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?

Margaretha Eriksson

doi:10.34080/os.v25.22541

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.

Highlights

The breeding success of Black-throated Diver Gavia arctica in Sweden has been monitored in the period 1994– 2014
Recent population estimates indicate a population size of around 22 000 pairs of Black-throated Diver Gavia arctica in Europe, of which about 98% is found in the Nordic countries
The prey is mostly caught in the breeding lakes, in contrast to the Red-throated Diver Gavia stellata, which often breeds in small lakes and ponds without fish, but catches fish for the chicks from larger lakes with clear water or at sea up to a travelling distance of ca 10 km, sometimes further

Summary

Introduction

The breeding success of Black-throated Diver Gavia arctica in Sweden has been monitored in the period 1994– 2014. For the Blackthroated Diver, the production of young has been in balance with the annual mortality, without any long-term temporal trend (Eriksson 2014) This is consistent with results from the Swedish Bird Survey, which indicate a slow but long-term increase in the size of the breeding population since the mid-1970s, the trend might have levelled off in recent years (Green & Lindström 2015). Despite the fact that levels and trends do not raise any immediate concern, there is a worrying indication that the percentage of broods with 2–3 chicks has decreased since the mid 1990s (Figure 1, Eriksson 2014) This might indicate a reduction in the survival of non-fledged chicks.

Methods

Results

Conclusion