FLOOD DISTURBANCE REGIMES INFLUENCE RAINBOW TROUT INVASION SUCCESS AMONG FIVE HOLARCTIC REGIONS

Abstract

There is growing awareness that predicting biological invasions will require the development of conceptual models for specific taxa at appropriate scales. Salmonids are ideal taxa for testing factors that influence invasions, because large numbers have been introduced worldwide for long periods and their ecology is well known. We evaluated the hypothesis that, among regions with suitable water temperatures, environmental resistance from flood disturbances that wash away trout fry strongly influence invasion success of rainbow trout (Oncorhynchus mykiss), the most widely introduced fish species. We predicted that flow regimes in regions where rainbow trout invasions are successful would match those in their native range and would differ from those in regions where invasions are moderately successful or failed. We tested six specific predictions about how timing, predictability, frequency, duration, and annual variability of floods, as well as timing of low flows, will differ relative to timing of rainbow trout fry emergence among five Holarctic regions. Analysis of hydrologic regimes for eight rivers each in the native range (Pacific Coast) and four regions where rainbow trout invasions varied from highly successful (Southern Appalachians) to moderate (Colorado, USA, and Hokkaido Island, Japan) or failed (Honshu Island) showed that winter flooding and summer low flows in Pacific Coast rivers that favor spring emergence of rainbow trout were closely matched by Southern Appalachian flow regimes. In contrast, the other three regions had spring or summer flooding that hampered rainbow trout recruitment to different degrees, and winter low flows. Rainbow trout invasion success was best explained by a match between timing of fry emergence and months of low flood probability. Alternatively, cold water temperatures, which hamper reproduction, and biotic interactions with brown trout (Salmo trutta) and whirling disease parasites may account for low invasion success in European regions. However, differences in genetic makeup of donor stocks and propagule pressure are unlikely mechanisms to explain invasions. Understanding how abiotic disturbances interact with timing of critical life history events to limit nonnative species will help ecologists develop more robust theories to predict invasion success.