Population trends and threats from ship traffic to long-tailed ducks in the Baltic Sea

The long-tailed duck Clangula hyemalis is a small sea duck that breeds in Arctic tundra regions and winter in marine and brackish waters. Surveys indicate substantial declines in numbers in recent decades and the species is now classified by IUCN as globally threatened in the category “vulnerable”. The largest of the four recognized long-tailed duck populations is the West Siberian / North European population. Birds belonging to the WS/NE population breed in northern Russia and northern Scandinavia and overwinter mainly in the Baltic Sea. An International Single Species Action Plan for the long-tailed duck has also recently been developed by specialists under the auspices of AEWA (Agreement on the Conservation of African-Eurasian Migratory Waterbirds) (HEARN et al. 2015) Two Baltic wide surveys have shown that the WS/NE population has decreased very rapidly from approx. 4.3 million birds in 1992-93 to approx. 1.5 million birds in 2007-2009 (DURINCK et al. 1994; SKOV et al. 2011). Although there is some uncertainty regarding the overall level of the population size estimates it is believed that the difference between the estimates accurately reflects the trend between the mid 1990s and late 2000s. A further decline also after 2009 can be assumed as the mean proportion of juveniles in the wintering population has been low since 2009. The recent decline of the WS/NE population can most likely be explained by a combination of factors affecting both the productivity at the Arctic breeding grounds and the adult mortality in the wintering areas in the Baltic Sea. Four important anthropogenic threats affecting the wintering birds have been recognised, namely, 1. operational oil spills from ships at core wintering sites, 2. by-catches in fishery, 3. hunting and 4. disturbance at and exploitation of offshore mussel banks. Mortality due to by-catches has decreased but is still high (BELLEBAUM et al. 2013). Hunting mortality is fairly well known and can be regulated if agreements are reached. Displacement of wintering long-tailed ducks from good feeding areas might be more important in future if planned large scale wind farms will be established at core wintering sites, i.e. at offshore banks. In this note one of the four recognised anthropogenic threats, i.e. the threat from intensive shipping activities is elucidated. More specifically, an analysis of ship traffic within and close to two marine Natura 2000 sites is presented. The analysed sites, the Hoburgs bank and the Northern Midsjö bank, are two of the most important wintering sites for long-tailed ducks in the Baltic Sea. Several hundred thousand long-tailed ducks, which is a significant part of the global population, have been observed wintering within these areas in recent years (SKOV et al. 2011, NILSSON 2012). Possible methods to reduce the threat from ship traffic are also discussed