Brodeur and Rugen: Vertical distribution of ichthyoplankton in the northern Gulf of Alaska 



225 



salinity were measured continuously in real time dur- 

 ing the tow and stored for later analysis. 



To examine diel variations in density and size of 

 larvae with depth, collections from the 22 tows were 

 grouped into one of four time periods (hours): dawn 

 (0530-0830), day (0830-1830), dusk (1830-2130), 

 and night (2130-0530). Diel-depth variation in den- 

 sity of eggs and larvae at each depth was examined 

 by using a two-way ANOVA on log-transformed data. 

 The log (X+l) transformation was used to achieve 

 homogeneous variances (Bartlett's Test, all P>0.05). 

 In addition, a weighted mean depth of occurrence 

 of eggs or larvae of the dominant species for each 

 time interval was calculated as follows: 



IIXa,, 



A- 





m 



where n ( = number of tows in time interval t , 



N = number of larvae in net j in tow i in 



'.it 



time interval t, 



D = midpoint depth of net j in tow i in time 



interval t with a variance equal to 



Var(D t ) = 



2>2(Z) a -A> 2 , 



K-i) 



& 



where N (/ = number of larvae in tow i in time 



interval t. 

 Differences in the weighted mean depths over the 

 four time periods were tested with ANOVA, and 

 Tukey multiple-comparison tests were conducted 

 when significant differences were observed. 

 Untransformed larval lengths for the three most 

 abundant species were entered as dependent vari- 

 ables in two-way ANOVAs, with time of day and 

 depth as factors. 



Results 



Species composition 



Eggs and larvae of species other than walleye pol- 

 lock were found in 134 of the 145 samples collected 



