TITLE OF THESIS: The Ecology and Life History of Non-Native Rainbow Trout in Rivers of Prince Edward Island, Canada

ABSTRACT: Humans have affected natural ecosystems in many ways, most prominently through widespread habitat destruction and the broad exchange of species between previously isolated areas of the world. Contemporary ecosystems are often fundamentally changed to the point where restoring historic ecological conditions may be logistically unrealistic. Researchers face the urgent challenges of understanding declines in native populations in addition to studying the dynamics and implications of contemporary ecological conditions. On Prince Edward Island (PEI), in eastern Canada, non-native rainbow trout (Oncorhynchus mykiss) were repeatedly introduced for almost a century and naturally spawning populations now persist in many rivers across the province. Meanwhile, within many of these rivers, native Atlantic salmon (Salmo salar) populations have declined to precariously few individuals. This thesis investigates the ecological space where rainbow trout have succeeded on PEI. A provincial rainbow trout distribution survey and reach scale surveys were conducted at a watershed level. It was hypothesized that rainbow trout thrive in anthropogenically modified watersheds, whereas Atlantic salmon prefer less impacted habitat and the other native salmonid brook trout (Salvelinus fontinalis) would dominate in headwater habitats. The study found juvenile rainbow trout, indicative of successfully spawning populations, in 12 watersheds where they were previously not documented, bringing the provincial total to 32. Examining habitat and watershed level variables with multivariate statistics showed that proximity to source populations was associated with newly established populations. Additionally, watersheds with rainbow trout had higher agricultural land use and higher slope. At the stream reach scale, rainbow trout presence was associated with higher stream slope and its correlate, streambed in motion. Atlantic salmon were found in wider stream reaches with abundant cobble substrate and brook trout were the dominant species in headwater reaches with fine substrate. The subsequent hypothesis was that rainbow trout are using coastal estuaries to undertake anadromous migrations. To elucidate migratory patterns, external Floy tagging, acoustic telemetry and otolith microchemistry analyses were conducted on rainbow trout in three watersheds: Dunk River, West River, and Montague River. In the three study populations, short distance movement within estuaries was the prominent movement pattern, although there were site specific differences in use of higher salinity habitats. Montague River fish had high Sr:Ca concentrations in otolith chemistry which are indicative of high salinity occupation. Populations on West River and Dunk River, however, had higher residency in low salinity tidal habitats and often exhibited highly frequent movements across the salinity gradient. Otolith chemistry was more ambiguous in these sites as the technique is most effective for broadly discriminating between explicitly marine or freshwater signatures. Otolith chemistry revealed that migratory individuals were significantly larger than freshwater counterparts and that migratory individuals were more likely to arise from migratory mothers. Concurrent water quality monitoring suggested that high temperatures and periodic anoxia could explain site specific differences in habitat use. This led to the hypothesis that rainbow trout movements are a balance between an incentive to occupy the estuary and avoiding adverse conditions that occur therein such as high temperature and anoxia. High resolution water monitoring and acoustic telemetry were used to explore the influence of abiotic conditions on habitat use. Discrete zones were generated across the salinity gradient and daily occupation at each zone was assessed. When perturbation increased downstream, tagged fish increased residence in low salinity reaches. Tagged fish spent 99% of time above 5.48 mg∙L-1 dissolved oxygen and in temperatures below 21.3 °C.  A boosted regression tree model demonstrated that dissolved oxygen, daily river discharge, and temperature were all predictors of rainbow trout occupancy. The study showed that rainbow trout behaviourally mediate risks associated with environmental perturbation in order to occupy productive tidal and estuarine environments. Given that rainbow trout have been introduced across the planet, studies exploring the ecology of these invasions on PEI are of fundamental interest as they may help predict the probability of invasion in other regions. The findings of these studies support the thesis that anthropogenic watershed disturbance results in ecological conditions that are favorable for rainbow trout. The implications of the work are that rainbow trout are passengers of anthropogenically driven ecological change, rather than the drivers of native species declines.

Anyone who wishes to attend the public presentation should contact the Grad Studies Coordinator at gsc@upei.ca to receive the link to the meeting.