Classification of Southern Ocean krill and icefish echoes using random forests

Niall G. Fallon, Sophie Fielding, Paul G. Fernandes

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23 Citations (Scopus)
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Abstract

Target identification remains a challenge for acoustic surveys of marine fauna. Antarctic krill, Euphausia superba, are typically identified through a combination of expert scrutiny of echograms and analysis of differences in mean volume backscattering strengths (SV; dB re 1 m−1) measured at two or more echosounder frequencies. For commonly used frequencies, however, the differences for krill are similar to those for many co-occurring fish species that do not possess swimbladders. At South Georgia, South Atlantic, one species in particular, mackerel icefish, Champsocephalus gunnari, forms pelagic aggregations, which can be difficult to distinguish acoustically from large krill layers. Mackerel icefish are currently surveyed using bottom-trawls, but the resultant estimates of abundance may be biased because of the species' semi-pelagic distribution. An acoustic estimate of the pelagic component of the population could indicate the magnitude of this bias, but first a reliable target identification method is required. To address this, random forests (RFs) were generated using acoustic and net sample data collected during surveys. The final RF classified as krill, icefish, and mixed aggregations of weak scattering fish species with an overall estimated accuracy of 95%. Minimum SV, mean aggregation depth (m), mean distance from the seabed (m), and geographic positional data were most important to the accuracy of the RF. Time-of-day and the difference between SV at 120 kHz (SV 120) and that at 38 kHz (SV 38) were also important. The RF classification resulted in significantly higher estimates of backscatter apportioned to krill when compared with widely applied identification methods based on fixed and variable ranges of SV 120–SV 38. These results suggest that krill density is underestimated when those SV-differencing methods are used for target identification. RFs are an objective means for target identification and could enhance the utility of incidentally collected acoustic data.
Original languageEnglish
Pages (from-to)1998-2008
Number of pages11
JournalICES Journal of Marine Science
Volume73
Issue number8
Early online date7 Apr 2016
DOIs
Publication statusPublished - Sept 2016

Bibliographical note

Acknowledgements
The authors thank the crews, fishers, and scientists who conducted
the various surveys from which data were obtained. This work was
supported by the Government of South Georgia and South
Sandwich Islands. Additional logistical support provided by The
South Atlantic Environmental Research Institute, with thanks to
Paul Brickle. PF receives funding from the MASTS pooling initiative
(TheMarine Alliance for Science and Technology for Scotland), and
their support is gratefully acknowledged. MASTS is funded by the
Scottish Funding Council (grant reference HR09011) and contributing
institutions. SF is funded by the Natural Environment
Research Council, and data were provided from the British
Antarctic Survey Ecosystems Long-term Monitoring and Surveys
programme as part of the BAS Polar Science for Planet Earth
Programme. The authors also thank the anonymous referees for
their helpful suggestions on an earlier version of this manuscript.

Keywords

  • acoustics
  • fish survey
  • South Georgia
  • target identification

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