Development of an index for assessing the role of propagule pressure on salmonid hybridization

Hybridization with introduced salmonids is one of several potential threats to native cutthroat trout populations.  Although introduced salmonid dispersal into native salmonid habitat can be successfully modeled from environmental variables, the amount of propagule pressure is increasingly seen as an important predictor of invasion success.  To assess the influence of propagule pressure on hybridization levels between native cutthroat trout and introduced rainbow trout in the upper Kootenay River, British Columbia, we developed a Propagule Pressure Index (PPI) by analyzing introduced rainbow trout stocking records within a GIS.  Using an information theoretic approach, we compared performance of 23 logistic-regression models predicting hybridization levels from combinations of PPI, environmental (biogeoclimatic zone, elevation, temperature, peak flow timing, and stream width), and spatial (migration barriers, distance to stocking site and Koocanusa Reservoir, and number of stocking sites within 10 km) variables.  Hybridization levels were best predicted by PPI and site biogeoclimatic zone (r² = 0.62).  PPI had the highest importance weight among variables (0.995) and was included in all of the 7 best performing models.  PPI demonstrates the importance of propagule pressure on levels of hybridization between a native and introduced trout, and can inform management decisions designed to protect native populations.