Sea otters originally ranged the Pacific coast from the Northern Islands of Japan to the central Pacific Coast of Baja California, with northern sea otters in Alaska ranging from Cape Yakataga to the Aleutian Islands. They were heavily exploited during the fur trade beginning in 1741, and by 1910 were near globally extirpated aside for thirteen remnant colonies. With protection under the 1911 International Fur Seal Treaty and translocations to extirpated Southeast stocks, Alaska’s Southeast, Southcentral, and Southwest rose by the 1980s. The 1989 Exxon crude oil spill caused more sea otter declines across Alaska, most heavily in Prince William Sound. Currently the Southeast and Southcentral stocks show recovery, increasing population trends, and regional expansions in the Southeast stock. The Southwest stock is declining and is protected under the Endangered Species Act, with threats believed to be, but not limited to, predation, contamination, disease, harvesting, habitat loss, and food limitations. Sea otters have additional protection under Marine Mammal Protection Act, and the 1911 International Fur Seal Treaty. The recovery plan for the Southwest stock under U.S Fish and Wildlife Service aims to maintain sustainable and ecologically functional populations and reduce threats to decline. Northern sea otter exploitation impacted nearshore community structure through the increase of epibenthic invertebrates, which have since been exploited by fisheries. Increasing sea otter abundance creates competition and conflict with fisheries and native Alaskans, with crabbing and sea urchin hunting the main concerns. Further research and monitoring are needed in all stocks to track population status and assess the impact of present and future threats.
Sea otters (Enhydra lutis) are a keystone species that maintain nearshore community structure and organization through predation (Simenstad, Estes, & Kenyon, 1978). They are indicators of productive communities since they reduce epibenthic macroinvertebrate abundance, thereby increasing macroalgae abundance and primary production (Simenstad et al., 1978).
This status review summarizes the biology, exploitation, impacts, population status, legislative framework, conflicts, and survey methods of three Alaskan northern sea otter populations (Figure 1 (Fish & Service, 2013)): Southeast, Southcentral, and Southwest. The geographical ranges for these stocks are as follows: Southeast runs from the Dixon entrance to Cape Yakataga; Southcentral includes Cape Yakataga to Cook Inlet, Prince William Sound (WSPS), Kachemak Bay, and the Kenai Peninsula; Southwest includes the Alaska Peninsula coasts, Bristol Bay coasts, and the Pribilof, Aleutian, Barren, and Kodiak Islands (Muto et al., 2019).
Figure 1: Northern sea otter stock boundaries from the 2013 Southwest Alaska Distinct Population Segment of the Northern Sea Otter (Enhydra lutris kenyoni) Recovery Plan
Physical Characteristics. The sea otter is the smallest of the marine mammals containing a dense dark brown insulating fur coat (Kenyon, 1969). Flattened hind feet and tail are used for propulsion and retractable claws on their front feet are used for grooming and eating (Kenyon, 1969). Their long, heavy body is highly adapted for narrow, nearshore environments and they can reach weights and lengths of 45kg and 145cm for males and 33kg and 140cm for females (Kenyon, 1969). Sea otters have flat rounded molars for crushing the external skeletons and shells of their prey (Kenyon, 1969) which consist of mostly epibenthic macroinvertebrates (Simenstad et al., 1978).
Habitat Requirements. Marine sea otters inhabit the open coast of the North Pacific, typically within 54m of the shore (Kenyon, 1969). Areas with rocky shores and bays of kelp beds are favored as these give protection, rest areas, and places to dive for food (Kenyon, 1969).
Behavior. When not diving for food, sea otters are feeding, resting, and grooming while floating on their backs (Kenyon, 1969). Grooming can last several hours a day and is essential for maintaining the waterproofness and insulation of their coat (Kenyon, 1969).
Reproduction. Females become sexually mature at 4 years and have an average annual birth rate of 50%, with two years as the period between births. Gestation lasts for 12-13 months with breeding occurring in all seasons but peaking in the fall (Kenyon, 1969). Births occur on land (Kenyon, 1969).
