Range extension in the period 1968 – 1970 of the alien amphipod, Gammarus tigrinus Sexton, 1939, in The Netherlands

Gammarus tigrinus was introduced in the Netherlands around 1964. Previous papers recorded the spreading of this amphipod in the successive years. In the period 1968-1970, it has invaded new regions in the provinces of Friesland, Groningen, Overijssel, Gelderland, Noord – Holland and Zuid – Holland, as well as in the Noordoostpolder and Oostelijk Flevoland. The maximum progress made since 1967 is about 36 kilometers. It is recorded for the first time south of the river Rhine.