Mortality of post-settlement clams Rangia cuneata (Mactridae, Bivalvia) at an early stage of invasion in the Vistula Lagoon (South Baltic) due to biotic and abiotic factors

The clam Rangia cuneata, originating from the Gulf of Mexico, was recorded in the Vistula Lagoon for the first time in the early 2010s, and quickly became the dominant component of the zoobenthic biomass. To assess mortality as a factor potentially controlling the growth of Rangia population, a year-long field experiment involving marked bivalves placed in sediment-filled trays deployed on the bottom was conducted in 2014 and 2015. Predator-induced mortality of the clams was low in summer, and very high in the winter–spring period. It was inversely proportional to the size of the clams. Such changes can be partially attributed to predation from at least five fish and three duck species, which contained clams in their digestive tracts. Non-predatory mortality particularly affected large individuals, and was highest in spring, several weeks after the end of winter. We hypothesize that it could be caused by persistent low temperatures over several winter months which led to considerable weakening of the condition of clams. A long winter could also reduce their resistance to environmental stress and potential effect of epibionts, as well as increase susceptibility to predation.