Antarctic krill under sea ice: direct observations with an autonomous underwater vehicle

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Antarctic krill under sea icedirect
observations with an autonomous underwater
vehicle

• Andrew Brierley Paul Fernandes Mark
  Brandon University of St Andrews Marine
  Laboratory, The Open University
• British Antarctic Survey Aberdeen

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Antarctic krill

• Keystone species in the Southern Ocean
• Harvested commercially

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Krill are associated with sea ice
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Krill are associated with sea ice
RRS Discovery
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Sea ice extent varies seasonally
February 14 1994
September 14 1994
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and inter-annually

• Antarctic
• Circumpolar
• Wave
• 6 / 7 year period
• ENSO driven?

White and Peterson, 1996
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YEAR 1
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YEAR 2
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YEAR 3
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YEAR 4
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YEAR 5
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YEAR 6
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YEAR 7
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YEAR 8
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YEAR 9
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YEAR 10
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YEAR 11
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YEAR 12
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YEAR 13
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A response by krill to sea ice?
Hewitt and Linen Low, 2000
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Krill seem to be linked to ice, but

• Sampling difficulties prevent direct observations

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Krill seem to be linked to ice, but

• Sampling difficulties prevent direct observations

Divers have limited range
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Krill seem to be linked to ice, but

• Sampling difficulties prevent direct observations

Divers have limited range
Ships are disruptive
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Krill seem to be linked to ice, but

• Sampling difficulties prevent direct observations
• Evidence is circumstantial

Divers have limited range
Ships are disruptive
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Autosub new sampling technology

• Autonomous Underwater Vehicle
• 7 m 1 m
• Unmanned
• 1,600 m depth capability
• 3.5 knots speed
• 40 hours
• 260 km

• Designed and built in the UK by NERC
• Battery powered
• Pre-programmable

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The USIPS Project
We equipped Autosub with a scientific echosounder
and made the first broad-scale, direct
observations of krill under sea ice
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Marginal Ice Zone, 2001
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The Survey

• 7 Missions
• 210 km under ice

• Autosub deployed in open water from RRS
  James Clark Ross and surveyed in to ice-covered
  zone (max. 27 km under ice)
• Ship surveyed adjacent open water

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Survey Grid
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Acoustic observations of krill
under sea ice
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Distribution of krill swarms
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Krill Under Ice c.f. Open Water
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Krill distribution and ice concentration
ANOVA P lt 0.0001 r2 0.85
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Conclusions

• Direct link between krill distribution and sea
  ice cover observed for the first time
• These observations would not have been possible
  without Autosub
• Krill are concentrated just inside the ice edge
  an example of risk balancing?
• Reductions in sea ice area may have less grave
  consequences for krill than previously assumed .

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25 reduction in ice area in 1960s?

• 25 reduction in area ?
• 9 reduction in edge length
• Not a 25 reduction in habitat?

de la Mare, 1997
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Acknowledgements

• British Antarctic Survey
• Marine Laboratory, Aberdeen
• The Autosub team at Southampton Oceanography
  Centre
• UK Natural Environment Research Council
• Trans-Antarctic Association
• GLOBEC

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Other Data
Autosub has enable us to collect a wealth of
physical data in the Antarctic Icebergs Sea ice
thickness Illumination under sea ice
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Observations of iceberg draught
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Depth, m
Depth, m
100
125
150
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Observations of sea ice thickness
Antarctic before Autosub
Arctic
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Large data set on sea ice thickness
Proportion, range 0 to 0.1 m
Thickness, m