In Washington State, a small vertical-axis tidal turbine was monitored for months to evaluate fine-scale interactions with wildlife. An automated underwater tracking array delivered 109 days of camera images for researchers to analyze.
Altogether, 1,044 sightings of fish, seabirds, and seals were captured.
Only four fish came into contact with the moving blades. And three swam away unharmed.
Engineers now have evidence that ocean wildlife can navigate around this kind of operational subsea hardware.
How to monitor a cross-flow tidal turbine
The rig was a cross-flow test unit with four blades functioning on a vertical axis. It was mounted on a gravity-seabed base off the coast of Washington.
The rotor was 3.9 feet tall by 2.8 feet wide, with a sweep area of 10.8 square feet. Its position sat at between 4.9 and 8.9 feet off the seafloor, with all blades, weighing 3.3 pounds each, operating under automated speed control.
A tip-speed ratio of two was maintained in tidal currents peaking at speeds of 5.6 miles per hour.
Researchers attached an adaptable monitoring package to the mounting frame with a high-resolution video camera to record animal movements on a fine scale.
The setup was intended to model off-grid power generation by remote marine equipment, operating with cut-in current speeds between 2.0 and 2.2 miles per hour.
The process combined automated machine-learning detection via the array with manual review of a 13.9-foot monitoring radius, which is equal to the rotor spans of five turbines.
Breaking down marine animal contact and blade impacts
There were 109 days of operational video to analyze. Not one collision involving diving seabirds or harbor seals was noted.
More than 50 times as many fish were able to move safely around the spinning blade assembly as the number that came into physical contact.
These records are evidence that marine wildlife can avoid active cross-flow turbine blades while going about their normal functions in high-velocity Pacific Northwest tidal channels.
Seabirds showed up only during daylight, while seals and fish visited day and night regardless of rotor movement.
Over the test period, 224 individual fish and five fish schools swam past or dodged the rotating blades safely. Only four fish hit the active rotor.
Following the blade strikes, three fish were seen swimming away (indicating no immediate mortality), while the fourth appeared motionless and sank out of view.
Distinguishing small fish from drifting seaweeds proved tough during low-visibility periods. Even so, the raw counts show that safe evasion happened over 50 times more often than blade contact.
These records establish concrete physical interaction baselines for small hydrokinetic sites.
Fine-scale behavioral drivers and environmental monitoring trade-offs
The 1,044 sightings showed marine animals entering the 13.9-foot turbine zone to carry out specific habits tied directly to current speed and rotor motion.
Diving seabirds used the still frame as an artificial reef during slack water, feeding only when zero current kept the blades stationary.
Harbor seals navigated the site during both active and idle periods, using the steel frame as a resting spot or hunting area inside the swift current.
Fish moved through the zone to follow natural tidal flows or took cover inside the low-velocity water pocket created behind the subsea foundation. While passing through, they altered their swim paths in real time to avoid the moving blades.
Moderate speeds minimize mortality
While observations of a single small cross-flow turbine revealed four fish collisions and non-collision passages with local fauna, the study’s authors note that sampling constraints prevent quantifying overall collision probabilities or drawing broader safety conclusions for other sites and turbine designs.
Because the rotor operated at a low tip-speed ratio in moderate current speeds, low relative collision velocities minimized kinetic energy transfer during impacts with passing fish.
Still, optical cameras depend heavily on water clarity, leaving smaller marine life hard to track amid floating plant debris.
While the study highlights the value of pairing optical cameras with multibeam sonar to improve detection around tidal farms, the authors offer this as a best-practice monitoring recommendation rather than a mandatory permitting requirement.
The full study can be found here: Cotter, E., Bassett, C., Murphy, P., Scott, M., Runyan, A., Almokharrak, J. M., … & McMillen, S. A. (2026). Observations of marine animal interactions with a small tidal turbine. PloS one, 21(1), e0338376.
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