Abstract
An underwater optical imaging system made from commercially available components can directly observe and record the migrating populations of anadromous river herring: Alewife (Alosa pseudoharengus) and Blueback Herring (Alosa aestivalis). River herring populations are depleted relative to historic levels due to decades of habitat loss and overexploitation; harvest is now widely prohibited. River herring in select river systems are monitored by human observers, and some have electronic counting systems. However, current fish-counting technologies are phasing out, and developing additional fish enumeration methodologies is a high priority. To record the river herring migration, multiple underwater optical imaging systems made from commercially available components were created and deployed. The cost for a single system in 2025, not including the underwater camera itself, was $1,200-1,300. A single-board computer controls the camera and produces digital video files that are stored on an external storage device. All electronics are securely housed inside a weatherproof hard-sided case, except for the solar panels, the camera, and external lighting. The system is solar powered and uses a 100 Ah LiFePO4 battery. For the 2024 river herring migration season, the system was deployed at the Santuit Pond fish ladder on March 16, 2024, and removed on June 6, 2024. The system was deployed for 1,968 hours and produced 1,426 hours of video during that time. The system produced video files for 72% of its deployment time at this site in 2024. For the 2025 season, the system was deployed on March 28, 2025, and removed on June 13, 2025. The system was deployed for 1,850 hours and produced 1,575 hours of video files during that time. The system produced video files for 85% of its deployment time at this site in 2025.For the 2024 river herring migration season, the system was deployed at the Coonamessett River on April 15, 2024, and removed on June 14, 2024. The system was deployed for 1,435 hours and produced 1,380 hours of video during that time. The system produced video files for 96% of its deployment time at this site in 2024. For the 2025 season, the system was deployed on March 28, 2025, and removed on June 13, 2025. The system was deployed for 1,846 hours and produced 1,768 hours of video files during that time. The system produced video files for 96% of its deployment time at this site in 2025. This portable field computer/camera system is intended to be an economical, robust, and modular device that can be easily customized to fit user needs. Video footage can be reviewed by humans and/or processed with computer-automated methods to produce spawning population count estimates and reveal patterns in migration events and population dynamics. When paired with machine learning methods to automate counting, this system can be used to rapidly provide data on the timing and magnitude of migration events. This system meets the research and monitoring needs identified by the Atlantic States Marine Fisheries Commission for maintaining and expanding sustainable river herring fisheries. The system can operate additional instruments besides a camera and can be deployed for ecological or environmental monitoring purposes other than river herring migrations. The portable design and self-sustaining solar-power supply make it suitable for long-term use in remote or otherwise difficult-to-access locations.