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Sea Otter Status March 2010

A year-end report based on 2009 population indicators

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A decade into the new millennium, how have California’s sea otters fared? This status update uses 2009 data to assess the health of the southern sea otter population. It can be difficult to assess the status of a population—different indicators may give us conflicting information. Taken as a whole, we can get an idea of population trends that can influence management decisions though.

In Summary

The Otter Project considers four factors when evaluating population status. Note that a positive or negative classification is describing the population impacts of this indicator (for example the number of dead strandings being low is deemed positive because it means less otters were found dead). For 2009 the factors indicated the following:

  • Spring survey: Negative. 2009 saw a drop in the number of otters counted in the spring survey. This year was particularly noteworthy in that we saw a drop in the 3 year average, suggesting a more serious decline.
  • Dead strandings: Positive. Through December 2009, the number of dead otters recovered was down from previous years.
  • Mortality by age-class: Negative. Increased mortality in pup and reproductive adult age classes.
  • Pup to independent ratio: Positive. From 2003 through 2007 the pup ratio modestly improved. In 2008 the pup ratio declined; in 2009 the pup ratio is back up, to a record high.

That gives us two positive and two negative indicators. Although this is a seemingly neutral status, the significance of a decline in the 3-year average should not be overlooked. The southern sea otter population has gone from stagnant growth to decline. Overall The Otter Project believes that this year’s findings, while ambiguous, are cause for concern. Management measures need to be taken expediently to address this decline in the 3-year average.

Sea Otter Survey

*** 2010 Spring Sea Otter Survey***

The 2009 spring survey found 2,654 otters, 3.8% lower than the 2008 count. This tipped the 3-year average into a decline for the first time since the mid 1990s. The 3 year average for 2009 is 2813, a decline of -0.46%. The US Geological Survey, the federal agency that coordinates the count, described the population as “stable or slightly declining”.

The sea otter population is surveyed twice a year, in spring and fall. The spring count is used to assess the health of the population because it is widely acknowledged that better spring viewing conditions and less surface kelp lead to a more accurate count. There is statistical uncertainty inherent to wildlife counts, and there are measures that could be taken to improve the statistical rigor of the sea otter count. During the 2009 fall count researchers experimented with sampling techniques that could lead to greater statistical accuracy in future counts. This does not impact the 3 year average, which is the number that is taken into account for consideration of up-listing[1] or de-listing.

Scientists have long noted that population growth for the southern sea otter was somewhat stagnant; the population has never grown at more than a 5% rate of increase. (USFWS, 2003) The theoretical maximum is 17-20%, and populations elsewhere such as Washington have seen average growth rates such as these. (Estes, 1990)

The 3 year running average is the number that is considered for up-listing or de-listing under the Endangered Species Act. It is the figure most widely used in considering management actions. This year the 3-year average was 2813, a decline of -0.46% from 2008.

Because the methodology of the fall 2009 survey differed from past years, the numbers do not allow for direct comparison. Instead of a range-wide count, surveyors counted otters in 40 subsamples. Statistical analysis of these numbers is still pending.

Dead Strandings

Otters found dead on beaches are collected throughout the year. Researchers perform necropsies to determine cause of death, which provides a greater understanding of threats to the population. Furthermore, the numbers of otters found dead can serve as a population indicator. It is necessary to note that not all dead otters end up on public access beaches convenient for scientific research. Because of the unknown disparity between the actual numbers of otters that die and the number of otters that are recovered, this indicator can be misleading, and should be considered in context of other indicators.

That said, an unusual number of strandings can indicate serious problem, as happened most recently in 2003.

There were 231 otters found dead through December 2009, one of the lowest numbers recorded since 2003. This year’s total number was near equal the 10 year average (1999-2008) of 232.2.

As the sea otter population grows, we would expect the number of otters found dead to also increase. However, we might expect the percentage of the population dying to be fairly stable. This appears to not be the case—the trend points to an increase in the percentage of the population dying. In 2008 and 2009 the increase in percentage of spring count found stranded stabilized a bit. From 2005 to 2008 the percentage of spring count found dead was 9.9%. In 2008 it dropped to 8.6%. This year the percentage of the spring count found dead was 8.7%. This is only slightly higher than the 10-year average (1999-2008) of 8.0%.

Mortality by Age Count

The Otter Project looks at the distribution of age and sex of otters stranded dead on the beach by averaging all previous years and comparing the current figures against that average.

In a healthy population, we would expect to see high levels of mortality for two key age groups—the very young and the very old. Mortality in aged adults should be the highest. Unfortunately, this rarely holds true for the southern sea otter population, and this year is no exception.

Researchers believe that female otters are demographically the most important. Studies suggest persistent high mortality amongst reproductive age female otters—a problematic trend with negative implications for recovery.

