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Marine Mammals; Finding on Petition To List the Alaska Stock of Sea Otters as Depleted

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Fish and Wildlife Service, Interior.


Finding on petition.


On August 21, 2001, the U.S. Fish and Wildlife Service (FWS) received a petition under section 115 of the Marine Mammal Protection Act (MMPA) from the Center for Biological Diversity. The petition requests that FWS list the Alaska stock of sea otters as depleted. The FWS finds that the petition does not present substantial information that the petitioned action is warranted. The FWS has determined that the statewide population of sea otters in Alaska is larger than presented in the petition. Furthermore, the best available scientific information indicates that multiple stocks of sea otters exist in Alaska.

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Douglas Burn, Wildlife Biologist, Marine Mammals Management Office, 1011 East Tudor Road, Anchorage, Alaska 99503, or telephone 907/786-3800 or facsimile 907/786-3816.

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The sea otter, Enhydra lutris, is the smallest species of marine mammal. Sea otters occur primarily in shallow, nearshore marine habitats (Rotterman and Simon-Jackson 1988). They eat a wide variety of benthic (i.e., bottom dwelling) invertebrates, including bivalves, molluscs, gastropods, crustaceans, echinoderms, and occasionally octopus and fish. This dependence on nearshore benthic invertebrates greatly influences sea otter distribution, and as a result, they are seldom found in deep water. Sea otters seem to prefer areas with kelp beds, but this is not an essential habitat requirement (Riedman and Estes 1990). Although predominantly marine, they will occasionally haul our on shore to rest.

Taxonomically, three subspecies of sea otter have been identified (Wilson et al. 1991). The northern sea otter contains two subspecies: Enhydra lutris kenyoni, which occurs from the Aleutian Islands to Oregon, and Enhydra lutris lutris, which occurs in the Kuril Islands, Kamchatka Peninsula, and Commander Islands in Russia. The third subspecies, Enhydra lutris nereis, occurs in California and is known as the southern sea otter.

Historically, sea otters occurred around the North Pacific rim from Hokkaido, Japan, through the Kuril Islands, Kamchatka Peninsula, the Commander Islands, the Aleutian Islands, peninsular and south coastal Alaska, and southward to Baja California (Kenyon 1969). Extensive commercial hunting of sea otters began following the arrival in Alaska of Russian explores in 1741 and continued during the 18th and 19th centuries. By the time sea otters were afforded protection from commercial harvests by international treaty in 1911, the species was nearly extinct throughout its range, and may have numbered only 1,000-2,000 individuals (Kenyon 1969).

The remaining sea otters were distributed as 13 isolated remnant populations scattered throughout the historic range. Once commercial harvests ceased, 11 of the 13 remaining Start Printed Page 55694populations began to grow and recolonize their former range. By the early 1960s, sea otters had not yet returned to southeast Alaska. In the mid-1960s, the Alaska Department of Fish and Game translocated sea otters from Amchitka Island in the Aleutians and Prince William Sound to several sites throughout southeast Alaska. Similar to the remnant populations, these translocated populations began to grow and expand their geographic range. By the mid-1980s, sea otters had recolonized much of their pre-exploitation range.

In April 1992, FWS conducted an aerial survey of sea otters throughout the entire Aleutian archipelago (Evans et al. 1997). The most striking results of this survey were that sea otter density and abundance in the Rat, Delarof, and western Andreanof Islands had unexpectedly declined by more than 50% since 1965. Boat-based surveys of sea otters at several islands in the Near, Rat, and Andreanof Islands further documented an ongoing decline of sea otters during the 1990s (Estes et al. 1998).

In April 2000, the FWS Marine Mammals Management Office replicated the 1992 aerial survey in the Aleutians. Overall, sea otters in the Aleutian Islands have declined by 70% during the 8-year period from 1992 to 2000 (U.S. Fish and Wildlife Service, unpublished data). The largest declines occurred in the Rat Islands (87%) and the central Aleutians (71%).