EXPLOITATION AND DECLINES
Global Population Exploitation. Hunted extensively from 1741 to 1911 (Burn, Doroff, & Tinker, 2003), global sea otter populations declined to thirteen remnant colonies consisting of 1,000-2,000 total individuals (Kenyon, 1969). Previous counts from their historic range of the Northern Islands of Japan to the central Pacific Coast of Baja California (Jameson, Kenyon, Johnson, & Wight, 1982), consisted of 150,000-300,000 individuals (Johnson, 1982). To re-occupy extirpated regions, translocations of 708 individuals from Amchitka Island and Prince William Sound occurred in Washington, Southeast Alaska, British Columbia, and Columbia from 1965-1972 (Jameson et al., 1982). With this, alongside protection under the International Fur Seal Treaty of 1911, global population estimates from 2004-2012 approximated 125,831 sea otters worldwide with populations in Alaska declining from 100,000- 125,000 individuals in 1973, to 89,073 individuals in 2013 (Doroff & Burdin, 2015).
Exxon Oil Spill of 1989. In 1989, 11 million barrels of crude oil spilled in Prince William Sound (WSPS), Southcentral Alaska (Degange, Doroff, & Monson, 1994), most heavily effecting WSPS and the Kenai Peninsula (Degange et al., 1994). An estimated 750-2,650 sea otters died in WSPS (Bodkin, Ballachey, & Esslinger, 2011), and an estimated total 3,905 otters died in Alaska (Degange et al., 1994). Through aerial surveys, WSPS showed an increase from 1993 first post- spill estimates of 2,054 to 3,958 in 2009 (Bodkin et al., 2011). Knight Island of WSPS annually increased 25% since 2003 with the most recent count of 116 individuals 30% less than pre- spill estimates of 165 individuals (Bodkin et al., 2011). Oil and gas production currently occur only in Cook Inlet, within the ranges of the Southwest and Southcentral stocks, and there has been no major crude oil spills since 1989 (Muto et al., 2019). Production and development cause non- crude oil spills, averaging 133, 46-gallon spills per year in the Southeast region and 64, 2-gallon spills in the Southcentral and Southwest regions between 2006-2010 (Muto et al., 2019). These minor spills show no indication of impact on the stocks (Muto et al., 2019).
DATA COLLECTION METHODS
Since sea otters rest and feed in large, easily seen groups, visible counts dominate survey methodology (Bodkin & Udevitz, 1999). Counts from the shore, boats, and aircraft occurred from the 1960s-1990s, but most of these methods didn’t standardized search intensity, altitude, environmental conditions, number of observers, or proportion of animals detected, creating large amounts of bias (Bodkin & Udevitz, 1999). Bias could also rise from weather, glare, or camouflage in kelp beds (Bodkin & Udevitz, 1999). To reduce this, Bodkin and Udevitz in 1999 created new survey methods utilizing aerial surveys that intensively search within strip transects. These methods reduce detection bias (Bodkin & Udevitz, 1999) and have been used most commonly for present population estimates (Muto et al., 2019).
The Method. The Bodkin and Udevitz method utilize preliminary aerial line- transect surveys to first determine survey altitude, then utilize shore observers to determine the effects of altitude, intensive search pattern, and search effort (Bodkin & Udevitz, 1999). Aerial and shore observers measure location, group size, number of pups, and activity within and outside of the transect. Shore observers additionally take measurements before the aerial trials (Bodkin & Udevitz, 1999). Three intensive search patterns (ICU’s) are also implemented, with sizes of 400m, 750m, and 400x800m oval (Bodkin & Udevitz, 1999). Ground and aerial data is then compared to calculate aerial sea otter detection probabilities and the use of group or systematic ICU’s is used to determine correction factors (Bodkin & Udevitz, 1999). Slight bias can arise in the correction factors as the precision of the ICU’s depend on whether the sampled section contains sea otters, but ICU’s overall adjust for and reduce detection bias (Bodkin & Udevitz, 1999).
