Chick mortality in lesser black-backed gulls

Contaminant-induced mortality in gull chicks

Over the past three to four decades, the Fennoscandian subspecies of the lesser black-backed gull, the nominate Larus fuscus fuscus, has experienced drastic population declines over most of its range. In the central Gulf of Finland, the fledging rate averaged only 0.13 during 1977-2002; only one in seven pairs managed to fledge one chick. This figure represents one-third of the calculated hypothetical minimum output required for a lesser black-backed gull population to remain self-sustained. As a consequence, the Finnish coastal population has been declining by an average of 8% per year during 1986-2002. Currently, nominate fuscus is listed in the Red Data Books of Finland, the Åland Islands, Sweden, Norway, East Fennoscandia, and Estonia, i.e. it is endangered over its entire range.

In our studies on chick mortality in three different gull species at Söderskär Game Research Station, Gulf of Finland, we demonstrated a high frequency of chick diseases due to degeneration and inflammation of various internal organs. These lead to bacteria entering the blood circulation. Sepsis is the final cause of death. Possibly, the degenerated liver is not capable of detoxifying the bacteria. Seventy percent of the chicks of the nominate lesser black-backed gull die showing these symptoms.

Organochlorines have been detected in elevated concentrations in lesser-blacked back gull chicks in our study areas, and previous studies suggest an association between organochlorine levels and chick mortality. However, the causal relationship between organochlorine residues and chick diseases is poorly understood. Lesser black-backed chicks from the Gulf of Finland and from the Bothnian Bay had very high concentrations of PCB in their liver; but the concentrations were not significantly higher than those of the healthy herring gull chicks, indicating a common exposure area for both species (i.e. the Baltic Sea). When compared to lethal dose values for toxic equivalents (TEQ) in bird eggs our TEQ levels clearly exceed most or all of the values associated with detrimental effects. Compared with published data on fish-eating waterbirds, the DDE concentrations in the diseased lesser black-backed chicks were well above the levels previously correlated with decreased reproduction, while the residues in apparently healthy herring gulls were below those levels. The DDE/PCB ratio in lesser black-backs was significantly elevated, indicating an increased exposure to DDTs as compared with most other Baltic and circumpolar seabirds.

Young and adult nominate fuscus leave Finland in August-September. Photo Martti Hario.

The nominate lesser black-backed gulls spend the winter in equatorial Africa, at western Rift Valley lakes, mainly in Uganda and to a lesser extent in Ethiopia, Kenya, and Tanzania in the south. They travel long distances with apparent ease and use the Black Sea and the eastern Mediterranean as stop-over sites. We do not know to what extent lesser black-backed adults are exposed to toxicants in Africa; e.g. almost nothing is known of their feeding ecology there. A joint project between Söderskär Game Reasearch Station, Finnish Environment Institute and BirdLife partners in Uganda and Finland aim to shed light on their secretive winter ecology.

After completing the 6000-7000-kilometre-long migration journey they reach the Rift Valley Lakes of Uganda, where the largest concentrations of more are 1000 individuals overwinter among hippos, storks and flamingos in Queen Elizabeth National Park. Photo Martti Hario.

Martti Hario
Game and Fisheries Research Institute
P.O. Box 2
FIN-00791 Helsinki
Finland
martti.hario@rktl.fi

Juha-Pekka Hirvi
Finnish Environment Institute
Hakuninmaantie 4-6
FIN-00430 Helsinki
Finland
juha-pekka.hirvi@ymparisto.fi

Achilles Byaruhanga
BirdLife Uganda
nature@natureugand.org

Publications:

Hario, M. & Rudbäck, E. 1996: High frequency of chick diseases in nominate Lesser Black-backed Gulls Larus f. fuscus from the Gulf of Finland. - Ornis Fennica 73: 69-77.

Hario, M. & Rudbäck, E. 1999: Dying in the midst of plenty - the third-chick fate in nominate Lesser Black-backed Gulls Larus f. fuscus. - Ornis Fennica 76: 71-77.

Hario, M. & Uuksulainen, J. 1993: Mercury load according to moulting area in primaries of the nominate race of the Lesser Black-backed Gull Larus f. fuscus. – Ornis Fennica 70: 32-39.

Hario, M., Himberg, K., Hollmén, T. & Rudbäck, E. 2000: Polychlorinated biphenyls in diseased lesser black-backed gull (Larus fuscus fuscus) chicks from the Gulf of Finland. - Environ. Poll. 107: 53-60.

Hario, M., Hirvi, J.-P., Hollmén, T. & Rudbäck, E. 2003: Organochlorine concentrations in diseased vs. healthy gull chicks from the northern Baltic. - Environ. Poll. (hyväksytty).

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