Comparison of acoustic measurements of zooplankton populations using an Acoustic Water Column Profiler and an ADCP

Self-contained, moored echo sounders are a means of monitoring the behavior of populations of zooplankton and small fish over extended periods of time. Such instruments, either moored at or near the seafloor looking upward, or mounted on a surface buoy looking downward, record profiles of acoustic backscatter as a time series, and thus can provide insights into the long-term behavior and distribution of these populations. Single-frequency instruments are not capable of identifying the source of acoustic backscatter as species, but nevertheless can provide valuable information with low-cost, easily-deployed instrumentation over extended periods of time. This type of data can be collected either with an echo sounder designed for the task, or as an auxiliary output of an ADCP, using the RSSI (Received Signal Strength Indicator) output. In each case, without precise instrument calibration, an estimation of volume backscatter strength can be made from the data recorded by the two types of instrument. In this paper, we will compare the capabilities of an example of each type of instrument in terms of their spatial and temporal resolution and deployment endurance for extended monitoring. Calibration issues will also be discussed. In June 2004, a 200 kHz Acoustic Water Column Profiler (Version 4) and a 300 kHz RDI ADCP were co-located in Saanich Inlet, BC. The instruments were mounted on a surface buoy looking downward in 150 metres water depth for a period of 10 days. The two instruments were configured to operate with similar range and time resolution. The ADCP recorded a 30-ping ensemble every minute in 1 metre range bins. The AWCP4 recorded a 3-ping average every 12 seconds with 0.5 m range resolution. The sampling regions of the two instruments were not exactly co-located, but were close enough to show the same larger scale features. Echograms from the two clearly show the two primary zooplankton populations in the Inlet, one migrating diurnally, and the other remaining at depth. Data from both instruments, when converted to volume backscatter strength are in agreement within the limitations of their approximate calibrations. Examples of these will be shown. The difference in performance between the two instruments appears when longer or more-frequently sampled deployments are considered. With the spatial and temporal resolution used in the Saanich Inlet deployment, the ADCP is limited to a 20-day deployment on a standard battery pack (80 days would be possible if 2 more batteries were added in an external case). The AWCP4, in contrast, would last over 8 months operating with those parameters. Operating the ADCP to achieve high-resolution backscatter data also degrades its performance in measuring current velocity in most cases. The AWCP4 therefore allows greater temporal and spatial resolution over the extended monitoring periods of many months that are one of the primary motivations for using a single-frequency instrument. The AWCP4 has recently been replaced by a new model, the AWCP5, which has increased data storage (up to 16 GBytes vs. 138 Mbytes), greater flexibility in choice of sampling strategies and 16-bit as opposed to 8-bit digitization for greater dynamic range. The AWCP5 offers even greater advantages in time and space resolution and length of operation for acoustically monitoring zooplankton populations.