Based on the results of this survey, on August 22, 2000, FWS designated sea otters in the Aleutians (from Unimak Pass to Attu) as a candidate species under the Endangered Species Act (ESA) (65 FR 67343). On October 25, 2000, FWS received a petition from the Center for Biological Diversity (CBD) to list sea otters in the Aleutians as threatened or endangered under the ESA. Due to a backlog of court-ordered listing and critical habitat designations, funds were not available to prepare a proposed rule in Fiscal Year (FY) 2001.

Immediately following the completion of the Aleutian sea otter survey, in May 2000 the FWS conducted an aerial survey of sea otters on the north side of the Alaska Peninsula from False Pass to Cape Seniavin. The FWS also conducted aerial surveys along the south side of the Alaska Peninsula in April 2001. The results of these surveys, which replicated a baseline study conducted in 1986 (Brueggeman et al. 1988), indicate that the sea otter population has also declined in these areas. In June 2001, the FWS conducted an aerial survey of the Kodiak Archipelago for comparison with data collected in 1994. A comparison of the two surveys, which used the same aircraft, pilot, and observer, indicate the sea otter population has declined in the Kodiak area as well. Based on the results of surveys conducted in the past year, the Alaska Region of the FWS has requested funding in FY2002 to prepare a proposed rule to list sea otters in southwest Alaska under the ESA.

On August 21, 2001, FWS received a petition from CBD to list sea otters throughout their range in Alaska as depleted under the MMPA (September 6, 2001, 66 FR 46651). Section 115(a)(3)(B) of the MMPA requires the FWS to publish a finding in the Federal Register as to whether the petition presents substantial information indicating that the petitioned action may be warranted.

Identification of Sea Otter Stocks in Alaska

Findings of depleted status must be made on the species or population stock level. Amendments to the MMPA in 1994 included section 117 (16 U.S.C. 1386), which mandated preparation of stock assessments for each marine mammal stock that occurs in waters under the jurisdiction of the United States. In 1995, FWS published a final stock assessment for the northern sea otter in Alaska as a single stock (60 FR 52008). Section 117(c) requires that stock assessments be reviewed: (A) At least annually for stocks which are specified as strategic stocks; (B) at least annually for stocks for which significant new information is available; and (C) at least once every 3 years for all other stocks. If the review indicates that the status of the stock has changed or can be more accurately determined, the stock assessment shall be revised according to the process outlined in section 117(b). The first revision to the stock assessment occurred in 1998.

In February 1998, FWS published a draft revision of the northern sea otter stock assessment that identified three stocks of sea otters in Alaska (63 FR 10936). The revision identified a southeast Alaska stock (Cape Yakataga to Dixon Entrance), a southcentral Alaska stock (Cook Inlet to Cape Yakataga), and a southwest Alaska stock (Cook Inlet to Attu Island, including the Kodiak archipelago).

In August 1998 the Alaska Sea Otter Commission (ASOC) requested a proceeding on the record as outlined in section 117(b)(2) of the MMPA to contest the identification of multiple stocks of sea otters in Alaska. After considerable discussion, FWS and ASOC signed a Memorandum of Agreement (MOA) in July 1999 to further investigate the issue of stock structure of sea otters in Alaska. On August 12, 1999, the ASOC (now the Alaska Sea Otter and Steller Sea Lion Commission) withdrew the request for a formal proceeding on the record. A revised stock assessment for the northern sea otter in Alaska has not been finalized.

The identification of multiple stocks of sea otters in Alaska in our 1998 draft revision was based on an analysis of existing data on distribution, population response, phenotypic data, and genotypic data according to Dizon et al. (1992). One element of the MOA concerned scientific peer review of the analysis that identified multiple stocks of sea otters in Alaska. That element has been satisfied by the publication of Gorbics and Bodkin (2001), who applied the criteria of Dizon et al. (1992) and identified three stocks of sea otters in Alaska: Southwest, southcentral, and southeast. Another element of the MOA involved the completion of additional genetics analysis using both mitochondrial and nuclear DNA, as well as a larger sample size. This study is complete and a manuscript is in preparation for scientific peer review and publication (Cronin et al. in prep.). The results of this study also support the identification of multiple stocks of sea otters in Alaska.

