The impact of water level fluctuation on the breeding success of the Black-throated Diver Gavia arctica in South-west Sweden

Abstract

We evaluated the impact of water level fluctuation on the breeding success of the Black-throated Diver in South-west Sweden by using results from a one-year study of 43 lakes (49 pairs) in 1996 and surveys of a population of 16–22 pairs at the lake system Fegen-Svansjöarna in 1997–2000. At this lake system, the water level is regulated (for hydropower production) with a maximum amplitude of 1.75 m. In 1997–2000, an attempt was made to maintain a stable water level during the period 1 May–15 June. Flooding was the most important cause of breeding failure at the 43 lakes, whereas no failure could be related to this factor at Fegen-Svansjöarna. In the 43 lakes, the change in median water level during incubation was +6 cm for five flooded nests, compared to –5 cm for 14 hatched clutches and –6 cm for 13 clutches that failed because of other or unknown causes. The mean breeding success at Fegen-Svansjöarna was on the same level as for South-west Sweden in general during 1997–2000 (0.44 and 0.38 chicks per pair and year, respectively), but was higher than for the four years before the attempt to keep the water level stable (0.22 chicks). We conclude that flooding is a main cause of nesting failure, that rainfall is the main factor behind the water level fluctuations, and that the regulation of the water level for hydropower production may have an additional negative impact on some lakes. Control of the water level during the incubation period may help to reduce the risk of failure caused by flooding, and our results support previous recommendations to allow for a rise of only a few centimetres or a lowering of a maximum of 20–30 cm during the incubation period.