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Between 1961 and 1999 the number of adult sooty shearwaters found dead on beaches in northern New Zealand declined by about 64.4% and the number of fledglings by about 77.7%. Only 2 factors that we know about have been acting on the sooty shearwater population during the period studied and could have caused such a dramatic decline; a rise in sea temperature perhaps as a result of movement of the Sub-Antarctic Front and increase in harvest. Two other more recent phenomena, north Pacific fisheries mortality and climatic variation (El Niño Southern Oscillation and Pacific Decadal Oscillation), may be involved, but we cannot find any direct evidence of their impact in our data. The impact of this decline has been recently found on the breeding islands. More study is required to fully understand how weather, patrol frequency, deposition rate, persistence rate, and live bird numbers vary and interact. Deposition and persistence experiments similar to those reported from overseas need to be done in New Zealand.