Abstract
The effects on fish of large tidal in-stream energy conversion (TISEC) devices deployed in very high flow environments (>2 m/s) are generally unknown. This uncertainty is concerning to regulators, scientists, and other stakeholders of the marine environment (e.g., fishers), particularly in areas where species of special concern (e.g. endangered, threatened, or commercial) are present. To help address these concerns, FORCE developed the Fundy Advanced Sensor Technology (FAST) platforms, equipped with sensors to monitor physical and biological characteristics of the FORCE test area. The first deployment of a FAST platform at the FORCE site occurred from December 2015 to January 2016, and it included an upwardfacing echosounder, the ASL Acoustic Zooplankton and Fish Profiler (AZFP).
To better understand fish use of this site and their potential for interaction with TISEC devices, we examined how fish density and vertical distribution (measured by the AZFP) varied with respect to environmental factors, in particular tidal stage and time of day, and how these factors influence spatial overlap of fish with a TISEC device. The TISEC device considered here was the Cape Sharp Tidal (CST) device (OpenHydro design), which was later deployed at the FORCE site from November 2016 to June 2017.
The AZFP echosounder was found to perform well in the FORCE high-flow environment. All AZFP data were processed in Echoview® software to remove non-target hydroacoustic backscatter, most of which was from entrained air. This resulted in omission of the upper 10 m of the water column from analysis. Cleaned data were subsequently split into time-depth cells, echo-integrated, and exported for statistical analysis. The processing steps and templates created for this project can be applied to future AZFP data collected with the FAST platform.
The presence (relative density) and vertical distribution of fish were examined with respect to tidal and diel stages. We found that fish were almost constantly present during the data collection period, with higher densities during the flood tide than the ebb tide. Fish density was highest in the upper portion of the water column analyzed (above 15-20 m from the sea floor), though fish were more evenly spread throughout the water column at night than during the day. Species of fish could not be determined from the acoustic data, so we recommend using multiple acoustic frequencies in the future, alongside general knowledge of which species are in the area during data collection periods (e.g., by drawing on local knowledge of the fish species present, their migration timing, and their behaviors).
The observed vertical distributions of fish were used to generate basic spatial overlap probability models, which estimated the probability that fish within the passage cross-section might spatially overlap with (and therefore potentially encounter) a TISEC device under different vertical distribution scenarios. While there were apparent differences in the vertical distribution of fish in relation to tidal stage (e.g. from ebb to flood tide), the estimated overlap probabilities for a single TISEC device were very low (< 0.002). Spatial overlap probabilities, however, may become important for arrays of devices. The determination of encounter probability will require additional information, including a determination of the probability that a fish (preferably of known species) will pass through the Minas Passage during some defined time interval. Hydroacoustic data on the horizontal distribution of fish (across the Minas Passage), as well as data on nearfield fish behavior in response to TISEC devices in high-flow environments, would also aid the development of models of in-stream turbine effects on fish.