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Marine Mammal Protection Act; Stock Assessment Report

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Fish and Wildlife Service, Interior.


Notice of availability of final 2008 revised marine mammal stock assessment report for the southern sea otter in California; response to comments.


In accordance with the Marine Mammal Protection Act of 1972, as amended (MMPA), and its implementing regulations, we, the U.S. Fish and Wildlife Service (Service), announce that we have revised our stock assessment report (SAR) for the southern sea otter (Enhydra lutris nereis) stock in California State, including incorporation of public comments. We now make our complete final 2008 revised SAR available to the public.

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For information on the methods, data, and results of the stock assessment, contact Lilian Carswell by phone at (805) 612-2793 or by e-mail at

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Send requests for printed copies of the SAR to: Field Supervisor, U.S. Fish and Wildlife Service, Ventura Fish and Wildlife Office, 2493 Portola Road, Suite B, Ventura, CA 93003. You may also view or download it at​ventura/​speciesinfo/​so_​sea_​otter/​.

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Under the MMPA (16 U.S.C. 1361 et seq.) and its implementing regulations in the Code of Federal Regulations at 50 CFR part 18, we regulate the taking, possession, transportation, purchasing, selling, offering for sale, exporting, and importing of marine mammals. One of the goals of the MMPA is to ensure that stocks of marine mammals occurring in waters under the jurisdiction of the United States do not experience a level of human-caused mortality and serious injury that is likely to cause the stock to be reduced below its optimum sustainable population level (OSP). OSP is defined as “the number of animals which will result in the maximum productivity of the population or the species, keeping in mind the carrying capacity of the habitat and the health of the ecosystem of which they form a constituent element.”

To help accomplish the goal of maintaining marine mammal stocks at their OSPs, section 117 of the MMPA Start Printed Page 79896requires us and the National Marine Fisheries Service (NMFS) to prepare a SAR for each marine mammal stock that occurs in waters under the jurisdiction of the United States. A SAR must be based on the best scientific information available; therefore, we prepare it in consultation with established regional scientific review groups. Each SAR must include: (1) A description of the stock and its geographic range; (2) minimum population estimate, maximum net productivity rate, and current population trend; (3) estimate of human-caused mortality and serious injury; (4) commercial fishery interactions; (5) status of the stock; and (6) potential biological removal (PBR) level. The PBR is defined as “the maximum number of animals, not including natural mortalities, that may be removed from a marine mammal stock while allowing that stock to reach or maintain its OSP.” The PBR is the product of the minimum population estimate of the stock (Nmin); one-half the maximum theoretical or estimated net productivity rate of the stock at a small population size (Rmax); and a recovery factor (Fr) of between 0.1 and 1.0, which is intended to compensate for uncertainty and unknown estimation errors.

Section 117 of the MMPA also requires us and NMFS to review the SARs (a) At least annually for stocks that 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.

A strategic stock is defined in the MMPA as a marine mammal stock (A) For which the level of direct human-caused mortality exceeds the PBR; (B) which, based on the best available scientific information, is declining and is likely to be listed as a threatened species under the Endangered Species Act of 1973, as amended (16 U.S.C. 1531 et seq.; ESA), within the foreseeable future; or (C) which is listed as a threatened or endangered species under the ESA, or is designated as depleted under the MMPA.

Before releasing our draft SAR for public review and comment, we submitted it for technical review internally and also for scientific review by the Pacific Regional Scientific Review Group, which was established under the MMPA. In a June 10, 2008 (73 FR 32732), Federal Register notice, we made available our draft SAR for the MMPA-required 90-day public review and comment period. Following the close of the comment period, we revised the SAR based on public comments we received (see below) and prepared the final 2008 revised SAR. Between publication of the draft and final revised SARs, we have not revised the status of the stock itself (i.e., strategic). However, in response to a public comment, we revised Nmin to base it on the 20th percentile of the log-normal distribution of the average count for the 3-year running average for 2006-2008. In addition, rather than listing the Nmin, of the mainland and the San Nicolas Island populations separately, we combined them into a single Nmin, for the stock as a whole. We used an Rmax of 6 percent, that of the mainland population, because this rate reflects the threats and limitations to which approximately 98 percent of the stock is exposed. We revised the PBR level from 9 to 8 based on an Rmax of 6 percent and the revised Nmin. We addressed most of the public comments we received by adding text for clarity.

The following table summarizes the final 2008 revised SAR for southern sea otters in California, listing the stock's Nmin, Rmax, Fr, PBR, annual estimated human-caused mortality and serious injury, and status:

Summary of Final Revised Stock Assessment Report for the Southern Sea Otter in California

StockNminRmaxFrPBRAnnual estimated average human-caused mortalityStock status
Southern sea otters2,7230.060.18UnknownStrategic.

Responding to Public Comments

We received comments on the draft SAR (73 FR 32732) from the Marine Mammal Commission, the Center for Biological Diversity, Friends of the Sea Otter, Defenders of Wildlife, and one private citizen. We present issues raised in those comments, along with our responses, below.

