TITLE:    Validation of bioacoustic surveys for Antarctic krill,
          Euphausia superba

AUTHORS:  LB Quetin, RM Ross and CM Lascara (presented by LB Quetin 
	  and RM Ross)

MEETING:  ICES International Symposium on Fisheries and Plankton Acoustics

DATES:    12-16 June 1995


ABSTRACT: The Palmer Long-Term Ecological Research (LTER) program includes
acoustical surveys as part of the multidisciplinary sampling at stations
within the study region.  The study region (900 km by 200 km) is in
primarily shelf and slope waters west of the Antarctic Peninsula.  The
primary target species for these surveys with a 120 kHz BioSonics
echosounder is Antarctic krill, Euphausia superba.  However, 120 kHz also
detects other sound scatterers such as salps, pteropods, other euphausiids
and extremely heavy concentrations of phytoplankton.  Validation of the
acoustic surveys for the Palmer LTER is accomplished in two ways:  1) a 2 m
square net is towed simultaneously with the downward-looking 120 kHz
dual-beam transducer during acoustic surveys; and 2) targeted tows are
aimed at specific layers or patches seen acoustically.
   The 2 m net is towed obliquely from the surface to 120 m, the depth above
which virtually all krill schools are found.  Total volume of the catch and
the volume and number of the community of possible scatterers is determined
either on board or from preserved samples.  Echointegration estimates of
volume back-scattering from 6 to 190 m from the BioSonics echosounder is
converted to krill biomass with estimates of krill target strength and the
relative volumes and target strengths of the alternate scatterers from the
2 m net analysis.  Average krill biomass from the nets and acoustic surveys
is compared for four cruises throughout the year:  spring (Novebmer 1991),
summer (January 1993), fall (March/April 1993) and winter (August/September
1993).  Simultaneously towing the transducer and the net provides optimal
validation for a specific acoustic survey.  Targeted tows are used to
validate the interpretation of a specific type of trace or pattern of
back-scattering volume as belonging to a specific group of organisms.