POPULATION STATUS AND TRENDS
Southeast. The commercial fur trade left no remnant colonies in Southeast Alaska, thus translocations of 408 animals occurred from 1965-1969 from Prince William Sound and Amchitka Island to Cape Spencer, Yakobi Island, Khaz Bay, Biorka Island, the Maurelle Islands, and the Barrier Islands (Esslinger & Bodkin, 2009). Surveys have shown these translocated populations increased 16-23% between 1975 and 1988 (Esslinger & Bodkin, 2009). Populations trends since have shown increase, with the total Southeast population increasing from 10,563 individuals in 2008 to 25,712 individuals in 2014 (Muto et al., 2019). Figure 2 (Muto et al., 2019) shows site-specific counts within this region. 2010-2011 surveys showed an annual rate of increase of 12-14% in South- Southeast and North- Southeast areas (Muto et al., 2019). Expansion of the South- Southeast sea otters has reached the northwest coast of Kuiu Island, Keku Strait, the southern tip of Admiralty Island, and north from the Barrier Islands through Telvak Strait (Muto et al., 2019). Within Yakutut Bay sea otters have expanded to the western shores and have annually increased by 14.6% from 2005-2014 (Muto et al., 2019).
Figure 2. Site- specific population counts, with minimum population estimates and correction factors, from the NOAA 2018 Marine Mammal Stock Assessment Report for the Southeast sea otter stock.
Southcentral. Southcentral populations have shown population increase with a most recent total of 18,297 sea otters compared to the previous SAR total of 15,090 sea otters (Muto et al., 2019). Specific site estimates can be shown in Figure 3 (Muto et al., 2019). Rates of increase through 2002, 2007, and 2008 survey collection in Kachemak Bay indicate an annual rate of increase of 26% (Muto et al., 2019). The annual rate of increase for Prince William Sound between 1993 and 2009 was 2.6%, suggesting a positive recovery from the Exxon 1989 oil spill (Bodkin et al., 2011). Kenai Fjords National Park, 2002, 2007, and 2010 aerial surveys showed an overall stable, possibly increasing population (Muto et al., 2019).
Figure 3. Site- specific population counts, with minimum population estimates and correction factors, from the NOAA 2018 Marine Mammal Stock Assessment Report for the Southcentral sea otter stock.
Southwest. Current population estimates per specific region can be found in Figure 4 (Muto et al., 2019). The total abundance count for the Southwest region is 54,771 individuals, which is slightly higher than the previous SAR of 47,676 individuals (Muto et al., 2019). This increase is thought to be due to the inclusion of the Katmai region (Muto et al., 2019). Though the SAR total has increased, overall population trends are decreasing, with populations in the Aleutian Islands decreasing 70% between 1992-2000 to a total count of 2,442 individuals (Doroff, Estes, Tinker, Burn, & Evans, 2003). The southern Alaskan Peninsula decreased 93-94% from 13,900-17,500 in 1986 to 1005 in 2001(Burn & Doroff, 2005). In one range of the northern Alaskan Peninsula decreased 91-94%, and increased 95-385%, leading to an overall decrease of 27-49% (Burn & Doroff, 2005). Along island coastlines of the southern Alaskan Peninsula, sea otter populations declined 63% and along the southern peninsula to the west of Castle Cape, sea otter density declined 35% (Burn & Doroff, 2005). From 2003-2011 Aleutian Island populations have stabilized but show growth rates of 0 and no evidence of recovery (Muto et al., 2019). Within the Kodiak Archipelago, Kamishak Bay in the lower western Cook Inlet, and Alaska Peninsula coast from Castle Cape to Cape Douglass, populations seem stable and possibly increasing (Muto et al., 2019). Overall the Southwest stock has declined 56-68% since the 1980s (Burn & Doroff, 2005), but is thought to since have stabilized with no signs of recovery (Muto et al., 2019).
Figure 4. Site- specific population counts, with minimum population estimates and correction factors, from the NOAA 2018 Marine Mammal Stock Assessment Report for the Southwest sea otter stock.
IMPACTS OF EXPLOITATION
Alternate Stable- State Communities. As a keystone species, sea otters influence nearshore communities through heavy predation (Simenstad et al., 1978). They reduce herbivorous epibenthic macroinvertebrates, allowing for macroalgae growth on rocky substrate which provides high primary productivity to the ecosystem (Simenstad et al., 1978). In sea otter absence, productivity is low as herbivorous invertebrates overgraze macroalgae, creating bare substrate (Simenstad et al., 1978). Fish dynamics are also impacted with near- shore fish thriving with sea otters presence but not without (Simenstad et al., 1978). Southcentral and Southeast regions evidence this impact as, following exploitation, benthic prey species containing abalones, sea urchins, crab, and clams increased (Pitcher, 1989). Additionally, when comparing the Southeastern Rat Islands, that survived exploitation, to the extirpated Near Islands, there were higher counts of sea otters, macroalgae, nearshore fish, and less benthic macroinvertebrates, suggesting the heavily exploited areas underwent ecosystem changes to alternate stable- states (Simenstad et al., 1978).
