Top predators, such as the Striped Bass Morone saxatilis and Muskellunge Esox masquinongy, can impact food webs and alter ecosystem structure through the regulation of prey populations. Within the Saint John River, New Brunswick, Canada, both predators have long been hypothesized to impart significant mortality on smolts of the endangered Atlantic Salmon Salmo salar. As a consequence, current management practices entail euthanizing Muskellunge (an introduced species) that enter fish passage facilities along the Saint John River. Furthermore, the recovery and protection of a native Striped Bass population have largely been ignored. To assess seasonal diet, gastric lavage was performed on Striped Bass (n = 244) and Muskellunge (n = 96) captured in the downstream proximity of the Mactaquac Dam from April to November 2016. Clupeids dominated the stomach contents by number (92% for Striped Bass; 49% for Muskellunge) and mass (71% for Striped Bass; 96% for Muskellunge). Other prey species included White Perch Morone americana, Yellow Perch Perca flavescens, American Shad Alosa sapidissima, and American Eels Anguilla rostrata. No Atlantic Salmon smolts or other regionally recognized recreational fish species were identified in any of the stomach samples (n = 340) examined. Concurrently, this study observed little temporal overlap between the smolt migratory period and the arrival of Striped Bass to the Mactaquac Dam. Some Striped Bass (n = 33) were observed to be in spawning condition, releasing eggs and milt when handled, although reproduction by this species in the Saint John River was thought to have ceased long ago.

Introduced predators can have harmful top-down effects on their newly colonized system through competition with and direct predation on native species. Following an initial introduction of muskellunge in Lac Frontière, Québec in the 1970s at the headwaters of the Wolastoq/Saint John River, the species rapidly migrated downstream, expanding its range by $500 km over $20 years. Despite this expansive colonization and concern over possible threats to native species, little is known about the basic ecology of muskellunge in this system. The last downstream barrier is the hydroelectric facility, Mactaquac Generating Station (MGS), 150 km upstream of the sea. While there are no downstream fish passage facilities at MGS, adult muskellunge have been recorded downstream. In this study, muskellunge (n = 23) were surgically tagged with very-high-frequency (VHF) radio or combined acoustic radio telemetry (CART) tags and tracked over two spawning seasons. We sought to determine if there was a reproducing population downstream of MGS and tracked Tagged muskellunge over two spawning seasons. We tracked fish to locate and confirm spawning sites, and followed up with egg and/or juvenile sampling surveys. Tagged muskellunge (90%) moved upstream towards the MGS during the spawning period in each year (2016 and 2017), where they remained throughout the entire spawning period.No spawning or nursery sites were confirmed near MGS, but in 2016 three distinct spawning locations and six distinct nursery sites were confirmed 10-12 km downstream amongst a chain of flooded islands. In 2016, eggs, sac-fry and juveniles were collected and confirmed as muskellunge by genetic sequencing, providing the first empirical observation of successful spawning downstream of MGS.