Bacterial contamination related to harbor seals in Puget Sound, Washington

High concentrations of fecal coliforms have been reported at several sites in northern Hood Canal where hundreds of harbor seals congregate to haul out and rest. A preliminary study concluded that harbor seals had the potential to be major sources of fecal coliform concentrations in these area. Even though the management options are limited for controlling contamination from a natural source, such as seals, information on their role in bacterial contamination is important because:

  1. Effective mitigation of human and domestic animal sources of contamination cannot be conducted without information on whether these sources are indeed responsible for the problem in an area.
  2. Public support for measures to reduce human and domestic animal sources is compromised if there is a perception that a major alternate source of contamination is not being examined.
  3. Information is needed to determine the areas where future problems for shellfish production are likely to occur.
  4. If fecal contamination in an area is predominantly of seal origin then the degree to which this contamination represents a health hazard needs to be determined.

In this study we evaluated harbor seal contributions to bacterial contamination and the health risk, if any, they pose. The objectives of the research were as follows:

  1. Determine the number of harbor seals occurring at haul-out areas in northern Hood Canal and evaluate the trend in population size.
  2. Determine fecal coliform concentrations in harbor seal feces and identify factors that may alter contamination.
  3. Examine fecal coliform contamination in water and shellfish at a site with high concentrations of seals and negligible other sources of contamination.
  4. Examine the bacterial contamination contributed by seals in a closed captive environment.
  5. Evaluate the evidence that harbor seals carry diseases transmissible to humans.

Our censuses indicated harbor seal numbers have significantly increased since 1984 at study sites in northern Hood Canal, once the effect of other variables including season and tide were taken into account. Two aerial survey counts indicated a minimum of 1,400 seals in Hood Canal, with up to 403 seals in Quilcene Bay.

Fecal coliform densities in harbor seal feces varied significantly by site, with dramatically lower concentrations in feces from captive seals. Significant differences also were found among the three sites where feces of wild seals were collected. Sampling variables may have accounted for some of these observed differences. Captive seal studies provided useful information on the dissolution of feces in the water column in a closed environment and demonstrated some of the limitations in estimating the contribution of fecal coliforms from seals in the environment. The large differences in fecal coliform densities in captive seals compared to those in the wild, however, limited the comparability to the natural ecosystem. 

High levels of fecal coliforms were found in water and shellfish in Still Harbor, an embayment of McNeil Island that is the largest haul-out area for harbor seals in Puget Sound. Fecal coliform concentrations in both water and shellfish were highest at stations closest to the haul-out area. Bacteria also entered the bay from several small seasonal streams entering the harbor. The fecal coliform loading of these streams was far less than that calculated for seals, and the distribution of contamination was not consistent with these streams being the major source of fecal coliforms.



We conclude that the bacterial contamination at Dosewallips River Delta and at Still Harbor appears to be caused primarily by harbor seals. The role of seals in the bacterial contamination at Quilcene Bay is harder to determine because a number of other sources of contamination have been 
identified and there is no evidence that contamination is highest at the seal haul-out areas. Continued increase of harbor seal populations in Puget Sound will only increase the potential for conflicts involving harbor seals and shellfish operations. The human health threat posed by seal fecal contamination cannot be determined with existing data.

Three avenues of future research are required to identify further the degree to which seal-related contamination poses a problem:

  1. Develop techniques to identify whether bacteria in marine water and shellfish are from seals or from humans or domestic animals.
  2. Examine the distribution of bacterial contamination in water and shellfish at Quilcene Bay and other sites with large numbers of seals to evaluate the source of contamiantion, as was done at Dosewallips and Still Harbor.
  3. Test seal wastes for the presence of pathogenic organisms to evaluate the human-health risks of this contamination.


Calambokidis, J., G.H. Steiger, and B.D. McLaughlin. 1989. Bacterial contamination related to harbor seals in Puget Sound, WashingtonFinal report to Jefferson County and the Washington Department of Ecology, Olympia, Washington. 74pp.

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