The FWS is currently in the process of revising our original 1995 stock assessment for northern sea otters in Alaska. Based on the best available scientific evidence, FWS anticipates publishing draft stock assessments identifying multiple stocks of sea otters in Alaska. The drafts soon will be available for public review and comment.

Current Population Size

The Petition presents an estimated statewide sea otter population of fewer than 38,000 individuals. This figure was calculated using population estimates from the 1998 draft stock assessments, along with the estimated abundance for the Aleutians presented in the Candidate Species announcements (65 FR 67343). The statewide population estimate presented in the petition is inaccurate for several reasons. First, available population estimates are omitted from substantial portions of the State, including the Alaska Peninsula and Kodiak archipelago. Second, the estimate used for the Aleutian Islands is incorrect. Further analysis of the Aleutian aerial survey data has resulted in a revised population estimate of 8,742 rather than the previously reported value of 5,812 (Doroff et al. in prep.). Finally, most population Start Printed Page 55695estimates presented by the petitioners are not corrected for detection probability. Sea otters may either be at the surface and missed by observers, or below the surface and unavailable for counting. In order to calculate the best estimate of current population size for determination of depleted status, these data must be corrected for detection probability.

Detection probability is often specific to survey design, personnel, and weather conditions. The survey technique used in Prince William Sound, Kodiak, and Yakutat generates a detectability correction factor for each survey (Bodkin and Udevitz 1999). Detection probabilities for this technique ranged from 52-72%. Similarly, the Aleutian Islands survey in 2000 used sea otter counts made from skiffs at six islands to estimate a detection probability of 28% (Doroff et al. in prep).

For aerial and ship-based surveys for which no correction factor exists, the results of similar studies can be used as approximate values. For fixed-wing aerial surveys with one observer on each side of the aircraft, Evans et al. (1997) calculated that observers saw 42% of the sea otters within a known area. This equates to a detectability correction factor of 2.38 (CV=0.087). For surveys conducted from small boats, Udevitz et al. (1995) estimated that observers saw 70% of the sea otters present, for a correction factor of 1.43 (CV=0.071). Detection of sea otters during boat surveys is higher than aerial surveys because the survey platform is moving slower, which gives observers more time to visually search for otters. The additional search time also reduces the likelihood that otters below the surface may be missed. Using the most applicable correction factors for detection probability available, the best estimate for the Aleutians Islands, and including all areas of the State, the current best estimate of the Alaska sea otter population size is 74,143 with a 95% confidence interval of ±15,739 (Table 1).

Table 1.—Current Statewide Population Estimates of Sea Otters in Alaska. Original Estimates for Locations in Italics Did Not Include a Survey-Specific Detection Probability Correction Factor. Adjusted Estimates for These Locations Use a Correction Factor of 2.38 for Fixed-Wing Aircraft and 1.43 for Small Boat Surveys

LocationSurvey yearSurvey typeOriginal estimateCoefficient of variationAdjusted estimateReference
Southeast Alaska1994Small boat8,1800.39211,697Agler et al. 1995.
Yakutat Bay1995Fixed-wing aircraft4040.339404Doroff and Gorbics 1998.
North Gulf of Alaska (Cape Yakataga to Cape Spencer)1996Fixed-wing aircraft223531Doroff and Gorbics 1998.
Lower Cook Inlet1993Small boat5,9140.2678,457Agler et al. 1995.
Kenai Peninsula1989Helicopter2,3300.1202,330DeGange et al. 1995.
Prince William Sound1999Fixed-wing aircraft13,2340.19813,234USGS unpublished data.
N. Gulf of Alaska (Cape Hinchinbrook to Cape Yakataga)1996Fixed-wing aircraft271645Doroff and Gorbics 1998.
Aleutian Islands2000Fixed-wing aircraft8,7420.2158,742Doroff et al. in prep.
Unimak Island2001Fixed-wing aircraft42100FWS unpublished data.
North Alaska Peninsula (False Pass to Port Heiden)2000Fixed-wing aircraft5,7560.32713,699FWS unpublished data.
South Alaska Peninsula (False Pass to Pavlof Bay)2001Fixed-wing aircraft9390.8092,235FWS unpublished data.
South Alaska Peninsula (Seal Cape to Cape Douglas)2001Fixed-wing aircraft2,1905,212FWS unpublished data.
South Alaska Peninsula Islands2001Fixed-wing aircraft405964FWS unpublished data.
Kodiak Archipelago2001Fixed-wing aircraft5,8930.2285,893FWS unpublished data.