CONFLICT IN SOUTHCENTRAL AND SOUTHEASTERN POPULATIONS
Fisheries. Macroinvertebrate numbers increased after sea otter exploitation, leading to the development of commercial, sport, and subsistence fisheries (Johnson, 1982; Pitcher, 1989). Crabbing is the most common subsistence resource, with 71% of native households participating in subsistence crabbing (Pitcher, 1989). It is also a major economic resource, with commercial dungeness crab harvests from 1976-1985 valuing $15,906,538 (Pitcher, 1989). Increasing sea otter abundance now causes conflict in Prince William Sound, where higher otter counts caused Northeast Prince William to prohibit clamming (Johnson, 1982). The increased competition has caused resentment towards sea otters and resource management facilities (Johnson, 1982). Additionally, sea urchins are a first preference prey for sea otters, with counts showing red sea urchins comprising 83% of sea otter diet off west Chichagof Island in Southeast Alaska (Pitcher, 1989). With increased sea otters, the density of sea urchins will decline, impacting commercial exploitation of sea urchins, which has reached past harvesting amounts of 653,000lbs (Pitcher, 1989).
Legislative intervention began in 1911 with the International Fur Seal Treaty that gave protection to sea otter populations from commercial hunting worldwide (Fish & Service, 2013). Under the Marine Mammal Protection Act, Alaskan Natives can take marine mammals so long as hunting is not wasteful and is used for subsistence purposes or for selling authentic handcrafts or clothing (Muto et al., 2019). The same is true under section 10e of the Endangered Species Act, which listed the Southwest population as threatened 2005, however restrictions may increase for threatened stocks if harvesting proves to be a negative impact (Fish & Service, 2013). Though they can hunt sustainably, under the guidelines of the U.S Fish and Wildlife Service (FWS), Alaskan natives are required to partake in mandatory Marking, Tagging, and Reporting Program to track harvesting counts (Fish & Service, 2013). Current legislation focuses on the declining Southwest stock, with threats to decline believed to be increased predation, particularly from killer whales (Orcinus orca), infectious disease, oil spills, contaminants, fishery bycatch, illegal take, subsistence harvest, food limitations, and habitat loss (Fish & Service, 2013). The plan looks specifically at the Western and Eastern Aleutian Islands, South Alaska Peninsula, Bristol Bay, and the Kodiak, Kamishak, Alaska Peninsulas, with goal to reduce threats in the southwest regions so that these areas no longer need to be protected under the ESA (Fish & Service, 2013). The main objectives include: gaining and maintaining sustainable populations within the designated regions, maintaining enough sea otters so that they are contributing functionally to their ecosystems, and reducing/ controlling threats to where the populations can sustainably persist (Fish & Service, 2013). Researching the threats of decline is also a high priority of the plan as the direct cause of decline is unknown (Fish & Service, 2013). This plan is to be implemented over 5 years and will cost 15 million USD. Securing funding is presently a high priority (Fish & Service, 2013).
CONCLUSION AND RECOMMENDATIONS
Overall, the Alaskan northern sea otter populations have increased since exploitation, but continued monitoring is recommended to maintain accurate data and population trajectories. Southeastern and Southcentral northern sea otter stocks have recovered substantially and show to be increasing, however threats with fisheries and Alaskan residents exist and may worsen with increasing sea otter abundances. It is recommended to continue researching these stocks for any signs of conflict- related decline. Additionally, researching the threats facing the Southwest region and monitoring their numbers is pivotal due to their declining trend. Since the most recent population counts comprise data from the early 2000s to 2011, and there is slight variance in the survey methods used, it would be highly recommended to re-revaluate the sites for any disturbances or population changes utilizing the aerial survey method described by Bodkin and Udivetz in 1999 for uniform and comparable results.
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