Scientists filmed 1,044 animal encounters around a tidal turbine in Washington, and only four fish actually struck the spinning blades while every seal and seabird recorded around the machine managed to avoid a collision

Washington’s Sequim Bay was the location of a research study involving underwater camera systems. The point was to learn how local marine life interacts with subsea renewable energy infrastructure. Continuous monitoring was carried out over 109 days, and over 1,000 animal encounters were recorded.

Despite the high velocity of the flow, harbor seals, diving seabirds, and scores of fish still navigated the tidal turbine zone. These observations present energy developers and ocean regulators with firm evidence of collision risks during power generation. 

How a small cross-flow tidal turbine was monitored

The rig used for the study was a small vertical-axis cross-flow model fitted with four blades. It was mounted on a heavy gravity foundation off the coast of Washington State. It stood between 4.9 and 8.9 feet above the seabed. The unit’s dimensions were 3.9 feet tall and 2.8 feet wide, with a rotor sweep area of 10.8 square feet. Peak tidal currents in the area reached 5.6 miles per hour.

Researchers needed a way to track aquatic behavior without constant human oversight, so they decided on an automated monitoring system. It combined acoustic sensors, optical cameras, and learning software customized for this particular study. These technologies worked together to distinguish between living marine creatures and non-animal debris drifting in the water, like seaweed.

The setup was automated to switch on LED strobe lights and record a high-definition video every time a marine animal entered the monitoring zone. The system operated day and night for 109 days. The results provided accurate baseline data on wildlife movements around tidal turbines in channel flow conditions.

Marine animal interactions and their evasion patterns

A total of 1,044 animal sightings were reviewed. Not a single collision between a harbor seal or a diving seabird and the turbine equipment was recorded. Researchers logged 406 visits from seabirds, which were mainly double-crested cormorants and pigeon guillemots, and only during the day. The birds settled on the frame during slack water at high tide, using it like an artificial reef. This is when the rotor blades were left motionless in a lack of current. 

Harbor seals made 92 separate appearances. The time of day and tidal stage were irrelevant to them. Individual fish were logged visiting the turbine range 224 times. Five schools of fish were recorded. The figures show that fish managed to escape the rotor zone 50 times more often than make contact with the blades. Four fish hit the blades, and three swam off without any injuries visible. Only one appeared motionless, then it sank out of the camera view. The post-impact survival rates demonstrated were remarkably high.

Study findings, environmental constraints, and industry conclusions

The fact that the blades did so little harm could be explained by low kinetic energy transfer. The turbines were designed to run at a low tip-speed ratio in moderate currents. This means these blades did not have the velocity it takes to cause immediate fatal injuries to smaller fish.

There were still relevant environmental factors that placed constraints on the monitoring. Sometimes the water was cloudy, so camera visibility was low. Small fish could not always be easily distinguishable in the water conditions. This made researchers think that their count of successful passages was low, and collision risk is actually lower than recorded.

Not all tidal technology can be declared safe

The authors of the study say these findings cannot be applied across the board to all commercial marine projects, even though the numbers are positive. This specific test involved a 3.28-square-foot rotor, but full-scale utility turbines have blade sweeps up to 20 times bigger. Universal collision probabilities cannot be extracted because of size differences and site-specific factors. Not all tidal technology can be called safe.

Still, as North America’s first comprehensive optical dataset tracking animal interactions with an operating turbine, the study establishes an essential physical baseline. While environmental assessments tailored to specific sites are necessary, this study presented clear proof of low collision rates. It is a promising indication to base future regulatory approvals of commercial tidal power on.

All the details of 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.