This year we saw 110 females out of the 231 stranded. Of these, the majority were prime aged adults, followed by an equal number of sub-adults and pups.

Past trends suggest higher levels of mortality amongst male sea otters, for unknown reasons. This year slightly more females were found than males.

Pup to Independent Ratio

Researchers believe that sea otter population growth is stymied by high levels of mortality rather than poor reproductive success. That said, recovery is impossible without successful pupping, which is why we track pup trends closely. High levels of mortality in reproducing adults will impact pup production, and is cause for concern.

Past years have seen a decline in the pup to independent ratio.


2009’s spring count saw an increase in the pup to independent ratio, with 17.3 pups for every 100 independents. The actual number of pups counted is 391.

Looking at fall pup numbers can sometimes suggest how well the spring pups have done—high fall puppage could indicate that many pups were lost, as sea otter mothers who lose pups will immediately mate again. Given the new counting methodology in the works this year, however, the numbers so far are incomparable.

Range distribution

A note about range distribution: the center of the range has the highest density clusters of sea otters; lower density is found at the edge of the range. The Otter Project is increasingly concerned about the status of sea otters at the southern end of the range, which includes the no otter zone. Although the count of otters in the no otter zone was low this year, otters have been seen south east of Point Conception regularly for years. This year the US Geological Survey noted a record number of pups in areas adjacent to the edge of the range.

Our Interpretation: What The Otter Project thinks is happening

Based on the above analysis, The Otter Project believes that the southern sea otter population is in decline. It is too soon to tell how steep the decline will be, however any change in direction of demographic trends should be taken seriously by management agencies.  While a higher than average pup-count is certainly positive, continued high levels of mortality in reproductive age adults continue to be cause for concern—if pups don’t make it to reproductive age then population recovery will continue to be a pipe dream.

Regarding survey techniques, we are encouraged by efforts to improve the statistical rigor of the count with new methodology as was experimented with this fall, and The Otter Project would be glad to support additional efforts to improve surveying techniques and statistical analysis. This year, efforts are also being made to address new technologies that could help scientists recover information about otters that do not necessarily wash up on popular public beaches. The Otter Project is hopeful that these efforts will result in information that will lead to a better understanding of otter mortality and what we can do about it.

Why are sea otters dying and what can we do about it?

Disease continues to be the number one known cause of sea otter death, according to a considerable pool of data. Ideally, we could pinpoint a single predominant disease that is disproportionately impacting sea otters; unfortunately reality is not quite so neat. A wide range of diseases impact sea otters, including parasites, viruses and bacteria. Different diseases appear to be predominant in different geographical ranges, and research about the relationship between preferred prey species and disease transmission adds a new dimension to what we know about disease in sea otters.

This overwhelmingly suggests that the large scale degradation of natural coastal systems around human urban centers poses the largest threat to sea otters. Addressing water quality and the conveyance of contaminants, toxins and pathogens into the ocean continues to be a priority for sea otter recovery. This requires that resources be focused on preventing pollution from a myriad of sources from entering the water—work that requires advocacy, expert and public support, and initiative on the behalf of management agencies. While researchers struggle to identify the predominant causes of otter death, The Otter Project believes that we should use the best available science to manage for a healthier coastal ocean along the otter range.

Although the focus of otter recovery has shifted from being singularly about oil spill prevention, this aspect of protection continues to be important. The translocation program of 1987 that was implemented to protect the species from extinction in the face of a catastrophic oil spill has failed, leaving the population as vulnerable as ever. Minimizing oil related activity along the sea otter range should remain a priority for otter conservation.

In addition to improving water quality, it is essential that sea otters are permitted to naturally expand their range, as is already occurring. Providing a regulatory environment that is conducive to this expansion is paramount to protecting otters in the waters of southern California.

The Otter Project is exploring other ways of addressing the persistent trend of ecosystem degradation along the California coastline by supporting implementation of Marine Protected Areas, and considering other methods of habitat protection that could be useful in the future.


The following report was prepared using public agency and peer reviewed data. Surveys are conducted by researchers from a variety of organizations, orchestrated by USGS. Stranding numbers are provided by the Sea Otter Stranding Network, a coalition of agencies and organizations, including Department of Fish and Game, USGS, Monterey Bay Aquarium, Cal Academy of Sciences, Santa Barbara Museum of Natural History, and more; also orchestrated by USGS. Survey data from 2009 is made publicly available on USGS' website. Strandings are provided to The Otter Project by Brian Hatfield of USGS. All agency information is available for public use.

Data tables to support graphs and figures are available on request.


[1] Up-listing is defined as designating the population as “endangered” rather than “threatened” under the Endangered Species Act.

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