Combining Imaging Sonar Counting and Underwater Camera Species Apportioning to Estimate the Number of Atlantic Salmon and Striped Bass in the Miramichi River, New Brunswick, Canada

A combined method incorporating an imaging sonar and underwater cameras was tested for assessing the size of adult Atlantic Salmon Salmo salar and Striped Bass Morone saxatilis populations in one of the main tributaries of the Miramichi River, New Brunswick, Canada. The number of fish recorded with the sonar in October 2019 was apportioned using the species ratio from the underwater camera data. The combined method estimated 358 Atlantic Salmon and 255 Striped Bass when the species ratio was applied every day and 274 Atlantic Salmon and 337 Striped Bass when the monthly species ratio was applied. The counts were compared to catches in a downstream index trap net using estimated values for trap-net catchability and for the proportion of fish ascending to the same tributary. Depending on the estimated values, the sonar-camera counts were between 40% and 190% of the estimated Atlantic Salmon numbers in the index net. For Striped Bass, the same estimated catchability and proportion values produced a lower agreement (sonar-camera count = 5-24% of the adjusted catch) because unlike Atlantic Salmon, Striped Bass do not deterministically migrate up the tributary in autumn. The fish were mostly detected overnight, and the trends in daily numbers of fish detected with the combined sonar-camera method were similar to the catches in the index net, with most Atlantic Salmon being detected mid-month and most Striped Bass being detected at the end of the month. The similarity of the fish counts with the adjusted trap-net catch and the new information about migration timing demonstrate that the sonar-underwater camera combination can provide tributary-specific and timely information on the Atlantic Salmon population in the Miramichi River.