The alien signal crayfish (Pacifastacus leniusculus) was introduced into Finland, and then more broadly in Europe, with the expectation that it would be a suitable substitute for the native crayfish that was eradicated by crayfish plague (Aphanomyces astaci) epidemics. Signal crayfish introductions were promoted and the initial results indicated the stocking was successful. Later, there have been several cases showing that the initial conclusions were premature, and that signal crayfish stocks were struggling in Nordic countries. Here we present the first data on wild signal crayfish population declines and crashes in Kanta-Hame, Finland, where a minimum of 29 signal populations have collapsed during the past decade (a total of 39% of stocked populations). These populations were mainly inhabiting small water bodies (less than 1000 ha) and were all commercially productive (> 1 CPUE) before the collapse. Our data indicates that success of signal crayfish introductions should only be assessed after this alien species has achieved the carrying capacity of the aquatic ecosystem. In light of our results, the early assessments of the suitability of the signal crayfish to successfully inhabit European aquatic ecosystems should be carefully reconsidered. Copyright © 2014 by The Author(s). Published by the International Association of Astacology. This article is distributed under the terms of the Creative Commons Attribution License (CC-BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. ARTICLE INFO