Table of Contents

  • This report summarizes the recent trends in five species of diving ducks breeding in, or wintering within, the Baltic Sea, and compares trends verified with trends outside the Baltic Sea, in Norway and Iceland. The focal species are Greater Scaup (Aythya marila), Common Eider (Somateria mollissima), Long-tailed Duck (Clangula hyemalis), Common- (Melanitta nigra) and Velvet Scoter (M. fusca). All focal species have recently been reported to decline rapidly in winter in the Baltic Sea, and they all feed on mussels, to a large extent on Blue Mussels (Mytilus edulis x M. trossulus).

  • The publication of the report on the population status of waterbirds wintering in the Baltic Sea (Skov et al. 2011, Waterbird Populations and Pressures in the Baltic Sea, TemaNord 2011:550) highlighted a rapid decline in a number of seaduck species wintering in the Baltic Sea, including Greater Scaup (Aythya marila), Common Eider (Somateria mollissima), Long-tailed Duck (Clangula hyemalis), Common Scoter (Melanitta nigra) and Velvet Scoter (M. fusca). While these species are not exclusive examples of declines reported, these seaducks are numerically very dominant among the wintering birds, and all feed mainly on mussels, such as Blue Mussels (Mytilus edulis x M. trossulus). This common feature in the ecology of these ducks, generates a second central question on possible changes in the abundance of their preferred food reflecting changes in the Baltic Sea food chains on a more general level (e.g. Ottwall 2012).

  • We follow the delineation of the Baltic Sea (henceforth) wintering area by Skov. et al. 2011), including Kattegat and bordering the North Sea at the northern tip of Jutland, Denmark. This area may also be defined as the HELCOM area (see HELCOM 2013). The total area of the Baltic/ Wadden Sea flyway is, however, larger, including the Wadden Sea and other areas on the North Sea / Atlantic coast. Other areas (Norway, Iceland) are mentioned when appropriate. The wintering areas of Norwegian and Icelandic species are indicated as appropriate.

  • The wintering populations of all five focal species in the Baltic have reportedly declined between 1987 and 2009 (Skov et al. 2011). The recent results rely on data from surveys performed in 2007–2009, which were compared to data collected in 1992–1993 (Durinck et al. 1994). These surveys aimed at obtaining densities of birds, i.e. to identify the relatively most important areas for each species using land-based counts, aerial total counts, aerial transect counts and ship-based transects. Estimated population sizes from these data were made by integrating density estimates for discrete areas of different density levels. Further, to assess changes in numbers, these were compared with the previous density data created by Durink et al. (1994). Thus, the large-scale decreases of wintering seaducks are based on two large-scale surveys of the birds both producing a total estimate of the population size, which are compared.

  • This section gives an overview of the current estimates of breeding pair number of the focal species, as well as an indication based on national monitoring schemes on the recent trend for each species. All estimates of breeding numbers and trends are taken from sources which are as up to date as possible. The table also indicates the national IUCN classification of the species.

  • Both the Finnish and the Swedish national monitoring schemes covering coastal populations of the focal species identify a similar decrease in the Common Eider – the breeding populations in the Baltic Sea have about halved during the last 15 years, a the decline has been rapid especially in the last 10 years (Ekroos et al. 2012a). The trend is still continuing.

  • Trend verification ideally comes from data tailored for the purpose of tracking changes in populations over time. The tailored solutions over the nations involved in the trends analyzed here are the national monitoring programs on both breeding and wintering birds, some of which are also multinational ventures such as the study by Skov et al. (2011).