We documented estuarine predation by an immature striped bass (Morone saxatilis) upon an Atlantic salmon smolt (Salmo salar) that had previously been tagged with an ultrasonic transmitter on the Narraguagus River, Maine. That observation revealed the potential confounding effects that fish predation can have upon a telemetry study if the behavior of predatory fish cannot be distinguished from that of the target species. To investigate this effect, we also tracked the movements of immature striped bass in the same river system. We found that the striped bass alternately moved upstream and downstream of the release site, independent of tidal flows and light conditions, whereas salmon movements were generally passive and in the direction of tidal flows. These differences can be incorporated into our data analysis to better recognize and explain anomalous behavior of tagged smolts. Salmon abundance in Maine rivers has steadily declined since the mid-1980s, whereas Atlantic coast striped bass stocks have been increasing in abundance. Although the potential exists for significant smolt predation by striped bass in any one year, striped bass abundance in eastern Maine rivers is highly variable between years, suggesting that striped bass predation is not the proximate cause of the persistent decline in salmon populations.