Cormorant chicks and eggs. Photo: Maria Ovegård, SLU.

As part of the national environmental monitoring program selected species are collected annually from lakes and the sea to be analyzed for environmental contaminants. Because many contaminants bio-accumulate, their concentrations increase higher up the food chain so that higher predators often have concentrations that are easier to detect and are more comparable to human exposure levels.

However, species higher up the food chain are also often rare, protected and/or have a patchy distribution. Bird eggs are considered a good matrix for contaminant monitoring for several reasons, not least because lethal sampling of adults or chicks is not required and potential effects on populations can be less severe (where birds lay several eggs per clutch). Guillemot eggs have been collected from Gotland since the late 1960s, giving unique insights into how contaminant concentrations change over time. Currently eggs are also collected from Eurasian oystercatchers and common tern from Sweden´s west coast as part of the national contaminant monitoring program. In comparison, the cormorant is a common species with a widespread distribution. Douglas Jones, environmental analysis specialist at the Swedish Species Information Center (SLU Artdatabanken), who initiated the study whilst working with contaminant monitoring at NRM, explains:

– Great cormorants are common over large parts of Sweden. They eat fish and are quite high up the food chain, making their eggs a good potential candidate for contaminant monitoring.

Cormorant nest. Photo: Douglas Jones, SLU Artdatabanken.

But great cormorants commonly spend their winters in Europe and around the Mediterranean coast so that contaminants measured in their eggs might not reflect the local environment where they breed. From a contaminant monitoring perspective it’s important to know if they use stored body reserves when forming eggs (capital breeding strategy) or if they use the fish they eat around around the breeding colony (income breeding strategy), according to Douglas Jones.

The study shows that great cormorant eggs appear to be a suitable candidate for monitoring of contaminants from a range of aquatic environments (marine, fresh, and brackish water) across the country. It is also a good example of collaborative research combining expert knowledge on species, contaminants and ecology with analytical competence across a range of institutes.

Cormorant egg. Photo: Douglas Jones, SLU Artdatabanken.

Stable isotopes of certain elements can help to decipher where animals have foraged. In this study great cormorant eggs were collected from several places; the northern Bothnian Sea, Stockholm and Blekinge archipelagos, the Bohuslän coast and lake Roxen in Östergötland. The eggs were analysed for stable isotopes of carbon, nitrogen and sulfur as well as for mercury. The isotope data revealed that great cormorants appear to produce eggs using local resources (fish caught around their breeding grounds) and not from stored reserves. They also found that mercury concentrations are related to diet.

– Sulfur isotopes are excellent for detecting if cormorants have eaten from lakes or the sea and nitrogen isotopes, especially when carried out on amino acids, give information on an individual’s trophic position, says Agnes Karlsson, researcher at the Department for Ecology, Environment and Plant Sciences (DEEP), and Baltic Sea Fellow.

Mercury bio-accumulates up the food chain and, as expected, the highest concentrations of mercury were found in eggs where cormorants are known to eat predatory fish such as pike and perch.

Douglas Jones in a cormorant tree. Photo: Henrik Dahlgren, NRM.

The article “A multi-isotope approach to evaluate the potential of great cormorant eggs for contaminant monitoring” is published in the journal Ecological Indicators, with the authors Douglas Jones, Swedish Species Information Center at SLU, Maria Ovegård, the Department of Aquatic Resources (SLU Aqua), Henrik Dahlgren, Sara Danielsson, the Swedish Museum of Natural History (NRM), Maria Greger, Tommy Landberg, the Department of Ecology, Environment and Plant Sciences (DEEP) at Stockholm University, Andrius Garbaras, Center for Physical Sciences and Technology i Litauen, Agnes ML Karlson, the Department of Ecology, Environment and Plant Sciences (DEEP) at Stockholm University and the Baltic Sea Fellows at the Stockholm University Baltic Sea Centre.

The article is published in open access:

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