Field Evaluation of Fish Interactions with a Commercial-Scale Marine Hydrokinetic Device

Fish are a key part of the marine ecosystem likely to be affected by marine hydrokinetic devices.  Laboratory studies exist in which fish are introduced to the region of a turbine in a flume or other enclosed space; however, less is known about fish behavior around a turbine in the natural environment.  Cobscook Bay, Maine is currently host to development of the largest tidal power project in the United States.  Ocean Renewable Power Company (ORPC) is applying to install a pilot tidal energy device as early as fall of 2011.  Part of pre-deployment testing included a full-scale test turbine mounted 6 m below a floating platform moored in Cobscook Bay.  We observed this test turbine using two DIDSON acoustic cameras, mounted fore and aft of the device.  24-hour sampling sessions were carried out in February, March, May, June, September, and October of 2010.  Each session spanned day, night, and two complete tidal cycles.  Over 200 hours of footage were collected: 158 hours with the turbine in the water (122 moving and 36 still), and 50 hours with no turbine present (mainly in February).  The sampling periods covered current speeds of up to 2.5 m s^-1 (~5 knots) and a wide range of environmental conditions.  Fish behavioral responses to the turbine were quantified in order to understand how fish will respond to this marine hydrokinetic device when deployed.  Responses were classified (e.g., as avoidance, entrance, and exit), and interacting fish were categorized (small, <10 cm; medium, 10 to 30 cm; or large, <30 cm).  We examined the correlation of factors such as fish size, current speed, time of day, tidal stage, and time of year with the number of fish-turbine interactions and the type of behavioral response.  Given the poor visibility in the bay and the need for both day and night observation, the DIDSON has proven to be a useful tool for the device assessment.