Sanctuary Integrated Monitoring Network
Monitoring Project

Concentrations and Effects of Environmental Contaminants on the Health of California Sea Otters

Principal Investigator(s)

  • Kurunthachalam Kannan
    New York State Department of Health
  • Nancy Thomas
    United States Geological Survey (USGS)


  • SIMoN
Start Date: October 01, 2003
End Date: June 30, 2006

Recent research on adult southern sea otters suggests that the lack of population growth is not due to low reproductive output, but rather a high incidence of adult mortality. It is possible that the prevalence of disease has increased in recent years. Southern sea otters may be more susceptible to diseases for a number of reasons. Some have hypothesized that environmental contaminants compromise the otters’ immune response or weakens them in some other way, predisposing sea otters to disease-induced mortality.

Concentrations of four perfluorinated contaminants were measured in liver tissue from 80 adult female sea otters collected from the California coast during 1992-2002. Concentrations of one contaminant in the livers of these sea otters were among the highest values reported for marine mammals to date. To examine the association between exposures to these contaminants and their potential effects, concentrations were compared among the adult female otters that died from 1) infectious diseases, 2) noninfectious causes, and 3) from apparent emaciation. Concentrations were significantly higher in females in the infectious disease category. Concentrations of were not significantly different between noninfectious and emaciated otters, suggesting that the poor nutritive status of emaciated otters did not affect the concentrations of perfluorochemicals in livers. Significant association between infectious diseases and elevated concentrations of perfluorochemicals in the livers of female sea otters is a cause for concern and suggests the need for further studies to understand how these compound contribute to increased mortality.

Summary to Date

As a top-level predator in the marine food web, the California sea otter may be particularly vulnerable to the accumulating effects of environmental pollutants. The otter’s intensive use of the nearshore marine habitat, its predation on benthic invertebrates, and narrow home range make it potentially an excellent indicator of detrimental human-induced aquatic pollution within its range along the central California coast (Riedman and Estes 1990). This highly protected, threatened otter population has not been thriving. After its near extirpation in the early part of the last century, the California sea otter slowly expanded in numbers and range from a nucleus of animals at Big Sur. However, the growth rate of the California population has been strikingly lower at 5% than expanding northern sea otter populations (17-20%) (Estes 1990; Estes et al. 1996). From 1994-99 the growth rate in California was reduced to 0%, and, although population growth has taken a slight up-turn recently, biologists’ continue to be concerned about the population’s poor rate of recovery. The major factor limiting the population’s growth appears to be an unusually high rate of mortality affecting pre-weaning juveniles as well as adults (Riedman et al. 1994; Estes et al. 1996, Estes et al. 2003).

In the course of providing basic diagnostic services for Federally-listed endangered and threatened mammals and birds, the U. S. Geological Survey’s National Wildlife Health Center intensively investigated the causes of mortality in California sea otters by performing postmortem examinations on every freshly-dead otter found along the coast from 1992-96 (Thomas and Cole 1996), and continued this monitoring at a reduced rate (every fourth freshly-dead otter) through 2002. We found that California sea otters have an unexpectedly high rate of mortality from infectious diseases, and die from an unusually wide variety of infectious diseases. Forty percent of the otters we examined died from infectious diseases, 19% from traumatic injuries, 9% were emaciated due to unknown causes, 13% had miscellaneous conditions such as neoplasia or gastrointestinal obstructions, and in 19% no cause of death could be determined. The infectious diseases of California sea otters included parasitic diseases (acanthocephalan peritonitis, protozoal encephalitis) (Cole et al. 2000, Lindsay et al. 2000, Miller et al. 2001, Mayer et al. 2003), a fungal disease (coccidioidomycosis), and a variety of different bacterial infections. Coccidioidomycosis was found only in otters dying at San Luis Obispo County (Thomas et al. 1996). Some of the infections, such as toxoplasmosis (due to the protozoan parasite, Toxoplasma gondii) are known to occur most commonly in individuals with inadequate immune function. We concluded that the otter population was undergoing unusually high exposure to many different disease agents, or may have an unusual susceptibility to diseases. Toxicity from some classes of environmental contaminants can cause immune suppression, leading to increased susceptibility to infectious disease. Here the sea otter may potentially be acting as an indicator of marine water quality and ecosystem health.

