Testing fish deterrents for use under-ice in the Mackenzie Delta area

Firstly, the density of the fish population in the target lake at the time of the study was not expected to be high enough to allow a meaningful distribution map to be built through a sequential echo-sounding process, as the motion of individual fish in a sparse population would undermine the sampling process; moreover the assumption of a “steady state” condition following the activation of the de [...] An ensemble photograph of the equipment layout as seen from the shore of the lake near the instrumentation site is shown in Figure 4. The pair of floats at the left edge of the image and their counterparts on the opposite side of the net pen maintained tension on the riser lines from the anchor blocks, to which the hydrophones were attached. [...] In actual practice, due to the difficulty of monitoring the instrument readouts and adaptively adjusting the aim from the unstable platform of a small boat, we found it more feasible to set the correct heading by visual observation of the light beam briefly tilted upward and then bring the unit to level attitude, relying on the suspension ropes at the front and back to maintain the orientation. [...] In the standard runs the tracking software was started about ten minutes ahead of the activation of the deterrent stimulus, thus providing baseline monitoring of the swimming behaviour of the subjects in the pen. [...] The HTI software inserts in the header of its output files the start time of the tracking based on the PC clock, and precisely times each data point relative to that reference; the stimulus period was also timed by the operator based on the PC clock or a chronograph synchronized with it, thus allowing correlation of the tracking data with the stimulus to within one second.