Propagule pressure and initial dispersal as determinants of establishment success of brook trout (Salvelinus fontinalis Mitchill 1814)

Abstract

Establishment success of invasive species depends largely on the size of the initial source populati on because small populations are more likely to fail due to stochastic or inverse density-dependent Allee effects. However, there are difficulties involved in using propagule pressure as an explanatory variable to account for establishment success because records concerning the size of initial source populations are typically (i) non-existent, (ii) only rough estimates and/or (iii) based on indirect measurements. The focus of this study was the establishment success of a deliberately introduced non-native salmonid, Salvelinus fontinalis as a function of actual site-specific propagule pressure. Additionally, we investigated whether newly released fish located suita ble habitat patches, potentially facilitating the spread of the invader across Finnish stream systems. We found that the propagule pressure leading to highly successful establishment amounted to approximately 8000 released individuals. Additionally we also noted that one fourth of the newly introduced fish rapidly located suitable habitat patches (small tributaries) during initial dispersal, thus potentially increasing establishment success and the spread of the invader. The results suggest that by considering both propagule pressure and initial dispersal it will be possible to understand the latter stages of the invasion process, eventually leading to an improved capability in predicting successful invaders and sites that will be invaded.