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Cetacean Assessment & Ecology Program

Passive Acoustic Monitoring to Study Belugas in Cook Inlet, Alaska: A Novel Approach or a Novel Nightmare?(pg. 2. 1,)

Research Reports
Winter 2015
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Figure 2. Schematics and images of the CIBA mooring packages containing an Ecologic Acoustic Recorder (EAR), a Cetacean and Porpoise Detector (C-POD), and an acoustical release (EdgeTech PORT) deployed in Cook Inlet, Alaska, in a) the sandwich configuration used in early 2010; b) the sphere and tripod configuration used in late 2010; and c) the small sphere tethered to the anchor configuration used in 2011. In the schematics, 1 = Edgetech PORT, 2 = C-POD, 3= EAR, 4 = (a) syntactic/(b and c) urethane foam float; 5 = sacrificial anchor made of pieces of 57.0 kg/m railroad track: (a) 70-kg single piece, (b) 204-kg multiple-piece tripod, and (c) 140-kg double piece.

The sandwich design proved much more effective throughout Cook Inlet. However, an even more compact and robust design was necessary to sample the very shallow waters in upper Cook Inlet (i.e., 30 ft at mean low water), particularly over winter; so, two more designs were created for these extremely shallow areas. A “sphere” design was developed in late 2010 to reduce the vertical profile and current drag; increase the protection of the instruments from collisions with ice, debris, pebbles, or coarse sediment; and increase the resistance to burial or sediment entrapment (Fig. 2b). A “small sphere” design was developed in 2011, based on the same concept as the initial sphere but reduced in size and weight to facilitate deployments and recoveries from the small vessels used to access the shallow waters (Fig. 2c). Both sphere designs replaced the sandwich design at deployment sites in upper Cook Inlet. The acoustic instruments and acoustical release were clustered inside the urethane foam sphere, which was 66 cm in diameter with a 68-kg in-air weight. The sphere was attached to a 204-kg sacrificial tripod made of 57.0 kg/m railroad track pieces. Both the sphere and the tripod were covered with dampening material to reduce the noise created by colliding debris in high current periods. Several dampening materials (such as burlap fabric, polyethylene fiber, and epoxy coating) were tested, but a 1/8-inch butyl rubber sheet had the best performance. The small sphere’s size and weight were reduced to a diameter of 51 cm and an in-air weight of 39 kg, and it was tethered to a sacrificial anchor, made of two 70-kg pieces (each 122 cm long) of 57.0 kg/m railroad track welded together, instead of to a heavier and bulkier tripod. The attachment point of the anchor was centered to increase drag during strong current periods. The battery cells and electronics from both acoustic instruments were extracted from their original housings and placed in custom-made, aluminum housings that were smaller and more compact.

After 2 years of continuous testing and mooring design improvement, the CIBA program was successful in collecting high-quality, year-round acoustic data throughout the inlet. These data, which will soon be published, are providing valuable information on Cook Inlet beluga seasonal distribution year-round. More than 80 mooring deployments were made between 2008 and 2013. A total of six moorings were lost in the process, and multiple instruments were damaged. Several lost moorings were later found, with the distance adrift “record” currently held by a sphere that beached on Middleton Island (in the Gulf of Alaska) 3 years after deployment in northern Knik Arm (upper Cook Inlet). The shallowest parts of Knik Arm, north of Anchorage, remain a challenge, particularly for overwinter deployments. The CIBA team’s most recent design to monitor this area has explored auger-assisted deployment of reinforced-housing instruments into the sediment at the water level during extreme low tides (Fig. 3). This “silo” design maintains the instrument at ground level with just the hydrophone, protected by solid aluminum bars, exposed to the water. Two silos were successfully tested for the first time in a 2012-13 overwinter deployment, providing the first winter detections of belugas visiting the upper end of Knik Arm. Silos are still being deployed in Cook Inlet as part of this research effort.

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Figure 3. Auger-assisted deployment of reinforced-housing instruments into the sediment at the water level during extreme low tides in shallow areas of Knik Arm, Cook Inlet, Alaska. The( a) acoustic instrument, covered by a reinforced housing, is( b) inserted into a drilled hole in the sediment, allowing just the hydrophone to be exposed to the water. A silo (c) deployed in fall 2013 and (d) recovered in spring 2014.

The CIBA program’s efforts to develop a “Cook Inlet resistant” acoustic mooring has prompted interest from other research groups working in shallow waters in Alaska and other states. The sandwich-mooring configuration is currently being used in a cetacean study in Kotzebue Sound, and researchers developing studies at St. Lawrence Island (in the Bering Sea) and in California are considering these mooring designs.

 

By Manuel Castellote 


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