Acoustic assessment of spawning walleye pollock, Theragra chalcogramma, off 
                         western Hokkaido, Japan, October 1996

 Hiroya Miyake*1, Tomonori Kaneta*1, Masami Mizuno*1, Hideo Yoshida*2, Ryotaro 
             Ishida*2, Masashi Natsume*3, Kohji Iida and Tohru Mukai*4

		*1: Hokkaido Wakkanai Fisheries Experimental Station
		*2: Hokkaido Central Fisheries Experimental Station
		*3: Hokkaido Abashiri Fisheries Experimental Station
		*4: Faculty of Fisheries, Hokkaido University

     In October 1996, a stock  of spawning walleye pollock (Theragra chalcogramma) 
was  acoustically surveyed  with an  EK500 and  a BI500  in  order to  estimate the 
distribution pattern of these pollock during the pre-spawning season in the Japan Sea 
off western  Hokkaido. Relatively high abundances  of walleye pollock were  observed 
in the southern area, and almost all were found within 30 nautical miles of the coast. 
The spawning season normally begins  in December, but the spawning migration had 
already begun  and walleye pollock  were nearing the  spawning grounds in  October. 
Walleye pollock were distributed in a layer at a depth of 350 - 500 m in the offshore 
region, and  were distributed  near the bottom  layer in  the continental shelf  region. 
Moreover, pollock in the offshore region were distributed in the same depth layer and 
in the same  school structure during both day  and night; thus pollock did  show diel 
vertical migration.
     Since walleye pollock were distributed near the coast, we do not need to  extend 
the survey area west of  139 degrees E, thereby keeping the cost of the  survey to a 
minimum. Moreover, since pollock  did not migrate vertically on a  diel basis, we can 
make surveys to  estimate the abundance of  walleye pollock all day  long. For these 
reasons we concluded that October is a good season  for stock assessment of walleye 
pollock