WESTSLOPE CUTTHROAT AND RAINBOW TROUT HYBRIDS: DIFFERENCES IN MIGRATION TIMING AND GROWTH

Westslope cutthroat trout are a species of concern across all of their native range. It is estimated that westslope cutthroat currently occupy only 10 percent of their native range. One of the main reasons for their decline is hybridization with non-native rainbow trout. I looked at the ecological differences between hybrids and pure westslope cutthroat and rainbow trout. I have collected 200 samples over a 3 year period on the Jocko River basin in Western Montana. I will discuss the importance of migration timing in the spread of hybridization and examine whether environmental factors (stream flow and temperature), or population characteristics (size and age of fish or extent of hybridization) explain differences in migration timing for these species. I will also explore how much variation in growth rate exists across the years in relation to the extent of hybridization. Otoliths (the inner earbone of a fish) were extracted from all the samples and used to determine the age. I then back-calculated the proportional growth per year to see how much and how fast each individual was growing. I then compared this growth rate across individuals with varying degrees of hybridization to determine if hybridization is influencing growth rate. Westslope cutthroat have been petitioned for listing under the Endangered Species Act and have been repeatedly denied. Understanding what ecological differences exist between hybrids and pure westslope cutthroat and rainbow trout is an important step in developing a management strategy to deal with this threat