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Marine Ecology Progress Series

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MEPS 279:63-72 (2004)  -  doi:10.3354/meps279063

Assessing impacts of dredge spoil disposal using equivalence tests: implications of a precautionary (proof of safety) approach

Russell Cole1,*, Graham McBride2

1National Institute of Water and Atmospheric Research, PO Box 893, Nelson, New Zealand
2National Institute of Water and Atmospheric Research, PO Box 11115, Hamilton, New Zealand

ABSTRACT: Equivalence tests evaluate whether a treatment effect lies within or outside a pre-determined equivalence interval. The equivalence interval might be determined in relation to a pre-cleanup value, reference values, or unimpacted controls. Such tests are little known in ecology, but offer advantages over the common tests of point-null hypotheses. They come in 2 forms. The first, testing the equivalence hypothesis, constitutes a proof of hazard approach by postulating that a difference lies within the interval. The second, testing the inequivalence hypothesis, also known as bioequivalence testing, constitutes a proof of safety approach by postulating that a difference lies beyond the interval. The proof of safety option provides a formal mechanism for the implementation of the precautionary approach. We demonstrate the usage of these tests for a subtidal rocky reef dataset evaluating the impact of dredge spoil disposal at New Plymouth, New Zealand. Sampling of conspicuous subtidal organisms was done once before, and twice after spoil disposal. Abundances of subtidal organisms were compared between 6 sites in the predicted path of the dredge spoil and 6 distant control sites. Comparison of mean numbers of species and of individuals on and off the dumpground before and after disposal demonstrated that greater effort was required for proof of safety, though there was no proof of hazard. The most important conclusion was that much greater sampling effort than is common in ecological study was required to demonstrate safety. Because equivalence tests are readily calculated, test a realistic hypothesis, and provide an outcome that is directly interpretable in terms of a biological endpoint, we suggest that they should be more widely adopted.

KEY WORDS: Dredge spoil disposal · Equivalence tests · Proof of hazard · Proof of safety · Tests of null hypotheses

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