Exposure to high concentrations of environmental contaminants has been implicated in immunosuppression and other adverse health effects on wildlife. Organochlorines, including PCBs, PCDDs, PCDFs, DDTs, HCHs, chlordanes, organotin compounds such as butyl- and phenyltins, and heavy metals such as mercury, cadmium, and lead are currently recognized as the most important classes of environmental pollutants from the standpoint of health effects. Reproductive or immunologic dysfunctions have been associated with elevated concentrations of organochlorines in the diets or bodies of harbor seals in the North and Wadden Seas (Helle et al. 1976; Hall et al. 1992; Heide-Jorgensen et al. 1992; Reijnders 1986), beluga whales in the St. Lawrence River (Martineau et al. 1987), striped dolphins in the Mediterranean Sea (Kannan et al. 1993; Aguilar and Borrell 1994), California sea lions in California (DeLong et al. 1973), and bottlenose dolphins along the U.S. Atlantic coast (Kuehl et al. 1994).

Tissues from California sea otters were analyzed in the past to assess environmental contaminant exposure (Risebrough 1989, Estes et al. 1997, Bacon et al. 1999) and more recently we have begun to investigate contaminant exposure and possible links to health effects (Kannan et al. 1998, Nakata et al . 1998, Kannan et al. 2001, Kannan et al. 2004 in press). Preliminary investigations of beach-cast southern sea otters have found higher concentrations of organochlorine compounds, particularly PCBs, DDT, and tributyltins, in those animals that died of infectious disease than in those that were apparently healthy at the time of death (Kannan et al. 1998; Nakata et al. 1998). To date we have analyzed tissues from a small number of the dead adult otters to investigate the otters’ exposure to two classes of aquatic contaminants with immunosuppressive properties, butyltins and organochlorine compounds (n=35, and n=20, respectively). Butyltins were found in fairly high concentrations in California sea otter tissues in comparison with other marine species (Kannan et al. 1998). Tributyltin (TBT) was first used in 1961 and by the 1970s was widespread as an anti-fouling agent in marine paint formulations. Effects on non-target organisms such as oysters and gastropods were first recognized in 1974. In the 1980s, it was shown that declines in European oyster populations due to “imposex” (e.g., females with male sex organs) were causally related to TBT exposure. Although the use of TBT in paints has been banned for certain classes of boats since 1988 in the United States, these contaminants persist in sediments for several years and are still being released into the marine environment as paint is abladed from the hulls of large ships (see Kannan et al. 1998). The International Maritime Organization is seeking a global ban on TBT use in marine paints by 2003, with the expectation that all ship hulls would be decontaminated by 2008. In California sea otters butyltin exposure had some geographic variation. Otters from Monterey Bay contained some of the highest concentrations of butyltins, with the highest detected in an otter from Moss Landing. Butyltin concentrations were greater in otters that died from infectious diseases than in apparently healthy otters that died acutely from trauma such as shark attack

