Reduced survival of Black-throated Diver Gavia arctica chicks - an effect of changes in the abundance of fish, light conditions or exposure to mercury in the breeding lakes?

Abstract

The breeding success of Black-throated Diver Gavia arctica in Sweden has been monitored in the period 1994– 2014. The production of young was in balance with the annual mortality, without any temporal trend. However, the percentage of broods with 2–3 chicks decreased significantly, indicating reduced survival of the unfledged chicks. Three plausible causes were investigated: abundance of fish, impaired water visibility, and exposure to methylmercury. There is no indication of any changes in abundance of the main prey fish, such as Perch Perca fluviatilis. Impaired light conditions have been recorded in lakes in South Sweden, but not in the central and northern parts, so increased difficulties in localising prey fish might have been a contributing factor but cannot fully explain any reduction in chick survival. Exposure to methylmercury, however, deserves further study. For Perch, which is the main prey in most breeding lakes, the average mercury levels in muscle tissue exceeds levels for proposed screening benchmarks, with reference to risk of behavioural aberrations or impaired reproduction in Common Loon Gavia immer in North America.