

Trauma, especially vehicular trauma, seems to be an important cause of mortality for river otters in the Salish Sea (Gaydos et al., 2020) and in other places like the San Francisco Bay area (Bouley et al., 2015). Gaydos et al. (2020) also documented illegally gunshot animals presumably killed by disgruntled boat, dock, or home owners displeased with otters defecating on their property.

Oil Spills

The threat of oil spills to river otters has been well studied and documented in Alaska. River otter mortality related to the massive Exxon Valdez oil spill in Prince William Sound, Alaska may have occurred directly from toxicity of petroleum hydrocarbons and indirectly from hydrocarbon damage to the ecosystem. The 1989 spill appears to have reduced suitable habitat for river otters and changed prey availability, resulting in, among other things, river otters requiring larger home ranges in oiled compared to non-oiled areas (Bowyer et al., 2003). Otters living on oiled shores also had lower body mass, larger home ranges, and less diverse diets relative to those in non-oiled areas (e.g., Bowyer et al. 2003). After the Exxon Valdez oil spill, river otters had an increased probability of mortality likely due to the combined effect of direct physiological damage from chronic exposure to oil, decreased ability to dive and forage, and increased energy demands to avoid oiled beaches (Bowyer et al., 2003). Oil spills are bad for river otters, immediately and during the ecosystem’s long recovery.

Water quality

Water pollution and other degradation of aquatic and riparian habitats was thought to play a role in river otter declines across much of North America prior to 1900, and restoration of water and habitat quality played a While numerous river otter predators have been reported (Melquist et al., 2003), little of this work originates from the Salish Sea. Based on data from other regions, potential river otter predators in the Salish Sea include the bald eagle (Haliaeetus leucoephalus), killer whale (Orcinus orca), gray wolf (Canis lupus), coyote (Canis latrans), domestic dog (Canis familiaris), red fox (Vulpes vulpes), bobcat (Lynx rufus), mountain lion (Puma concolor), black bear (Ursus americanus) and brown bear (Ursus arctos), with most predation directed toward juveniles. in population recovery (Bricker et al., 2022). Looking forward, water quality and habitat destruction has the potential to limit distribution of otters and could pose long-term threats if enforcement of water quality standards is not maintained and enforced (IUCN, 2020).

Predators

While numerous river otter predators have been reported (Melquist et al., 2003), little of this work originates from the Salish Sea. Based on data from other regions, potential river otter predators in the Salish Sea include the bald eagle (Haliaeetus leucoephalus), killer whale (Orcinus orca), gray wolf (Canis lupus), coyote (Canis latrans), domestic dog (Canis familiaris), red fox (Vulpes vulpes), bobcat (Lynx rufus), mountain lion (Puma concolor), black bear (Ursus americanus) and brown bear (Ursus arctos), with most predation directed toward juveniles.

Human conflict

Despite data showing that most of their diet consists of non-game fish, in some states the media has reported on animosity about river otters due to their consumption of fish deemed valuable to recreational anglers (Melquist et al., 2003). At times state wildlife agencies responsible for managing trapping of river otters appear to have been complicit in fostering negative portrayals about river otter predation to gain public support for trapping seasons (IUCN, 2020; Bricker et al., 2022). This has not been noted in Washington State or British Columbia though a recent study reported that public perception of river otters in Washington State was divided between positive and negative (Bricker et al., 2022). Marine-foraging river otters have been known to consume prey and defecate on moored boats and docks and den under houses, both of which create conflicts with humans. It might be due to these types of conflicts that gunshot animals have been reported in Washington (Gaydos et al., 2020, Island, 2026).

In recent years, several human-otter conflicts have occurred contributing to negative public perception, where river otters have attacked people (Russo et al., 2026), dogs (Fenlon, 2024), and in one case, a river otter pulled a toddler from the Bremerton Marina dock into the harbor, one of 6 documented Washington river otter attacks in the last decade (WDFW, 2024).  These conflicts are rare, occurring where dogs are unleashed, along harbors where fishing remains are common, or during pupping season, when mothers are acutely attentive to vulnerable young (Island, 2026). The Washington Department of Fish and Wildlife provides information about how to address river otter conflicts non-lethally (see links at end). 

Ecosystem indicators

Coastal river otters in the Salish Sea rely heavily on marine habitats for foraging, feeding primarily on marine fishes and invertebrates, but they remain semi-aquatic mammals that spend considerable time in freshwater lakes and estuaries as well, moving between freshwater, saltwater, and land throughout the day, making them a good biomarker for environmental contamination and a potential resource for evaluating and remediating contaminants  (Guertin et al., 2012; Wainstein et al., 2022).  One such example includes the river otters in Point Reyes National Seashore, where, through the River Otter Ecology Project’s efforts and their Otters for Waters Program, river otter monitoring data was used to inform stream and watershed restoration of the Lagunitas Creek, by documenting otter recolonization and habitat use (Isadore and Island, 2025). Similarly, in Europe, the Eurasian otter (Lutra lutra) serves as a bioindicator and flagship species to promote clean-water initiatives across the continent (IUCN, 2020). As such, there is no reason why marine-foraging river otters should not be exemplars for similar clean water efforts in the Salish Sea.

Population monitoring

While many states are actively monitoring river otter populations (Erb et al., 2018; Roberts et al., 2020), Washington State and British Columbia are not (Bricker et al., 2022) and neither has river otter population estimates. If closer monitoring of marine-foraging river otter populations were to become a priority, available tools could be improved to non-invasively collect DNA from scat to monitor populations (Guertin et al., 2012; Wainstein et al., 2022).

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Links

WDFW: https://wdfw.wa.gov/species-habitats/species/lontra-canadensis#conflict

River Otter Distribution Data from Woodland Park Zoo Carnivore Spotter: https://carnivorespotter.org 

River Otter Distribution Data from iNaturalist: https://www.inaturalist.org/observations?subview=table&taxon_id=41777 

River Otter Ecology Project, Otter Spotter Program: https://survey123.arcgis.com/share/f06b7665970948a0af7aa58d6c232335

Acknowledgments

I really appreciate the help of Ysmine Hentati, Michelle Wainstein, Scott Viers, and Heide Island who provided important comments and edits that improved the quality of this species profile. (This river otter species profile was updated August 28, 2026.)

Licensing & attribution

N/A