Marine oil pollution has been an issue of concern for at least a century. The earliest reports contained outrage over oil-contaminated dead seabirds found ashore. This paper reports on observed trends in strandings and oil rates of Common Guillemots Uria aalge to illustrate the history of oil pollution and its effects in the North Sea. This paper is also a tribute to Peter Hope Jones, who brought systematic beached bird surveys and oil-spill impact assessments to a higher level, by implementing detailed research on affected wildlife. In recent decades, unexpectedly, the oil problem has disappeared almost completely from the North Sea. Given the global nature of current environmental issues, including the climate and biodiversity crises, it may be instructive to examine the background to this stunning success. A short history of major oil events is presented, including the measures taken to reduce the oil problem. It required a succession of major oil incidents to push the community into action. The process of international conventions was notoriously slow, and the implementation of concrete measures even slower, as economic arguments prevailed to prevent or delay immediate action.

Marine oil pollution has been an issue of concern for at least a century. One of the most dramatic and visible effects of oil pollution is the contamination and associated mass mortality of seabirds at sea (Bourne 1976; Croxall 1977; Reineking & Vauk 1982; Clark 1984; Dunnet 1987; Camphuysen 1989a; Danielsen et al. 1990). For decades, it seemed a tough nut to crack. Yet, miraculously, and in the course of only a decade or two, the problem has now disappeared almost completely from the North Sea, and in fact from most other areas worldwide. Give or take a few oil incidents, chronic oil pollution is now, quite suddenly, something from the (recent) past. Now that many people get depressed and feel frustrated by the global nature of current environmental issues, such as the climate and biodiversity crises, the effects of agriculture or fisheries, plastic pollution, or ‘just’ the human population increase, it may be worthwhile to document this stunning success. How did it come about, how did it develop and escalate, and, most of all, how could it end?

Oil pollution (petroleum hydrocarbons, PHCs) enters the marine environment from a variety of sources, including transport (e.g. deliberate discharges or accidental leakages of bilge and fuel oil), tanker operations or shipping accidents, atmospheric emissions, releases or leakages from coastal refineries, offshore production instal- lations or marine terminals, from urban or river run-off (e.g. municipal or industrial waste), together with natural inputs (Wardley-Smith 1976; NRC 1985; Wolfe 1985; Clark 2001). Here I will focus on the situation in the North Sea, where the prime sources for most of the easily visible chronic marine oil pollution were (and in a way still are) transport, releases or leakages from offshore installations, and shipping incidents (e.g. Dunnet 1987; Dahlmann et al. 1994; Camphuysen 2007a).

The first written reports of marine oil pollution expressed outrage over oil- contaminated dead seabirds found on beaches (Anon. 1910; Dawson 1911; Verwey 1915). Even then it was clear that divers (loons), grebes, sea ducks and auks were the groups most severely affected by oil. This northern-hemisphere perspective was subsequently extended to include penguins and cormorants, and it was confirmed that species that spent the majority of their time swimming had a higher sensitivity to oil pollution than more aerial groups such as tubenoses, skuas, gulls and terns (King & Sanger 1979; Gandini et al. 1994; Camphuysen 2007a). Since the earliest accounts, the publication record on oil-related seabird mortality and oil incidents is massive.

Common Guillemots Uria aalge (hereafter ‘Guillemots’) have been over- represented in published reports of oiled birds in the North Sea. Abundant and highly vulnerable seabirds as they are, they commonly washed ashore and are still often found on beaches today. It was not until the early twenty-first century that the EU Marine Strategy Framework Directive insisted on the implementation of an independent indicator to evaluate the effectiveness of measures to reduce chronic oil pollution (OSPAR 2010). The so-called ‘oiled-Guillemot-EcoQO’ was adopted, an Ecological Quality Objective (EcoQO) to assess fluctuations, patterns and trends in chronic marine oil pollution, and beached bird surveys officially became that independent monitoring instrument. The oil rate in Guillemots was selected because the species was abundant enough to be useful as a ‘Canary in the coalmine’ in all North Sea countries (OSPAR Commission 2009).

The oiled-Guillemot-EcoQO came late, and was just a measuring instrument. At least 100 years had already been spent on attempts to reduce the oil problem, but initially with little effect. To refresh our collective memory, it seems timely to present a short history and a timeline of both the worrying signals, as published since the early twentieth century, and the measures taken to reduce the oil problem.

This paper reports on observed trends in strandings and oil rates (% oiled) of Guillemots to document the history of oil pollution and its effects in the North Sea. It uses data collected from birds that washed ashore along the Dutch coast over the past 120 years (1900–2020), or throughout the entire twentieth and early twenty-first centuries. But not only that. It was the late Peter Hope Jones who, in the late 1970s, brought both systematic beached bird surveys and incidental oil- spill impact assessments to a higher level, by implementing more detailed research on affected wildlife. He saw the ecological importance as well as the relevance for seabird conservation of collecting as much information as possible from animals that were otherwise largely out of reach, far out at sea. An important step was to collect and study carcasses such that, for example, age composition and the (likely) colonies of origin could be assessed. As a tribute to his legacy, I will therefore present and discuss results obtained by using his initial protocols (Hope Jones et al. 1978, 1982, 1984, 1985, 1988; Hope Jones & Morgan 1979), partly updated in later years (Kuschert et al. 1981; Camphuysen 1983, 1995a, 2007b; Maas 1983; Sandee 1983; Franeker 1983; Harris 2014). As a result of these ‘extra’ observations, regular beached bird surveys became more than just a demonstration of an existing environmental problem. We were able to learn things from birds that were otherwise notoriously hard to study. The results are valuable pieces of the jigsaw puzzle, especially when combined with results from studies on breeding grounds, at-sea surveys, seawatching, ringing studies, dietary studies using proxies such as eDNA, stable isotopes or fatty acids, and novel tracking studies.

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