In this study we investigate small scale distributi on of pelagic fish within a windfarm by means of a high resolution sonar (DIDSON, Dual frequency IDentification SONar; Soundmetrics). In addition we assess the bias of small scale variations induced by the effects of wind turbines (monopiles) on distribution of the pelagic fish community in the hydro acoustic surveys carried out on the OWEZ Near Shore Wind farm (NSW).
A deployment for the use of a DIDSON sonar is presented as well as a survey technique from onboard a RIB (inflatable boat). The qualitative results from this study clearly show that the fish concentrations a round the monopiles are much higher in the first 15 – 20 meters. Overall fish density was on average a factor 37 higher above the scour bed around the monopiles than in the open water habitats in between monopiles.
We calculate the possible bias in a large scale hydro acoustic survey for densities around the monopiles. Avoidance does not lead to a significant bias. A density higher than 100 times the density in the open water leads to an underestimation of 8%. For the OWEZ windfarm, the factor 37 found from the DIDSON data would lead to an underestimation of 3% of the fish density.
More insight in the spatial and temporal dynamics of occurrence of pelagic fish is needed for a complete understanding of the speciesspecific response of pelagic fish to wind farms and monopiles. For this, hydro acoustic surveys and DIDSON observations are comple mentary, focusing on different spatial and temporal scales ranging from meters to kilometres, and from tidal, daily to seasonal. Although DIDSON has more potential for species identification than other acoustic methods, additional techniques are needed to determine species composition at the monopiles at the time of the large scale acoustic surveys.