Population Status Relative to OSP

The worldwide population of sea otters in the early 1700s has been estimated at 150,000 (Kenyon 1969) to 300,000 (Johnson 1982). The size of the Alaska sea otter population prior to commercial depletion is unknown. Calkins and Schneider (1985) estimated the statewide sea otter population at 100,000 to 150,000 in 1976. Sea otter populations have potentially high reproductive rates (Riedman and Estes 1990). As a result, recovering otter populations may temporarily exceed carrying capacity (K) on a local level, before stabilizing at a lower equilibrium value (Estes 1990). These uncertainties make a current determination of K for sea otters in Alaska problematic. In the face of these uncertainties, the petitioners propose a conservative estimate of K for sea otters in Alaska of 100,000 individuals. Lacking specific information about habitat and K throughout much of Alaska, we believe this is a reasonable estimate of K at this time.

Determination of the Maximum Net Productivity Level (MNPL), which defines the lower bound of Optimum Sustainable Population (OSP) for any marine mammal stock is difficult. Initial studies on marine mammal populations using a generalized logistic population model resulted in a MNPL of half of K. Later studies based on the life history characteristics of marine mammals suggested that MNPL lies somewhat closer to K (Eberhardt and Siniff 1977, Gerrodette and DeMaster 1990). For most species of marine mammals, there is insufficient data available to estimate MNPL accurately (Taylor and DeMaster 1993). Where a Species-specific estimate of MNPL is unavailable, the best available information calls for applying 60% of K as an approximation (Barlow et al. 1995). Using this approximation of MNPL (60%) for the purpose of responding to this petition and in the absence of specific productivity levels for Alaska sea otters and an estimated K of 100,000, the petitioners present a lower threshold of OSP for sea otters in Start Printed Page 55696Alaska of 60,000. The best available population estimate for sea otters in Alaska (74,143) is above this threshold.

Finding on the Petitioned Action

Based on the best available estimate of current population size, the statewide sea otter population is above the conservative estimate of OSP. In addition, although the last finalized stock assessment in 1995 classified sea otters in Alaska as a single stock, the best scientific information currently available indicates that multiple stocks exist. This information suggests that three stocks occur in Alaska: Southwest, southcentral, and southeast. The best available scientific information shows that the population in southeast Alaska is growing (Bodkin et al. 1999), and the population in southcentral Alaska is either stable or growing. While these two populations are either stable or growing, the FWS acknowledge that sea otters in southwest Alaska have undergone widespread, dramatic declines in the past 10-15 years.

The FWS is in the process of revising the stock assessment of sea otters in Alaska under the MMPA, and as part of this process will make a final determination on the number and geographic range of Alaska sea otter stocks. Stock identification is a defined process under the MMPA and while the currently available biological data indicates that three stocks are appropriate, the Service needs to complete the stock assessment process properly and in close cooperation with our partners. The genetics study conducted by the FWS was just recently completed and we expect the new stock assessments to be completed soon. Once these stock assessments are finalized, the status of each stock will be evaluated and designation of a southwest stock as depleted may be warranted at that time.

The FWS acknowledged the decline of sea otters of the Aleutians by designating them a Candidate Species under the ESA in August 2000. In the candidate species designation, the FWS treated the sea otters in southwest Alaska as a distinct population segment under the ESA and its implementing regulations. Once funding is available the FWS will proceed to propose the southwest Alaska sea otters for Federal listing under the ESA. This action would be more applicable to the sea otter than a depleted designation. The primary benefits that accrue to a depleted species is the requirement for a conservationn plan which is already in place for the sea otter, and the ability to publish regulations to regulate harvest if harvest is negatively affecting the population; we do not believe harvest is affecting the population. Also, under section 3(1)(C) of the MMPA (16 U.S.C. 1362), a species or population stock that is listed as an endangered species or a threatened species under the ESA is automatically classified as depleted under the MMPA. While continuing to evaluate the sea otter under both statutes, the FWS will also continue to monitor population status and further assess causes of the decline, to the extent possible within available resources.

