|Beached bird surveys indicate decline in chronic oil pollution in the North Sea|In: Marine Pollution Bulletin. Macmillan: London. ISSN 0025-326X, more
trends; power analysis; oil rates; seabirds; strandings; operational oil discharges; spatial variation; temporal variation
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Strandings of oiled seabirds have been a signal of the ongoing problem of chronic oil pollution in the North Sea since the beginning of the twentieth century. Overall numbers of beached birds are subject to enormous fluctuations, being the result, for example, of changes in the amount of oil spilled in the marine environment, currents, the frequency of onshore winds and variations in the numbers of seabirds in a given region. In contrast, oil rates, being the fraction of oiled birds of the total stranded, appeared to be relatively constant while specific for different species and regions. These species-specific oil rates are thought to reflect the risk for birds or corpses of birds to become oil fouled at sea and hence to mirror fluctuations in the amount of oil spilled in different regions. High oil rates are typical for mainly swimming, highly exposed seabirds in areas with frequent oil spills (e.g. around shipping lanes and near the major harbours); low oil rates are typical for mainly flying seabirds away from the busiest shipping lanes. A power analysis of the results of beached bird surveys demonstrated the sensitivity of these data as a tool to monitor trends in oil rates of stranded birds. Rather subtle changes in oil rates could be demonstrated, indicating positive results of attempts to protect certain sea areas (e.g. the Wadden Sea) and a decline in oil rates over time.