In order to explain the large variation in levels of halogenated organic contaminants in great skuas, both between and within colonies in the Northeast Atlantic, researchers tested blood samples from more than 200 adult skuas. The results indicate that the level of toxins accumulated in the body is determined by individual behaviour of the birds.

Toxic at the top of the food chain

It is well known that the concentration of environmental toxins in wild animals increases with trophic level. The higher up in the food chain you are, the higher the level of toxins, since these chemicals accumulate at every step in the chain. The great skua Stercorarius skua is a territorial predator feeding mainly on fish, but it can also eat bird eggs and chicks, and even adult birds in the breeding period. Therefore, it is not unexpected that relatively high levels of contaminants are found in great skuas. It is surprising, however, that the concentration of such substances varies greatly, both between and within colonies in the Northeast Atlantic. In a recently published scientific study, researchers measured the levels of environmental toxins in blood samples from great skuas breeding across the Northeast Atlantic in order to explain the variation in the concentration of halogenated organic contaminants (HOCs).

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Great skuas feed mainly on fish, but they also eat eggs, chicks and adult individuals of other bird species, especially in the breeding period. Some may even specialize in catching other seabirds. This great skua is about to kill an adult northern fulmar.
Photo: Erlend Lorentzen

Highest concentrations on Bjørnøya

In 2008 and 2009, blood samples were collected from 204 breeding great skuas on Svalbard (Kongsfjorden and Bjørnøya), Iceland, the Shetland Islands, and islands along the Norwegian coast (Hjelmsøya and Runde). The concentrations of a wide range of contaminants were measured in each sample. The researchers expected to find decreasing concentrations of toxins with increasing latitude, if long-range transport via atmosphere and ocean currents was the main determinant of HOC levels in great skuas. On the contrary, it turned out that the concentration of HOCs was highest in birds breeding on Bjørnøya, the second northernmost colony. This result refutes the hypothesis that location relative to emission sources is the most important factor for the uptake of toxins in birds. Instead, the researchers found that great skuas feeding on other birds had significantly higher levels of contaminants in the blood than those feeding mainly on fish. These findings clearly indicate that occurrence of contaminants in great skuas is determined to a large extent by individual behaviour of the birds, such as migration strategies and choice of prey.

Contact person: Jan Ove Bustnes, NINA