The FWS finds that the petition did not present substantial information that the petitioned action is warranted. The FWS has determined that the statewide population of sea otters in Alaska is considerably larger than the conservative estimate of OSP presented in the petition. Furthermore, the best available scientific information indicates that multiple stocks of sea otter exist in Alaska.


Agler, B.A., S.J. Kendall, P.E. Seiser, and D.B. Irons. 1995. Estimates of marine bird and sea otter abundance in lower Cook Inlet, Alaska, during summer 1993 and winter 1994. Migratory Bird Management, U.S. Fish and Wildlife Service, Anchorage, Alaska. 121 pp.

Agler, B.A., S.J. Kendall, P.E. Seiser, and J.R. Lindell. 1995. Estimates of marine bird and sea otter abundance in southeast Alaska during summer 1994. Migratory Bird Management, U.S. Fish and Wildlife Service, Anchorage, Alaska. 90 pp.

Barlow, J., S.L. Swartz, T.C. Eagle, and P.R. Wade. 1995. U.S. marine mammal stock assessments: guidelines for preparation, background, and a summary of the 1995 assessments. U.S. Department of Commerce, NOAA Technical Memorandum NMFS-OPR-6. 73 pp.

Bodkin, J.L. and M.S. Udevitz. 1999. An aerial survey method to estimate sea otter abundance. Pages 13-26. In: Marine Mammal Survey Assessment Methods. G.W. Garner et al. (Eds.) A.A. Balkema, Rotterdam.

Bodkin, J.L., B.E. Ballachey, M.A. Cronin, and K.T. Scribner. 1999. Population demographics and genetic diversity in remnant and translocated populations of sea otters (Enhydra lutris). Conservation Biology. 13(6)1378-1385.

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Calkins, D.G. and K.B. Schneider. 1985. The sea otter (Enhydra lutris). Pages 37-45. In: Marine Mammals Species Accounts, J.J. Burns, K.J. Frost, and L.F. Lowry (Eds.). Alaska Department of Fish and Game, Technical Bulletin 7.

Cronin, M.A., W.J. Spearman, W. Buchholz, S. Miller, L. Jack, and L.R. Comerci. In prep. Microsatellite DNA and mitochondrial DNA variation in Alaska sea otters.

DeGange, A.R., D.C. Douglas, D.H. Monson, and C.M. Robbins. 1995. Surveys of sea otters in the Gulf of Alaska in response to the Exxon Valdez oil spill. Natural Resource Damage Assessment Marine Mammal Study 6-7. Final Report. 11 pp.

Dizon, A.E., C. Lockyer, W.F. Perrin, D.P. DeMaster, and J. Sisson. 1992. Rethinking the stock concept: a phylogeographic approach. Conservation Biology, 6: 24-36.

Doroff, A.M. and C.S. Gorbics. 1998. Sea otter surveys of Yakutat Bay and adjacent Gulf of Alaska coastal areas—Cape Hinchinbrook to Cape Spencer 1995-1996. Minerals Management Service, OCS Study MMS 97-0026. pp.

Doroff, A.M., J.A. Estes, M.T. Tinker, D.M. Burn, and T.J. Evans. In prep. Sea otter population declines in the Aleutian archipelago.

Eberhardt, L.L. and D.B. Siniff. 1977. Population dynamics and marine mammal management policies. Journal of the Fisheries Research Board of Canada, 34: 183-190.

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Authority: The authority for this action is the Marine Mammal Protection Act of 1972, as amended, 16 U.S.C. 1361 et seq.

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Dated: October 26, 2001.

Marshall P. Jones, Jr.,

Director, U.S. Fish and Wildlife Service.

End Signature End Supplemental Information

[FR Doc. 01-27495 Filed 11-1-01; 8:45 am]