ALBERT

Description: The round goby (Neogobius melanostomus) is extremely invasive fish species, capable to reach high abundance and make significant impact on invaded ecosystem. First round gobies occured in the Lithuanian part of the Baltic Sea in 2002. The population passed establishment (2002-2010), expansion (2011-2012) and adjustment (2013-2015) phases. Rapid round goby abundance increment induced dramatic decline of its major prey - the blue mussel (Mytilus edulis), which in turn negatively affected population of wintering long-tailed duck (Clangula hyemalis). Food competition between round goby and long-tailed duck was evaluated analysing changes in their diet composition, feeding efficiency and feeding niche overlap during different round goby invasion phases. During establishment both species preyed mainly on M. edulis; during expansion, the diet of round goby was dominated by M. edulis, Crangon crangon and Macoma balthica, while in the last invasion period it shifted to polychaetes. Long-tailed duck shifted its diet from epibenthic blue mussel to fish prey during round goby expansion and adjustment phases. Feeding efficiency of the round goby decreased from 100% (percentage of full guts) determined during establishment to 80% and 68% during expansion and adjustment phases, respectively. The highest percentage of feeding long-tailed ducks was observed in the beginning of round goby invasion (74%), it dramatically declined during 2011-2012 (26%) and recovered (53%) during 2015-2016. Feeding niche overlap between round goby and long-tailed duck was biologically significant during round goby establishment, but it lost significance after drastic decline of the blue mussel in natural environment.

Description: The invasive round goby has established a viable population within 9 years of its first introduction to Lithuanian coastal waters (SE Baltic Sea). During its expansion phase, abundances increased 23-fold, which led to the near complete eradication of its main prey, the blue mussel, at 〈 20 m depth. The round goby population showed a stabilizing trend after blue mussel biomass was depleted; however, their abundance has not declined. The round goby feeds efficiently on newly settled mollusks, causing a severe constraint for blue mussel recovery. Changes in blue mussel availability and size structure induced a dietary shift in wintering long-tailed duck towards fish prey. An energetically dense food source sustains a good body condition in long-tailed ducks, however the change in trophic position (from 3.1 to 4.3 trophic level) suggests the potential for a reduction in their carrying capacity. Results from this study also show that coastal habitats with low and unpredictable population dynamics of blue mussel become less attractive wintering sites for long-tailed duck in the Baltic Sea. We also document a cascading effect of invasive species in the food web.