Detection Range of Acoustic Transmitters and Receivers in Deep Waters of Southeast Alaska

Advances in acoustic technology provide increasing ability to illuminate varied aspects of the lives of marine organisms, including their abundance and movements.  Detailed knowledge of movement dynamics can aid the assessment and management of commercially-important species such as sablefish (Anoplopoma fimbria), Pacific halibut (Hippoglossus stenolepis), and spiny dogfish (Squalus acanthias).   As a first step in evaluating the potential for establishing arrays of acoustic receivers to monitor movements of these and other marine fish species, we conducted tests to determine the detection range of transmission between VEMCO brand acoustic V16 transmitters and VR2W receivers in waters of Southeast Alaska.  These tests were necessary to help determine the effective receiver spacing for arrays in these waters.  Deep water (600 + m), large (5+ m) tide ranges, strong currents (3+ m/s), high energy storms and sea state, and periodic heavy surface vessel traffic are among the factors that can affect the detection of acoustic signals from fish instrumented with acoustic tags. We established six range test stations, each consisting of six subsurface moorings: one receiver mooring, with five transmitter moorings deployed at regular, increasing distances from the receiver mooring.  Minimum and maximum distances between transmitter and receiver anchors were 200 and 1200 m, respectively.  Water depth among stations varied from 195 to 585 m.  Transmitters were placed at 0.5 and 5 m off bottom at each mooring distance, and receivers were moored at 150 m, and at two stations with sufficient depth also at 400 m, below the surface.  Stations were distributed at locations that varied from a relatively shallow (154 m), narrow (3.8 km) ocean pass, through the mouth of a wide (20 km), deep (600 + m) fjord, to open ocean over the continental shelf up to 25 km offshore.  Receivers were deployed for periods ranging from 162 to 595 days. Total number of acoustic transmissions detected decreased with distance from the receiver, mostly monotonically; at first gradually, from transmitters at distances of 300 or 400 to 800 m from the receivers, and then more steeply at distances beyond 800 m.  The effects of tidal current, wind and vessel traffic on acoustic transmission-reception are addressed.