The magnitude, frequency, and duration of harmful algal blooms (HABs) are increasing worldwide primarily due to climate change and anthropogenic activities. Prymnesium parvum is a euryhaline and eurythermal HAB forming species that has expanded throughout North America resulting in massive fish kills. Previous ecotoxicological work supported an understanding of conditions resulting in HABs and fish kills; however, the primary endpoint selected for these studies was acute mortality. Whether adverse sublethal responses to P. parvum occur in fish are largely unknown. To begin to address this question, fish molecular and biochemical oxidative stress (OS) responses and behavioral alterations in two common fish models were investigated. Varying nutrient and salinity conditions influenced P. parvum related OS and fish behavioral responses of two common fish models, and these responses were heightened by conditions nonoptimal for P. parvum growth. Such sublethal observations present important considerations for future assessment and management of P. parvum.