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

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 fur seals 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.

[1]  E. John Simmonds,et al.  Fisheries Acoustics: Theory and Practice , 2005 .

[2]  P. A. Prince,et al.  Diet, provisioning and productivity responses of marine predators to differences in availability of Antarctic krill , 1999 .

[3]  Julian Priddle,et al.  Scales of Interaction Between Antarctic Krill and the Environment , 1988 .

[4]  J. Fischer,et al.  Seasonal variation of the daily zooplankton migration in the Greenland sea , 1993 .

[5]  K. Foote.  Calibration of acoustic instruments for fish density estimation : a practical guide , 1987 .

[6]  Andrew S. Brierley,et al.  Interannual variability in krill abundance at South Georgia , 1997 .

[7]  Kyounghoon Lee,et al.  Application of acoustic Doppler current profiler combined with a scientific echo sounder for krill Euphausia pacifica density estimation , 2004 .

[8]  P. Trathan,et al.  Spatial variability of Antarctic krill in relation to mesoscale hydrography , 1993 .

[9]  Andrew S. Brierley,et al.  A post-processing technique to remove background noise from echo integration data , 1996 .

[10]  E. Murphy,et al.  Antarctic ecosystem monitoring: quantifying the response of ecosystem indicators to variability in Antarctic krill , 2005 .

[11]  Robert J. Urick,et al.  Principles of underwater sound , 1975 .

[12]  Marine Zooplankton Colloquium.  Future marine zooplankton research-a perspective , 1989 .

[13]  L. V. M. Traykovski,et al.  Effects of orientation on acoustic scattering from Antarctic krill at 120 kHz , 1998 .

[14]  M. Godlewska.  Vertical migrations of krill [Euphausia superba Dana] , 1996 .

[15]  E. Murphy,et al.  Physical oceanographic conditions to the northwest of the sub‐Antarctic Island of South Georgia , 2000 .

[16]  Parkes,et al.  Interannual variation in condition of the mackerel icefish , 1997, Journal of fish biology.

[17]  P. Brehmer,et al.  Fisheries Acoustics: Theory and Practice, 2nd edn , 2006 .

[18]  R. Locarnini,et al.  Krill transport in the Scotia Sea and environs , 1998, Antarctic Science.

[19]  Ronald Pedersen,et al.  A comparative acoustic-abundance estimation of pelagic redfish ( Sebastes mentella ) from hull-mounted and deep-towed acoustic systems , 2003 .

[20]  Temporal variability of euphausiid concentrations in a Nova Scotia shelf basin using a bottom-mounted acoustic Doppler current profiler , 1994 .

[21]  E. Murphy,et al.  Southern Antarctic Circumpolar Current Front to the northeast of South Georgia: Horizontal advection of krill and its role in the ecosystem , 2004 .

[22]  G. Tarling.  Sex-dependent diel vertical migration in northern krill Meganyctiphanes norvegica and its consequences for population dynamics , 2003 .

[23]  Andrew Constable,et al.  Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR) , 2018, Yearbook of International Cooperation on Environment and Development 1998–99.

[24]  G. Jolly,et al.  A Stratified Random Transect Design for Acoustic Surveys of Fish Stocks , 1990 .

[25]  M. Naganobu,et al.  CHARACTERISTICS OF SEASONAL VARIATION IN DIURNAL VERTICAL MIGRATION AND AGGREGATION OF ANTARCTIC KRILL (EUPHAUSIA SUPERBA) IN THE SCOTIA SEA, USING JAPANESE FISHERY DATA , 2005 .

[26]  P. Ward,et al.  Differences in backscattering strength determined at 120 and 38 kHz for three species of Antarctic macroplankton , 1993 .

[27]  Spatial and temporal variability of Antarctic krill: implications for stock assessment , 1994 .

[28]  M. Brandon,et al.  Tracking passive drifters in a high resolution ocean model: implications for interannual variability of larval krill transport to South Georgia , 2004 .

[29]  Andrew S. Brierley,et al.  Acoustic discrimination of Southern Ocean zooplankton , 1998 .

[30]  William M. Hamner,et al.  Behavior of Antarctic krill (Euphausia superba): schooling, foraging, and antipredatory behavior , 2000 .

[31]  I. Everson,et al.  ACOUSTIC ESTIMATES OF KRILL DENSITY AT SOUTH GEORGIA, 1981 TO 1998 , 1999 .

[32]  A. Brierley,et al.  Concordance of interannual fluctuations in acoustically estimated densities of Antarctic krill around South Georgia and Elephant Island: biological evidence of same-year teleconnections across the Scotia Sea , 1999 .

[33]  Michael J. McPhaden,et al.  Correcting moored ADCP data for fish-bias errors at 0⁰, 110⁰W and 0⁰, 140⁰W from 1990 to 1993 , 2000 .

[34]  Andrew S. Brierley,et al.  Oceanographic variability and changes in Antarctic krill (Euphausia superba) abundance at South Georgia , 2003 .

[35]  A. Brierley,et al.  The Southern Antarctic Circumpolar Current Front: physical and biological coupling at South Georgia , 2002 .

[36]  Egil Ona,et al.  Acoustic sampling and signal processing near the seabed: the deadzone revisited , 1996 .

[37]  R. Hewitt,et al.  Bias in acoustic biomass estimates of Euphausia superba due to diel vertical migration , 1995 .

[38]  Verification of the acoustic techniques used to identify Antarctic krill , 2002 .

[39]  Andrew S. Brierley,et al.  An assessment of the utility of an acoustic Doppler current profiler for biomass estimation , 1998 .

[40]  V. Loeb,et al.  Krill (Euphausia superba) density, proportional and absolute recruitment and biomass in the Elephant Island region (Antarctic Peninsula) during the period 1977 to 1997 , 1998, Polar Biology.

[41]  P. Wiebe,et al.  Patterns and Processes in the Time-Space Scales of Plankton Distributions , 1978 .

[42]  Validation of the stochastic distorted-wave Born approximation model with broad bandwidth total target strength measurements of Antarctic krill , 2003 .

[43]  K. Deines,et al.  Backscatter estimation using Broadband acoustic Doppler current profilers , 1999, Proceedings of the IEEE Sixth Working Conference on Current Measurement (Cat. No.99CH36331).

[44]  C. Cotté,et al.  Formation of dense krill patches under tidal forcing at whale feeding hot spots in the St. Lawrence Estuary , 2005 .

[45]  A. Brierley,et al.  THE USE OF PREDATOR-DERIVED KRILL LENGTH-FREQUENCY DISTRIBUTIONS TO CALCULATE KRILL TARGET STRENGTH , 2006 .

[46]  Stéphane G. Conti,et al.  New target-strength model indicates more krill in the Southern Ocean , 2005 .

[47]  Ian L. Boyd,et al.  THE BEHAVIORAL BASIS FOR NONLINEAR FUNCTIONAL RESPONSES AND OPTIMAL FORAGING IN ANTARCTIC FUR SEALS , 2004 .

[48]  David A. Demer,et al.  Measurements of three-dimensional fish school velocities with an acoustic Doppler current profiler , 2000 .

[49]  John K. Horne,et al.  Acoustic approaches to remote species identification: a review , 2000 .

[50]  A. Brierley,et al.  Interannual variability of the South Georgia marine ecosystem : biological and physical sources of variation in the abundance of krill , 1998 .