Counting at low concentrations: the statistical challenges of verifying ballast water discharge standards

Authors

  • Melanie Frazier,

    Corresponding author
    1. Western Ecology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, 2111 SE Marine Science Drive, Newport, Oregon 97365 USA
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  • A. Whitman Miller,

    1. Smithsonian Environmental Research Center, P.O. Box 28, Edgewater, Maryland 21037 USA
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  • Henry Lee II,

    1. Western Ecology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, 2111 SE Marine Science Drive, Newport, Oregon 97365 USA
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  • Deborah A. Reusser

    1. Western Fisheries Research Center, USGS, 2111 SE Marine Science Drive, Newport, Oregon 97365 USA
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  • Corresponding Editor (ad hoc): H. J. MacIsaac.

Abstract

Discharge from the ballast tanks of ships is one of the primary vectors of nonindigenous species in marine environments. To mitigate this environmental and economic threat, international, national, and state entities are establishing regulations to limit the concentration of living organisms that may be discharged from the ballast tanks of ships. The proposed discharge standards have ranged from zero detectable organisms to <10 organisms/m3. If standard sampling methods are used, verifying whether ballast discharge complies with these stringent standards will be challenging due to the inherent stochasticity of sampling. Furthermore, at low concentrations, very large volumes of water must be sampled to find enough organisms to accurately estimate concentration. Despite these challenges, adequate sampling protocols comprise a critical aspect of establishing standards because they help define the actual risk level associated with a standard. A standard that appears very stringent may be effectively lax if it is paired with an inadequate sampling protocol. We describe some of the statistical issues associated with sampling at low concentrations to help regulators understand the uncertainties of sampling as well as to inform the development of sampling protocols that ensure discharge standards are adequately implemented.

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