Defining Seasonal and Spatial Survival, Growth, and Emigration of Native Brook Trout in Sympatry with Brown Trout

Although laboratory studies have provided evidence for competition between brook trout and brown trout, it is unknown how this competition affects larger scale demographics in a natural setting.  We examined seasonal demographics of Minnesota brook trout populations in the presence of high and low brown trout densities.   We estimated survival and movement while testing for effects of stream reach and season using a multistrata Cormack-Jolly-Seber model in Program MARK.  Multistrata models were analyzed for three age groups (age-0, age-1, and age-2+) to estimate apparent survival, capture probability, and movement.  Survival was dependent on season rather than study reach and was lower during flood events.  Age-0 brook trout emigrated from upper reaches to lower reaches, whereas, adult brook trout emigrated out of the downstream brown trout dominated reaches.  Growth in spring and summer did not differ across streams or treatments for the youngest age classes.  For age-2+ brook trout, however, growth was higher in areas where brown trout were less abundant. Competition can be age or size dependent; our results show evidence for adult interactions, but not age-0.  Our results suggest that brown trout may be influencing adult brook trout distribution through forced emigration.  Also, decreased growth rates of adult brook trout in the presence of brown trout warrants further research on possible mechanisms. A brown trout removal experiment is currently underway to test these two hypotheses.