Monitoring Trends

  • Exposure of California sea otters to organochlorine compounds, particularly PCB’s, also appears to have geographic variation in distribution and PCB’s were present in high concentrations in some of the animals (Nakata et al. 1998; Estes et al. 1997). Again, otters from Monterey Bay contained the highest concentrations of these contaminants. In our most recent study, a subset of sea otter livers and their prey species from Monterey Harbor were additionally analyzed for individual polychlorinated biphenyl congeners to estimate 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents (TEQs), an assessment of their potential effects (Kannan et al. 2004 in press). The results of these preliminary studies suggested that the concentrations of butyltins, PCBs and their TEQs and DDTs are at or above the threshold for adverse effects in certain individuals. The threshold concentrations have been derived from various toxicological studies using confamilial species such as mink or marine mammals.
  • Our results indicate that California sea otters are being exposed to some classes of immunosuppressive marine contaminants. However, the number of samples analyzed in previous studies was not adequate to establish an unequivocal relationship between contaminant burdens and health of sea otters. With this limited number of samples, statistical correlations with the causes of mortality, geographic or temporal patterns of exposure are inconclusive.
  • These preliminary results (Kannan et al. 1998; Nakata et al. 1998; Kannan et al. 2004 in press) and interpretations point to a need to investigate the relationship between contaminant burdens and health status in a greater number of dead otters and to account for age, sex, and location-specific differences in contaminant concentrations (Reeves 2002). Although dead otters represent the failed individuals in the population (the “worst case scenario”), mortality has been pinpointed as the factor hindering this population’s growth. It is critical to maximize the amount of contaminant exposure information that can be obtained from this important segment of the population. The use of a large sample set can help identify maximum tissue concentrations achieved in nature and distinguish geographic risk factors. In addition, salvaged sea otter carcasses provide a large resource for retrospectively investigating contaminant exposure without handling or sampling living otters. Liver samples from >300 southern sea otters found freshly dead and necropsied from 1992 through 2002 are currently available for analyses. These otters equally represent both sexes, were found across the sea otters’ range, and are comprised of approximately 70% independent (post-weaning) animals (sufficient age for contaminant accumulation). Factors such as sex, age (biological variables) and environmental variables (location, diet availability) can influence contaminant accumulation in sea otters. In order to control the effects due to these confounding variables in relating chemical concentrations to sea otter health, we will target adult female otters in our analyses. Female sea otters maintain narrow home ranges to feed and raise pups in comparison with males who may roam through the ranges of multiple females or to the ends of the population range seasonally. Adult females therefore present the best opportunity to monitor localized environmental contaminants.
  • In addition, we will analyze tissue from all adult otters found dead at Moss Landing, its harbor, or in adjacent Elkhorn Slough, the largest coastal wetland in sea otter range. This area receives freshwater run-off from the Salinas River drainage and is the subject of concern for potential accumulation of industrial and agricultural pollutants. Sediment concentrations of organic contaminants (including DDTs, PCBs, PAHs) in these areas were documented previously (Rice et al. 1993) and high butyltin concentration was detected in an otter from this site in our previous study. Male sea otters predominately use this area (Feinholz 1998) and females are rare, therefore, of necessity, we will include male otters from Elkhorn Slough and Moss Landing in order to have an adequate pool of samples from this important location. In past studies pollutant concentrations did not differ substantially between adult male and female otters, so adult males may substitute for females in this site with cautious consideration of the wider-ranging potential in this cohort.


For a detailed discussion of the findings, please read the paper listed below.

Study Parameters

  • Mortality
  • Age structure
  • DDT
  • Other pollutants

Study Methods

Sea Otter Tissues

Sea otter liver tissue archived from necropsies at the National Wildlife Health Center will be used for chemical analyses. In previous studies, liver tissue was shown to be a suitable site for detecting organic pollutants in sea otters (Kannan et al. 1998, Nakata et al. 1998). The archived tissues were removed from the carcasses of wild sea otters that were found freshly dead along coastal California. All such carcasses found from 1992 through 1996 were examined and every fourth such carcass was examined in 1997 through 2002. Samples archived from 1992-2002 include at least 80 adult female otters of which approximately 30 sea otters were found dead in Monterey Bay, and 6 adult otters from the Moss Landing area.

Each sea otter carcass was rapidly chilled at the time of collection from the field, and shipped refrigerated or frozen to the National Wildlife Health Center by commercial overnight carrier. A postmortem examination (necropsy) was performed on each carcass to determine the cause of death. A variety of tissues from most animals were formalin-fixed and examined by light microscopy. Selection of other diagnostic laboratory tests was based on the history and gross lesions and included various microbiologic, virologic, and parasitologic procedures. At necropsy liver from each otter was collected, wrapped in aluminum foil and/or whirlpac bags, and stored frozen at -20C. This tissue bank represents a unique resource of biological material from individually identified animals with fully documented causes of death and corresponding geographical locations.


The following analyses will be performed:
1. Petroleum= PAHs
2. Butyltins = Mono-, di-, and tributyltin (MBT, DBT and TBT)
3. Organochlorines = DDT and its metabolites, lindane and other HCH isomers, chlordane compounds, hexachlorobenzene, and PCBs (isomer specific)
4. Brominated flame retardants: Polybrominated diphenylethers (congener specific)
5. Perfluorinated compounds: Perfluorooctanesulfonate (PFOS), perfluorooctanoic acid (PFOA).
6. Heavy metals =Hg,Cd,Pb,Zn,Cu,Fe,Mn,Co,Ni,Ag
Contamination by OC pesticides and PCBs in sea otters can be indicative of exposure to agricultural and industrial run-off. Metals such as Ag can be suitable indicators of contamination by municipal sewage discharge. Chemical contaminants will be analyzed following standard and validated analytical procedures. Dr. Kannan, co-investigator, is an environmental chemist and has worked for several federally sponsored projects on monitoring chemical contaminants in various environmental media and has published papers on this subject. He has experience in following strict quality assurance and quality control (QA/QC) protocols in the chemical analysis.

