Ecological effects of the mass occurrence of the Ponto-Caspian invader, Hemimysis anomala G.O. Sars, 1907 (Crustacea: Mysidacea), in a freshwater storage reservoir in the Netherlands, with notes on its autecology and new records

A new Ponto-Caspian invader, the mysid Hemimysis anomala G.O. Sars, 1907 (Crustacea: Mysidacea) was recorded for the first time in the Netherlands in 1997. In the summer of 1998 extremely high densities (>6 ind. l−1) of this neozoon were recorded in one of the Biesbosch reservoirs (Honderd en Dertig). This high abundance can not be explained by a recent invasion. Either H. anomala reached the Netherlands via the River Rhine, probably aided by shipping, or through transport with ballast water from the Baltic or Black Sea. The invasion had dramatic effects on the zooplankton composition and abundance: from the end of August onwards hardly any Anomopoda, Ostracoda, Rotifera and invertebrate predators (Leptodora kindti and Bythotrephes longimanus) were present. Copepod densities, however, were not influenced. Chlorophyll-a concentrations were significantly lower compared to previous years, possibly the result of mysids feeding on the algae. Laboratory experiments revealed that H. anomala is a voracious predator and also an omnivorous feeder. Life history characteristics and behaviour of H. anomala fit into general mysid ecology. The hidden life style of H. anomala during the day makes it difficult to assess its present geographical distribution. More attention should therefore be given to appropriate sampling of this mysid. Because of its broad prey-size range H. anomala may pose a threat to existing lake restoration programmes (biomanipulation) in the Netherlands. In addition to the records of the Biesbosch reservoirs, H. anomala was found in the River Meuse and in reservoir Andijk, in the northern part of the Netherlands.