Seabirds as monitors of the marine environment

Abstract

Many studies have shown that seabirds are sensitive to changes in food supply, and therefore have potential as monitors of fish stocks. For most seabird species breeding parameters suitable for biomonitoring have yet to be measured over a wide range of prey densities. However, it is clear that responses vary among species and care must be taken when interpreting seabird data as a proxy for fish abundance. For many years seabirds have also been used as monitors of pollution, especially oil pollution. Beached bird surveys provide important evidence of geographical and temporal patterns, and, for example, show consistent declines in oil release into the southern North Sea over the last 15 years. Analysis of oil on birds can now permit fingerprinting of sources, allowing prosecution of polluters. As predators high in marine food webs, seabirds also have potential as monitors of pollutants that accumulate at trophic levels. Recent work on mercury in seabirds has permitted an analysis of spatial patterns and of the rates of increase in mercury contamination of ecosystems over the last 150 years, since mercury concentrations in feathers of museum specimens can be used to assess contamination in the birds when they were alive. Surprisingly, pelagic seabirds show higher increases than most coastal ones, and increases have been greatest in seabirds feeding on mesopelagic prey. This seems to relate to patterns of methylation of mercury in low-oxygen, deeper water. Accurate measurement of long-term trends in mercury contamination depend on the assumption that seabird diet composition has not changed. This can be assessed by analysis of stable isotopes of N and C from the same feathers used for mercury measurement, a technique that also permits the monitoring of trophic status over time or between regions. While high mercury contamination of seabirds in the southern North Sea is unsurprising, we cannot yet explain certain unexpected results, such as high levels in seabirds from north Iceland compared with those from south Iceland or Scotland.