548 



Fishery Bulletin 92(3). 1994 



a 



Q 



100 



150 



200 



4.0 



4.5 



5.0 



5.5 



6.0 



250 



4.0 



4.5 



5.0 



Temperature CO 



5.5 

 Temperature CO 



6.0 



a 



24.5 



25.0 



25.5 



26.0 



25.0 



25.5 



Density (sigma-t) 



Density (sigma-t) 



Figure 7 



Mean depth of occurrence of walleye pollock, Theragra chalcogramma, 

 eggs in series one, two, and three in relation to temperature (A) and 

 density (B). Series numbers are indicated next to data points. Tem- 

 perature (C ) and density (D I profiles during series one, two, and three. 



Q 



4,00 4 50 



Temperature 



Figure 8 



Mean depth of occurrence of walleye pollock, Theragra 

 chalcogramma, larvae by tow in relation to tempera- 

 ture. Series numbers are near appropriate data points. 



Diel changes in larval depth 

 distribution 



There was no clear diel pattern in the 

 depth distribution of 5-6 mm larvae in 

 series four ( Fig. 4 ). During the first diel 

 sampling period the mean depths var- 

 ied from 25 to 37 m; the deepest mean 

 depth occurred at dusk. During the sec- 

 ond diel period, night was not sampled 

 and an aberrant catch occurred during 

 the noon sampling period (discussed 

 above). The mean depths varied from 

 24 to 58 m, if the catch in the deep net 

 of the noon tow is included, and from 

 21 to 30 m if that catch is excluded. 



When proportions of larvae at each 

 depth from series nine are examined, a 

 clear pattern of vertical distribution 

 emerges despite differences in overall 

 density among tows. Although the deep- 

 est stratum sampled was 125-150 m, 

 mean depths of occurrence of larvae 

 were between 14.6 and 38.1 m (range 

 of one SD=8.8-36.4 ml, and fewer than 

 V/c of all larvae were collected below 60 

 m. Within the upper 60 m, the larvae 

 showed evidence of limited diel vertical 

 migrations (Fig. 10). The observed mean 

 depth of larvae was greater at noon than 

 at other times ( 38 m and 33 m ) and shal- 

 lowest at dusk ( 15 m and 23 m). At night 

 the larvae were found somewhat deeper 

 ( 23 m and 29 m ). Like the dusk sampling, 

 the distribution at night appeared deeper 

 during the second day compared with the 

 first. At dawn the larvae were found be- 

 tween the night and noon depths (30 m and 31 m). 



Although the same pattern of changes in vertical 

 position of the larvae was observed during both 24- 

 hour periods of series nine, differences in observed 

 mean depths between various times of day were con- 

 sistently greater on the first day than on the second 

 (Fig. 10). The average difference in mean depth be- 

 tween sequential time periods on the first day was 

 13.2 m versus 6.2 m on the second (Table 4). The 

 mean depths at dawn on the two days were within a 

 meter of each other, but at the other sampling times 

 there were differences of 4.8-8.4 m between mean 

 depths on the two days at the same time of day. Dif- 

 ferences were especially pronounced at dusk and 

 night. Examination of standard deviations of mean 

 depth and mean lengths of larvae among tows of se- 

 ries nine showed no consistent pattern of differences 

 with any of the variables under consideration (time 

 of day, depth, day). 



26.0 



