Acoustic survey of the diurnal vertical migration of Pacific hake off
      			south-west of Vancouver Island

     Akira Hamano (National Fisheries Univ.), Robert Kieser, Ken Cooke,
         Gordon A. McFarlane ( Fisheries and Oceans, Canada)
                  and Toyoki Sasakura(Sasa laboratory) 

    Diurnal vertical migration of Pacific hake (Merluccius productus) at the 
outer edge of the continental shelf off south-west of Vancouver Island was 
acoustically investigated in early summer of 1994, by the two vessels sailing 
along parallel courses, one of which was equipped with the quantitative sounder 
system using 120 kHz quasi-ideal beam and 50 kHz conventional one while the other
 with 38 kHz conventional one.
  The species composition responsible for mid-water echoes in daytime was 
verified to be Pacific hake by the mid-water trawl.  The results of the 24 hour
 echo survey confirmed a well-defined, dense day time layer at 150 to 250 m 
depth, that often extended outward from the continental shelf edge. At dusk this
 layer would dissipate, as Pacific hake rose to near surface waters.
 Intense wide spread scattering was observed  below the hake aggregations at 38
 and 50 kHz. This DSL (deep scattering layer) was sampled by a mid-water trawl 
with a fine cod-end liner. Samples from about 350 m depth were dominated by two
 species of micronekton (Stenobrachius leucopsarus and Thysanoessa spinifera). 
These species are assumed to be primarily responsible for the observed 
scattering.