Abstract
The European population of Greater Scaup Aythya marila has experienced an alarming, ~60% decline in numbers over the last two decades. The brackish lagoons of the Odra River Estuary (ORE) in the south-western Baltic Sea, represent an important area for the species during the non-breeding season in Europe. The lagoons regularly support over 20 000 Scaup, with peaks exceeding 100 000 (38%–70% of the population wintering in NW Europe and the highest number recorded in April 2011–105 700). In the ORE, Scaup feed almost exclusively on the non-native Zebra Mussel Dreissena polymorpha. This mussel was present in the ORE already in the 19th century and continues to be superabundant. Using the results of 22 Scaup censuses (November to April 2002/2003 to 2013/2014) from the whole ORE (523 km2 of water), we show that Scaup flocks follow areas with the greatest area of occurrence and biomass of the Zebra Mussel, while areas with low mussel densities are ignored. The numbers of Scaup in the ORE are primarily related to the area of Zebra Mussel occurrence on the lagoon’s bottom (km2) in a non-linear fashion. Zebra Mussels were absolutely prevalent (97% of biomass) in the digestive tracts of birds unintentionally by-caught in fishing nets (n = 32). We estimate that Scaup alone consume an average of 5 400 tons of Zebra Mussels annually, which represents 5.6% of the total resources of the mussel in the ORE. Our results provide a clear picture of the strong dependence of the declining, migratory duck species on the non-native mussel, its primary food in the ORE. Our findings are particularly important as they can form the basis for the conservation action plan aimed at saving the north-western European populations of Scaup.
Highlights
The Eurasian subspecies of the Greater Scaup Aythya marila marila (Fig 1c and 1d) breeds across the boreal zone from Iceland east to the River Lena
The study area is located in the south-western part of the Baltic Sea and forms the Polish part of the Odra River Estuary (ORE) system, which includes the Great Lagoon, Świna Backward Delta, Kamień Lagoon, Dziwna Strait and Lake Dąbie; it covers a total area of 522.58 km2 (Fig 2, Table 1) The average and maximum depths of the Lagoon are 3.8 and 8.5 m, respectively [27]
147 of the 209 ORE 2x2 km squares were occupied by the Zebra Mussel, and its occurrence was predicted in a further 8 squares; in total, 155 squares (74%) were considered as being occupied by the mussels. 33 of the squares surveyed were unoccupied, and we predicted the absence of mussels in a further 21 squares; a total of 54 squares were assumed to be unoccupied by the mussels
Summary
The Eurasian subspecies of the Greater Scaup Aythya marila marila (hereafter Scaup) (Fig 1c and 1d) breeds across the boreal zone from Iceland east to the River Lena. It lives both on seawaters, like other ducks of the tribes Somaterini and Mergini [4, 5, 6], and on freshwater bodies like typical Aythyini [7, 8]. Previous studies outside the ORE have shown that Scaup wintering in the Baltic area forage mainly on marine bivalves, including Sand Gaper Mya arenaria, Blue Mussel Mytilus edulis, and Baltic Clam Macoma balthica [5, 6, 20]. We predicted that the abundance of Scaup flocks is positively related to the areas of Zebra Mussel aggregations on the water bodies bed.
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