Fish ecology in regions of extreme tidal flows is poorly understood, but as these areas link on- and off-shore habitats, they are important to many marine and diadromous fish species. Strong tidal currents are also being targeted for energy extraction, but the effects of tidal energy devices on fish are unknown. The probability of fish encountering a tidal energy turbine is highly dependent on the vertical distribution of fish at the project site. In extremely tidal coastal areas, fish presence and distribution is heavily influenced by tidal, diel, and seasonal cycles. Understanding the vertical distribution of fish therefore requires sampling on a fine temporal and spatial scale. Stationary hydroacoustic surveys may be used to gather these data, as part of a BACI (Before, After, Control, Impact) type study design, to predict then monitor the effects of tidal energy devices on fishes.
Starting in May 2010, a down-looking, single-beam SIMRAD echosounder and a DIDSON (Dual-frequency IDentification SONar) unit were used to document the relative density of fish throughout the water column at a targeted pilot project site and a control site in Cobscook Bay, Maine. Stationary 24-hour surveys were carried out each season to examine variation in fish density and vertical distribution. Relative fish density was highest in spring and fall, and almost always increased near the bottom, regardless of tide or time of day. Tide and day/night had some effect on the vertical distribution of fish, but the effect was not the same each month. Results from these analyses will be used to predict the likelihood of fish encountering the turbine and to create a basis for comparison of data collected after device installation.
Direct observation of fish reactions to a full-scale test device was carried out in September of 2010. A test turbine suspended below a floating research platform was monitored for 24 hours using two DIDSON units. A higher proportion of fish interacted with the device when it was still than when it was rotating. A greater portion interacted at night, and the type of interaction shifted from avoidance during the day to passing into the turbine at night. This behavioral shift was most obvious in small fish (<00 cm), nearly all of which passed through the device at night; most large fish (>20 cm) still avoided the turbine. Most fish were present at night during the slack tide.
Combining the baseline knowledge of where fish are in the water column with knowledge of how they behave in close proximity to an operating tidal device will provide a more complete picture of the potential effects these devices could have once installed.