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Background  Translocation of native species and introduction of non-native species are potentially harmful to the existing biota by introducing e.g. diseases, parasites and organisms that may negatively affect the native species. The enemy release hypothesis states that parasite species will be lost from host populations when the host is introduced into new environments.    Methods  We tested the enemy release hypothesis by comparing 14 native and 29 introduced minnow ( Phoxinus phoxinus ) populations in Norway with regard to the ectoparasitic  Gyrodactylus  species community and load (on caudal fin). Here, we used a nominal logistic regression on presence/absence of  Gyrodactylus  spp. and a generalized linear model on the summed number of  Gyrodactylus  spp. on infected populations, with individual minnow heterozygosity (based on 11 microsatellites) as a covariate. In addition, a sample-based rarefaction analysis was used to test if the  Gyrodactylus -species specific load differed between native and introduced minnow populations. An analysis of molecular variance was performed to test for hierarchical population structure between the two groups and to test for signals of population bottlenecks the two-phase model in the Wilcoxon signed-rank test was used. To test for demographic population expansion events in the introduced minnow population, we used the kg-test under a stepwise mutation model.    Results  The native and introduced minnow populations had similar species compositions of  Gyrodactylus , lending no support to the enemy release hypothesis. The two minnow groups did not differ in the likelihood of being infected with  Gyrodactylus  spp. Considering only infected minnow populations it was evident that native populations had a significantly higher mean abundance of  Gyrodactylus  spp. than introduced populations. The results showed that homozygotic minnows had a higher  Gyrodactylus  spp. infection than more heterozygotic hosts. Using only infected individuals, the two minnow groups did not differ in their mean number of  Gyrodactylus  spp. However, a similar negative association between heterozygosity and abundance was observed in the native and introduced group. There was no evidence for demographic bottlenecks in the minnow populations, implying that introduced populations retained a high degree of genetic variation, indicating that the number of introduced minnows may have been large or that introductions have been happening repeatedly. This could partly explain the similar species composition of  Gyrodactylus  in the native and introduced minnow populations.    Conclusions  In this study it was observed that native and introduced minnow populations did not differ in their species community of  Gyrodactylus  spp., lending no support to the enemy release hypothesis. A negative association between individual minnow host heterozygosity and the number of  Gyrodactylus  spp. was detected. Our results suggest that the enemy release hypothesis does not necessarily limit fish parasite dispersal, further emphasizing the importance of invasive fish species dispersal control.    Electronic supplementary material  The online version of this article (doi:10.1186/s13071-016-1306-y) contains supplementary material, which is available to authorized users.

Gyrodactylus spp. diversity in native and introduced minnow (Phoxinus phoxinus) populations: no support for “the enemy release” hypothesis