Further observations on the range extension of the alien amphipod Gammarus tigrinus Sexton, 1939, in the Netherlands during the years 1974 to 1976

In the autumn of 1975 and in the spring of a new survey of the range of Gammarus tigrinus Sexton, 1939, was carried out, especially at the borders of the 1973 range. Considerable range extensions were found in the north-eastern parts of the province of Groningen, the Kampereiland (province of Overijssel), in the Deltaic Region and in some more or less isolated areas in the province of Zuid-Holland. In most of the newly invaded areas G. tigrinus has replaced the indigenous Gammarus species. Sampling along the Dutch-German border revealed that no invasion has taken place from any of the German populations. A fungus was found which appeared to be pathogenic for G. tigrinus.