Use of moored acoustic instruments to measure short‐term variability in abundance of Antarctic krill

Abstract

Upward‐looking acoustic Doppler current profilers (ADCPs) (300 kHz) and echosounders (125 kHz) were deployed on moorings at South Georgia to measure abundance of Antarctic krill continuously over several months. Echoes from krill were identified using the theoretical difference in echo intensity at 300 and 125 kHz and scaled to krill density using target strengths appropriate for krill in the region: krill size was determined from diet samples from furseals and penguins foraging near the moorings. A method using water flow past the moorings was developed to convert time‐based acoustic observations of krill to area‐based abundance estimates. Flow past the stationary moorings was treated analogously to motion along‐track of a research vessel through a nominally stationary body of water during a conventional survey. The moorings thus provide a Eulerian view of variation in krill abundance. This is ecologically instructive for South Georgia, where krill are generally passive drifters on currents and where temporal fluctuations in abundance have significant consequences for krill‐dependent predators. Moorings were positioned on routine research vessel survey transects, and validity of the mooring method was assessed by comparison of mooring and vessel observations. Krill density estimates from the moorings were not statistically different from vessel estimates in adjacent time periods. A time series of krill density from a mooring revealed step‐changes that were not seen during short‐term vessel surveys. Moorings deliver data over time scales that cannot be achieved from research vessels and provide insight on environmental factors associated with variation in krill abundance at South Georgia. Mooring data may aid ecosystem‐based management.

Use of bottom-mounted echo sounders in exploring behavior of mesopelagic fishes

“…Stationary splitbeam echo sounders can reveal the movements of individuals traversing the acoustic beam both horizontally and vertically (Ehrenberg & Torkelson 1996), and submerged echo sounders make it possible to establish the in situ swimming behavior of fish in deeper waters (Huse & Ona 1996). Autonomous systems may be battery powered and left at sea (Thomson & Allen 2000, Trevorrow 2005, Brierley et al 2006), or they can be cabled to land for power and transmission of data (Patel 2007). Simple, moored and cabled systems enable detailed, long-term and relatively low-cost studies, in contrast to studies made from research vessels.…”

Acoustic insights into the zooplankton dynamics of the eastern Weddell Sea

“…We followed Brierley et al (2006) and used beam-averaged data because the four beams together gave a better signal-to-noise ratio than individual beams. The noise level (E r ) of all four beams was determined from the minimum values of RSSI (Received Signal Strength Indicator) counts obtained in the remotest depth cell, when the sea surface was outside the ADCP range.…”

Toward a Global Ocean Ecosystem Mid-Trophic Automatic Acoustic Sampler (MAAS)

“…with current technology and methodology, it is now possible to provide ocean basin scale coverage similar to the successful ship of opportunity expendable bathy-thermograph network (Figure 2). In recent years new acoustic autonomous systems, stationary and drifting, have been tested and are giving genuinely new information [18][19][20][21] (Figure 3). …”