Organochlorines and brominated flame retardants

The method to be used is described in Nakata et al. 1998, and is briefly: Organochlorine analytical method consists of extraction of tissues using a Soxhlet apparatus, fat content determination, removal of fat from the extract by Florisil (dry column) chromatography, clean up of the extract with sulfuric acid, and fractionation of PCBs and pesticides by a wet Florisil column. The first fraction contains (hexane) HCB, PCBs, p,p’-DDE and trans-nonachlor. The second fraction (20% methylene chloride in hexane) contains HCH isomers (α-,β- and γ-HCH), chlordanes (trans-chlordane, cis-chlordane, cis-nonachlor and oxychlordane) and p,p’-DDD and p,p’-DDT.

Isomer-specific analysis of PCBs including highly toxic non-ortho coplanar congeners involve refluxing of tissues in 1N KOH-ethanol, followed by extraction in hexane, concentration and clean up by silica gel column chromatography.

These methods are sensitive with the detection limit of 100 pg/g, wet wt, for OC pesticides, and PCB congeners. Non-ortho coplanar PCBs can be detected at 1 pg/g, wet wt.

Polybrominated diphenyl ethers will be analyzed in an aliquot of the Florisil column extracts. The PCB fraction will be purified further by passing through silica-gel impregnated carbon column. The final extracts will be analyzed for PBDE congeners using a high resolution gas chromatograph interfaced with a high resolution mass spectrometer.


Polycyclic aromatic hydrocarbons will be analyzed by following the method described for organochlorines, except for the destructive steps such as sulfuric acid treatment and florisil dry column. This method is suitable for the determination of parent PAHs at 1 ng/g, wet wt. A gas chromatography/mass spectrometer (GC-MSD) and a high resolution gas chromatograph with electron capture detector (GC-ECD) will be used to quantify PAHs.


The method used is described in Kannan et al. 1998, and is briefly: Analysis of butyltins involves extraction of tissues with tropolone-acetone, derivatization by a Grignard reaction, florisil column clean up and quantification by GC-FPD (flame photometric detection). Quantitation of limits of up to 1 ng/g, wet wt, can be achieved by this method.

Perfluorinated compounds

Perfluorooctane sulfonate (PFOS and perfluorooctanoic acid will be measured in liver tissues by ion-pair extraction and final detection and quantification will be based on liquid chromatograph interfaced with a tandem mass spectrometer (LC-MS/MS) (Kannan et al. 2002).


Metals are analyzed by acid-digestion followed by atomic atomic absorption spectroscopy (AAS). When necessary, preconcentration with methyl isobutyl ketone (MIBK) and sodium diethyldithiocarbamate (DDTC) chelation technique will be performed. Mercury will be analyzed by cold vapor atomic fluorescence spectrophotometry (CVAFS). Detection limits of metals by these methods are in the range of 1-5, ng/g, wet wt.

Quality Assurance/Quality Control

Quality assurance and quality control protocols include the analysis of standard reference materials, matrix spikes, matrix spike duplicates, procedural blanks and other clean laboratory procedures as described in the Quality Assurance plan for Environmental Monitoring and Assessment Plan of Environmental Protection Agency (Heitmuller and Peacher 1995).

Data analysis/compilation

Multivariate and spatial statistical analyses will be conducted to look for correlations among the tissue concentrations of contaminants individually or in combination, the cause of death, time of collection (year), and geographic distribution of the otters.

Figures and Images

Figure 1. Southern sea otter (Enhydra lutris nereis) at the northern end of Moss Landing Harbor Beach on June 13, 2008. This otter was sluggish and it was unclear if it was healthy or not.

Figure 2. Box-and-whisker plots of perfluorinated chemical contaminant (PFOA and PFOS) concentrations in livers from central CA sea otters placed into one of three mortality categories: 1) disease (n=27), 2) emaciation (n=27), or 3) nondisease (n=26). The white line represents the median and the white circle is the mean; lower and upper limits of a box indicate 25th and 75th percentiles, respectively. The whiskers extend to the last observation within 1.5 times the interquartile range. (From Kannan et al. 2006)


  • Kannan, Perrotta & Thomas (2006)
    Association between perfluorinated compounds and pathological conditions in southern sea otters. Environmental Science and Technology 40:4943-4948.