Bioavailability, accumulation and effects of heavy metals in sediments with special reference to United Kingdom estuaries: a review

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.

Seabirds as monitors of the marine environment

Several authors of books on the monitoring of pollution have advocated the use of animals as monitors in terrestrial and aquatic environments (e.g. Phillips, 1980; Schubert, 1985). Such studies tend to emphasize the use of sedentary invertebrate animals as biomonitors. By comparison, birds suffer from several apparent drawbacks. They are mobile, so pollutants will be picked up from a wide, often ill-defined, area; they are long-lived, so pollutant burdens may be integrated in some complex way over time; and they have more complex physiology, and so may regulate pollutant levels better then invertebrates. Furthermore, birds tend to be more difficult to sample, and killing birds may be unacceptable for conservation or ethical reasons. However, some of these characteristics may at times be positively advantegeous. Integrating pollutant levels over greater areas or timescales or over food webs, may be useful, provided that species are chosen carefully. Less sampling may be necessary if birds can reflect pollutant levels in the whole ecosystem or over a broad area. In addition, since they are high in food chains, birds may reflect pollutant hazards to humans better than do most invertebrates. It is also significant that birds are extremely popular animals with the general public, so pollutant hazards to them are likely to receive greater attention than threats to invertebrates.

Birds as monitors of pollutants

To learn about the behavior and ecology of nesting birds, we often study them when they occupy nesting territories. But by visiting territories or nests, the investigator may to a greater or lesser extent disturb the birds and affect the parameters being studied. Thus, we want to learn about the behavior or performance of birds under natural conditions, but may alter these conditions by our presence in the field. This has been called the “uncertainty principle” in field biology (Lenington, 1979).

The Effects of Investigator Disturbance on Nesting Birds

Life history theory predicts a trade-off between current reproduction and survival to maximize fitness. In long-lived seabirds, one might expect adults not to jeop- ardize survival by excessive reproductive investment when environmental conditions for breeding are adverse. Some environmental factors have reduced the fecundity of Black- legged Kittiwakes (Rissa tridactyla) breeding in Shetland, UK, a metapopulation that has declined substantially during the last two decades. Two hypotheses are proposed to explain this decrease: (1) reduced availability of sandeels (the main prey for kittiwakes), which has occurred since the late 1980s; and (2) increased predation by Great Skuas (Catharacta skua), the main predator of kittiwakes. Detailed data on food supply (sandeel stock size, by age groups), kittiwake body condition and breeding success, and diet and productivity of Great Skuas allowed us to assess the influence of each of these factors on adult kittiwake survival from 1986 through 1997. Capture-recapture models were constructed to examine correlations between local adult survival rate and environmental parameters. Body condition at the end of the breeding season and availability of 0-group (juveniles ? 1 yr old) sandeels positively influenced adult survival, whereas survival was lower when Great Skua pro- ductivity was high. Adult survival varied greatly during the study, from 0.53 + 0.04 to 0.98 ? 0.01 (mean + 1 SD), and was independent of sex. Both hypotheses were supported. Contrary to predictions of life history theory, kittiwakes did not refrain from breeding in years of poor food supply or high predator activity to buffer their survival at the cost of not rearing chicks. Although 0-group sandeel availability affected breeding success of kittiwakes, its influence on adult survival was also significant. The discovery that food supply affects adult survival rate has important implications for fisheries management to minimize impacts on seabirds.

/pdf/influences-of-food-availability-and-predation-on-survival-of-4o17qwrqb5.pdf

Influences of food availability and predation on survival of kittiwakes

A minority of Kittiwake pairs lay clutches of three, but rarely raise all three chicks. There seems to be a limit to the rate of provisioning that the adults can maintain.

The diet and feeding ecology of breeding Kittiwakes Rissa tridactyla

Marking and visiting Lapwing Vanellus vanellus nests does not affect clutch survival

The Clyde Estuary is recovering from gross pollution. In the early 1970s high densities of a small number of species of intertidal invertebrates tolerant of organic enrichment and low oxygen levels provided rich feeding for large numbers of dunlin, redshank, lapwing, oystercatcher and curlew. Winter numbers of the first three species have now declined considerably. The declines cannot be attributed to national changes in wader numbers and are not due to changes in breeding habitats or lack of reproductive success in particular regions, since each species wintering on the Clyde Estuary breeds in a different area. The most likely explanation of the decline, which is most pronounced in the species highly dependent on Corophium volutator, is either that reduced organic pollution has reduced the densities of the main prey, or that higher oxygen levels over the mudflats have allowed fish, particularly flounders, to enter the estuary and compete with the waders for the available prey. Although there is evidence of some birds having moved to other wintering areas, the fate of most birds is unknown.

Recent changes in numbers of waders on the Clyde Estuary, and their significance for conservation

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The diet and feeding ecology of breeding Kittiwakes Rissa tridactyla

Marking and visiting Lapwing Vanellus vanellus nests does not affect clutch survival

Recent changes in numbers of waders on the Clyde Estuary, and their significance for conservation