Midwater Assessment & Conservation Engineering (MACE) Program
Bering Sea Ice Edge Cruise
Scientists from the Midwater Assessment and Conservation Engineering (MACE) program
conducted an acoustic-trawl survey in the vicinity of the marginal ice zone in
the eastern Bering Sea 20-27 April aboard the NOAA ship Miller Freeman.
The survey occurred primarily north of the Pribilof Islands. This research was
part of an interdisciplinary effort with other AFSC scientists from Recruitment
Processes/EcoFOCI, the National Marine Mammal Laboratory (NMML), and the Resource
Ecology and Fisheries Management (REFM) Division, and scientists from the Pacific
Marine Environmental Laboratory (PMEL), Institute of Marine Science of the University
of Alaska Fairbanks, and Scripps Institute of Oceanography who were working aboard
the University of Washington’s research vessel Thomas G. Thompson in
close proximity to the Miller Freeman. (See also Recruitment
Processes program report and the Seattle
Times sea-ice research article.)*
Acoustic data were collected over 1,000 nautical miles (nmi) of trackline aboard
the Miller Freeman and provisionally grouped into three different zones: an ice-free
zone, a marginal ice zone, and a zone along the ice edge itself. Acoustic data
were collected with a Simrad EK60 echosounder system and split beam transducers
operating at 18, 38, 120, and 200 kHz. Species-specific, frequency-dependent
differences in the intensity of the acoustic backscatter between 18-200 kHz were
used to determine the species classification of the scattering layers. Observations
of sea-ice characteristics and coverage as well as physical oceanographic data
were also collected.
Very little echosign was attributable to fish thoughout
the study area. An extensive and persistent sound scattering layer was observed
at 120 and 200 kHz. This layer exhibited diel vertical migrations in that it
was distributed primarily within 30 m of the seafloor during daylight hours and
moved upwards about 60-80 m to within 20 m of the surface during darkness. Methot
trawl hauls were used to groundtruth the acoustic data and confirmed that this
layer was dominated by backscatter from adult euphausiids and jellyfish (Chrysaora
melanaster).
