by Cassandra Burgess
Any man-made structure in a marine environment has the potential to impact the organisms living there. Previous research has shown that fish actively avoid trawlers and boats, and that abandoned oil platforms often become a place for fish to congregate. It has yet to be determined how fish will react to the presence of tidal energy generators. This may be an important design consideration. Vietnam and Zydlewski (2014) conducted research on fish behavior at a turbine in Cobscook Bay, Maine. This bay is known for high biodiversity, and provided the chance to study a range of fish species. The study found that over 50 percent of the fish they monitored in the area interacted with the turbine in some way, and that 34.8 percent were observed to enter or exit the turbine during the 22 hours study. They also found large fish (greater than 10 cm in length) were more likely to avoid the turbine at night than small fish. At night small fish had only a 0.002 probability of avoiding the turbine, while large fish had a probability of 0.109 of doing so.
This study was conducted using two DIDSON acoustic cameras. One tracked fish upstream of the plant, and the other tracked fish downstream. Unlike a conventional camera these allowed for tracking at night without using light, however, the range and resolution of these cameras is limited. Their greatest resolution was parallel to the current, so they were unlikely to detect fish moving at an angle to the current.. They also only tracked an area about 10 m away from the turbine. The study extended for 22 hours, including 11 hours in daylight and 11 hours at night. This period constitutes 2 tidal cycles, allowing for observations of both high and low tide during day and night.
The researchers found that turbine rotation greatly affected fish behavior, reducing the probability of fish entering by 35 percent, increasing the probability of fish avoiding the turbine by 120 percent, and increasing the probability of fish passing by the turbine to increase 97 percent. In this study avoidance was characterized by the fish entering the camera view on a course to enter the turbine, but then changing course to avoid doing so. Passing by was characterized as entering the camera view either above or below the turbine and continuing past it. It impossible that some of the fish that passed by the turbine actually exhibited avoidance, but did so out of range of the cameras. These are very large differences in fish behavior based on the motion of the turbine.
Based on these results, it is clear that day or night conditions, turbine motion, and fish size all have an impact on how fish will behave near a turbine. These factors should thus be taken into account in deciding where to place future turbines, and at what times they should operate. Previous studies have shown that fish of different sizes move to different parts of the water column throughout the day, so the height and time of operation of a turbine could have a large effect on what species of fish interact with it. This study was small, spanning only a 22 hour period, and monitoring a small area around the turbine. Fish behavior outside of the 10 meter radius could not be tracked, and so any fish avoiding the turbine from a greater distance were simply noted as passing. Because of these limitations of the study, and the importance of understanding how fish interact with turbines, the researchers believe that a larger study should be conducted to better define which parameters most influence fish behaviors.
Viehman, Haley A., and Gayle Barbin Zydlewski. “Fish Interactions with a Commercial-Scale Tidal Energy Device in the Natural Environment.” Journal of the Coastal and Estuarine Research Federation (2014): 30 Jan. 2014. Web. <http://link.springer.com/article/10.1007/s12237-014-9767-8/fulltext.html>.