Comment 1: Because of the uncertainty in population counts, the decline in the 2008 sea otter count, and the absence of routine updates to the SAR, the Service should take a precautionary approach and base the minimum population size estimate on the 20th percentile of the log-normal distribution of the average count for the 3-year running average for 2006-2008 rather than the latest single-year count.

Response: Our use of the latest single-year count in the draft SAR was based on the Guidelines for Preparing Stock Assessment Reports Pursuant to Section 117 of the Marine Mammal Protection Act (GAMMS II), published in 2005, which state that a direct count may be used as an estimate for Nmin. We acknowledge that there are considerable fluctuations in the population count from year to year, resulting in part from unquantifiable observation error. Because of this year-to-year variability, the 3-year running average is the metric recommended in the final revised recovery plan for the southern sea otter (68 FR 16305; April 3, 2003), and it is the metric we typically use to characterize population size and to track trends. However, use of the 3-year running average as a minimum population size estimate for the purposes of the SAR is not appropriate, because the MMPA defines Nmin as the number that provides reasonable assurance that the stock size is equal to or greater than the estimate. If a high count is followed by 2 years of declining counts, it is possible that the 3-year running average will not provide reasonable assurance that the stock size is equal to or greater than the estimate. Therefore, we adopt the precautionary approach recommended by the commenter, which applies the alternate guidelines for determining Nmin that are included in the GAMMS II guidance. To calculate Nmin for the stock, we combined counts for the mainland and San Nicolas Island. Given the log-normal distribution of the average combined counts for 2006-2008, the estimate corresponding to the 20th percentile of this distribution is 2,723. We have revised the minimum population estimate accordingly.

Comment 2: The Service should include an estimate of the average population size as well as a minimum population estimate.

Response: The data resulting from the annual spring surveys represent minimum population counts, with no associated correction factor or variance estimate. As a result, they include significant (but unquantifiable) observation error, probably caused mostly by year-to-year variance in survey conditions. In order to reduce potential influences from the vagaries of any single census, data are presented as Start Printed Page 798973-year running averages. In response to comments we received on our draft SAR (73 FR 32732), we now base the minimum population estimate on the 20th percentile of the log-normal distribution of the average count for 2006-2008 rather than on the most recent census. However, because correction factors or variance estimates are not available, we are unable to include an estimate of the average population size.

Comment 3: The SAR should clarify that the San Nicolas Island colony is considered to be a “non-essential experimental population” under the ESA because it was established during a translocation experiment (52 FR 29754; August 11, 1987). It should also clarify whether this population was included in the estimation of population parameters used to characterize the stock's status and to determine its PBR level.

Response: We have revised the SAR accordingly.

Comment 4: The Service should arrange for observer coverage of trap fisheries for lobster, crab, and fish, particularly in waters occupied by sea otters south of Point Conception, and of set and drift gillnet fisheries in the sea otter's range. Observer coverage should be augmented in the purse-seine fisheries.

Response: NMFS conducts observer programs. Since resources for these programs are fully utilized, no new programs may be initiated until other monitoring or conservation efforts are terminated so that resources can be redirected. A recent analysis has shown that a very high level of observer coverage would be required to see any indication of trap mortality, even if mortality levels were high enough to substantially reduce the rate of population recovery (Hatfield et al., in prep.). We are evaluating options for obtaining additional information on interactions between sea otters and fisheries that have limited or no observer coverage.

Comment 5: The Service assumes that mortalities from gill nets are “at or near zero” based on the closure of some areas to gill net use but lacks the observer and other independent data to back up this assumption. The Service cannot legitimately claim that entanglements are at or near zero based on the limited observer data available.

Response: We believe that southern sea otter mortalities resulting from interactions with gill nets are currently at or near zero because of the relationship between three factors: The depths that are closed to gill net fishing; the depths utilized by sea otters for foraging; and the current extent of the southern sea otter's range. Gill net fishing is prohibited in waters shallower than 70 fathoms (128 meters) from Point Reyes to Point Arguello, in waters generally within 3 nautical miles offshore of the mainland coast from Point Arguello to the Mexican border, and in waters shallower than 70 fathoms or within 1 mile, whichever is less, around the Channel Islands. Although sea otters occasionally dive to depths of 100 meters, the vast majority (more than 99 percent) of dives are to depths of 40 meters or less (M. Tim Tinker, pers. comm., 2008). The southern sea otter range currently extends from the mouth of the Tunitas Creek, in San Mateo County, to Coal Oil Point, in Santa Barbara County (​otters/​ca-surveyspr2008.htm). The closure from Point Reyes to Point Arguello, which includes most of the sea otter range, encompasses the depths to which southern sea otters are known to dive. The remainder of the range is located along the coast from Point Arguello to Coal Oil Point. The bathymetry of the area from Point Arguello to Coal Oil Point is such that the 3-mile closure translates into depths of approximately 100 meters. A preliminary analysis of sea otter dives in the southern portion of the range determined that a closure to 94 meters would include all dives of 95 percent of all sea otters, and a closure to 104 meters would include all dives of 99 percent of all sea otters (M. Tim Tinker, pers. comm., 2008). Because the likelihood of a sea otter diving to depths exceeding 128 or 100 meters is exceedingly small, we do not believe that, given the current extent of the range, sea otters are interacting with gill nets. However, we will continue to evaluate the risks to which sea otters are exposed by this type of gear.

