Hydroacoustic Analysis of the Effects of a Tidal Power Turbine on Fishes
Biological/Ecological Effects, PhD Thesis: Haley Viehman (2016/12)
Tidal currents help shape coastal marine environments and are essential in life cycles of many fish species. Areas with strong tidal currents are also targeted by humans for energy extraction via tidal energy turbines. The effects of these devices on fishes are difficult to predict because fish behavior within fast tidal currents is largely unstudied. Based at a tidal energy site in Cobscook Bay, Maine, this work sought to describe fish reactions to a tidal energy device, to understand the natural presence of fish at the site, and to provide guidance for future monitoring of tidal energy device effects in these difficult environments. A bottom-mounted echosounder was used to monitor the behavior of fish 7-18 m away from a stationary MHK device for several weeks. Fish moved with the current, but those approaching the device showed signs of avoiding it by adjusting their direction. The same echosounder was used to collect a two-year record of hourly fish passage rate at the depth of the turbine, after it had been removed. Fish passage rate, and therefore potential encounter rate with the turbine, changed dramatically over time with the dominant environmental patterns (tidal, daily, lunar, and seasonal cycles). By timing surveys of fish abundance at tidal energy sites with these cycles (e.g., carrying out 24-hr surveys at the same lunar stage throughout the year), the quality of results can be improved. Using this approach at tidal energy sites could therefore increase our ability to detect device effects without requiring expensive continuous sampling over a long time. Monitoring costs may be further reduced by using single beam echosounders, rather than the typical split beam systems, as statistical methods (deconvolution) were found to make certain single beam data comparable to that from the split beam. Depending on monitoring goals, the use of single beam echosounders could substantially reduce costs while supplying sufficient information on device effects for use in management decisions. Results from Cobscook Bay are likely to be useful at other tidal energy sites, but study designs and results need to be considered in the context of fish species and life stages present.