Evaluation of Fecal Coliform Samples from Oakland Bay, Washington, Using a New Sanitation Model

Article excerpt

Introduction

Aquaculture in the western U.S. is a viable industry with large-scale production in both finfish and shellfish. The term "shellfish" in this context is limited to oysters, clams, and mussels. The shellfish industry is tightly regulated under the National Shellfish Sanitation Program (NSSP), which is administered federally by the Food and Drug Administration and by departments of health or agriculture at the state level. These health regulations, which are necessarily stringent to protect public health, instruct state shellfish authorities to shut down the harvest of the shellfish if water quality indices drop below food safety levels.

Many pathogens associated with fecal material are discharged into coastal waters. Because monitoring for all human pathogens is not feasible, an indicator group of bacteria is used to assess the likelihood that human pathogens are present. Fecal coliform is the indicator group of bacteria used by NSSP. The state agencies routinely monitor fecal coliform and water quality parameters at established stations in each shellfish harvesting area. Subsurface water samples are collected, chilled, and shipped to a certified laboratory. The analysis for fecal coliform takes 24 hours, and numbers of bacteria are expressed in the units of most probable number (MPN) per 100 mL (Clem, 1994).

NSSP establishes bacteriological standards for shellfish harvesting area classification. For areas to be classified approved or conditionally approved, the level of fecal coliform in subsurface water samples must meet NSSP standards. The standards for growing area classification have two components. The first component establishes a geometric mean value expressed as MPN/100 mL. The second component establishes an estimated 90th percentile also expressed as MPN/100 mL. The limit for the estimated 90th percentile is 43 and 49 MPN/100 mL for the fecal coliform samples tested using the five- and three-tube, decimal dilution MPN procedures, respectively. The limit for the geometric mean is 14 MPN/100 mL for both tests. To meet the NSSP water quality standards, the fecal coliform sample's geometric mean and estimated 90th percentile must be less than the NSSP limits (Interstate Shellfish Sanitation Conference [ISSC], 2009).

The NSSP guidelines allow the shellfish-growing areas to be open for harvest when the 90th percentile values are below the NSSP limit of 43. We hypothesized that the estimated 90th percentile criterion, which is currently used by NSSP, is not sufficient to protect the public from risks caused by consumption of biologically contaminated shellfish. To reduce this risk, NSSP procedure should be augmented by applying the upper limit of the estimated 90th percentile of fecal coliform samples at the .05 significance level.

Materials and Methods

The study site for our project was Oakland Bay, Washington. This bay, which is located in South Puget Sound, is one of the most important commercial shellfish areas in the country, producing over three million pounds of manila clams a year. Oakland Bay and the adjacent Hammersley Inlet are typical of the narrow, shallow embayments that characterize South Puget Sound. While highly productive areas for shellfish, the bay's low flushing rates increases the area's sensitivity to human impacts. The city of Shelton, Washington, operates a domestic wastewater treatment plant that discharges treated sewage near the junction of Hammersley Inlet and Oakland Bay (Department of Ecology, State of Washington, 2011).

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The data set in our study consisted of 2,134 fecal coliform samples collected by the Washington Department of Public Health from 15 shellfish stations in Oakland Bay, Washington, over 10 years from January 13, 2000, to December 9, 2009. The 15 sampling stations are grouped into four growing areas: (1) growing area cen tral, which consists of stations 116, 117, 119, 120, 123, and 128; (2) growing area Chapman Cove, which consists of stations 615 and 639; (3) growing area north, which consists of stations 118, 121, 125, 126, 127, and 129; and (4) growing area north upper, which consists of station 614. …