Comment 6: The Service reports three non-lethal interactions in purse-seine fisheries over the past 5 years but assumes that no serious injuries or mortalities have occurred. This assumption seems overly optimistic.

Response: We have revised the SAR to reflect that no data are available to enable us to assess whether sea otter interactions with purse-seine gear are resulting in mortality or serious injury.

Comment 7: Because sea otters are not covered under section 118 of the MMPA, PBR does not apply to the governance of incidental take of southern sea otters in commercial fisheries. However, section 117 of the MMPA requires the calculation of PBR, and that calculation should be based on the best available scientific data. Therefore, the Service should use a value for Rmax of 5 percent rather than 6 percent to calculate PBR, because the average annual growth rate from 2001 to 2007 was approximately 5 percent.

Response: We have revised the SAR to clarify the status of southern sea otters with respect to section 118 of the MMPA. However, we have not used an Rmax of 5 percent as suggested by the commenter. The MMPA defines one-half Rmax as “one-half of the maximum theoretical or estimated `net productivity rate' of the stock at a small population size,” where the term “net productivity rate” means “the annual per-capita rate of increase in a stock resulting from additions due to reproduction, less losses due to natural mortality.” The maximum observed growth rate along the mainland is 6 percent annually. Although the maximum observed growth rate in any southern sea otter population is 9 percent annually, this rate has been seen only at San Nicolas Island, which is geographically removed from the mainland range and is subject to different threats and limitations than the mainland range. For the stock as a whole, we use an Rmax of 6 percent rather than 9 percent because that rate reflects the threats and limitations to which approximately 98 percent of the population is exposed.

Comment 8: It is misleading to say that the colony at San Nicolas Island “has grown by approximately 9 percent annually” since the early 1990s. It would be accurate to say that the colony has grown by “an approximate average of 9 percent annually” since the early 1990s.

Response: We have revised the SAR accordingly.

Comment 9: The Service does not provide an estimated number of non-lethal interactions or a precise estimate of observer coverage in the purse-seine fishery for 2006.

Response: The SAR has been revised to incorporate an estimated number of non-lethal interactions in 2006. A precise estimate of observer coverage in the purse-seine fishery for 2006 requires data on fishing effort derived from logbook and landing data. At the time the final SAR was prepared, logbook and landing data for purse seine fisheries targeting sardine, anchovy, mackerel, and tuna in 2006 were not available.

Comment 10: The SAR should reference the unpublished study that analyzed sea otter carcasses and their ability to fit through a variety of trap openings.

Response: We have included results from the referenced study (Hatfield et al., in prep.) in the final SAR.Start Printed Page 79898

Comment 11: The Service should take every action available to investigate and, where possible, mitigate the impact of infectious disease and should improve enforcement of the provisions of the MMPA that prevent the intentional shooting of marine mammals.

Response: We support and have provided funding for studies aimed at determining and mitigating the impact of infectious disease. We continue to investigate, and pursue actions in response to, intentional shooting of sea otters.

Comment 12: While section 118 of the MMPA does not govern the incidental taking of southern sea otters, the zero mortality rate goal (ZMRG) provisions in section 101 do apply to southern sea otters. The fact that the Service cannot make a status determination with respect to ZMRG confirms that ZMRG has not been achieved for sea otters and that the Service has not satisfied its requirements under the law. This failure strongly supports the need for [the Service] to aggressively place observers on fisheries that have the potential to take southern sea otters so that it can determine the status of the stock with respect to ZMRG.

Response: Please see our response to comment 4.

Comment 13: The SAR should provide additional discussion and references on the topic of food limitation and nutritional deficiency.

Response: We have included additional references in the final SAR and will expand our discussion as data become available.

Additional References Cited:

Bentall, G.B., 2005. Morphological and Behavioral Correlates of Population Status in the Southern Sea Otter: A Comparative Study Between Central California and San Nicolas Island. Masters Thesis, University of California, Santa Cruz, CA, unpublished.

Hatfield, B.B., J.A. Ames, J.A. Estes, M.T. Tinker, A.B. Johnson, M.M. Staedler, and M.D. Harris. Manuscript in preparation. The potential for sea otter mortality in fish and shellfish traps. 22 pp. + appendices.

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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 al.).

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Dated: December 17, 2008.

Kenneth Stansell,

Acting Director, Fish and Wildlife Service.

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[FR Doc. E8-31022 Filed 12-29-08; 8:45 am]