Steelhead Archival and Acoustic Tag Project

Objective 2

Measure temporal and spatial movement patterns of steelhead kelts returning to the marine environment. 

The ultrasonic gate surrounding the Ninilchik River and Deep Creek was constructed by mooring VR2 Single channel Monitors (right) around the combined boundary of each river mouth (below). Data loggers were suspended 5 meters from the bottom on short moorings with subsurface floatation and surface marking buoys. Disposable, degradable anchors were used to secure the moorings to the bottom.  Acoustic release units were used on three buoy moorings deployed at offshore locations as an experimental design for future arrays deployed at greater depths. The buoy array set up and recovery was accomplished by chartering a boat equipped with commercial fishing gear from a local charter fishing service. The VR2 data loggers are dedicated remote monitors designed to detect coded acoustic tags and will record the acoustic tag's individual identification and the date and time when a tagged animal comes into range. The VR2 monitors have a battery life of 180 days and can store 300,000 sonic tag detections.  They must be retrieved to be downloaded but can be redeployed at the same time.

The VR2 data loggers can receive and decode signals from any direction.  However, there are three factors that affect the reception radius of the data loggers: 1) distance the tagged animal is from the bottom, 2) swimming speed of the tagged animal, and 3) the pulse rate of the acoustic tags.  Increase in any of these factors results in the decrease of the reception radius of the data loggers.  Therefore, the exact spacing will be determined by the equation below.

The basic design goal is to determine the detection radius, r, at which an acoustically tagged animal can be identified under different oceanographic conditions.  Knowing r, it is possible to determine the spatial separation, D, for the receivers to ensure that an animal crossing the array at right angles has a high probability of being detected.

A previous water sample survey looking at the fresh water discharge and salt water mixing in the study area aided in determining  that the receivers have approximately a 400 meter detection radius.  The receivers were placed at 800 meter intervals covering nearly 6.4 kilometers of coast line extending out 1500 meters from shore, thus forming an acoustic gate around the near shore waters of the study area. In addition to the outer perimeter, a moored receiver was placed near the mouth of each river.

                                         A VR2 Acoustic Receiver Attached to a Mooring Line

(continued to Objective 3)

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