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



227 



found in the surface layer (0-15 m) during all four 

 time periods (Fig. 2). Although there were signifi- 

 cant (P=0.005) differences in density by depth 

 strata, neither the diel density differences alone 

 (P=0.838) nor the interaction between time and 

 depth (P=0.996) was significant. 



The majority of larvae, excluding pollock larvae, 

 from all collections combined were collected from the 

 upper three depth strata (Fig. 3). The maximum 

 density overall occurred at the second depth stra- 

 tum ( 15-30 m), below which larval density declined 

 with depth. However, this overall vertical distribu- 

 tion pattern was apparently confounded by higher 

 larval densities found during the night when the 

 larvae were mainly caught in the 15-30 m stratum; 



Dawn 



Day 



ro 



CD 



c 



Q. 



<D 

 O 



0-16 

 15-30 



30-45 

 45-60 

 60-80 

 80-100 

 > 100- 



} 



100 200 300 

 No/1000 m 3 



100 200 300 400 

 No/1000 m 3 



Dusk 



Night 



100 200 300 400 

 NO./1000 m' 



100 200 300 400 

 No/1000 m 3 



Figure 2 



Diel vertical distribution of Hippoglossoides elassodon eggs. Bars are 

 mean abundances per 1000 m 3 at each depth interval and error bars 

 are ± one standard deviation about the mean abundance. 



during the other three time periods the highest den- 

 sities were in surface waters (Fig. 4). The weighted 

 mean depth of larvae overall was significantly 

 (P<0.05) greater at night than during the other 

 three time periods (Table 3) and the interaction 

 between time and depth was marginally significant 

 (P=0.05; Table 4), suggesting that there were diel 

 differences in overall larval depth distribution. 



Four of the five most abundant larval taxa showed 

 the greatest weighted mean depths (Table 3) and the 

 lowest surface densities (Fig. 4) at night. This gen- 

 eral pattern was also evident in the two time peri- 

 ods examined for the fifth species, G. macro- 

 cephalus, but the diel differences were not signifi- 

 cant (Table 3). Only A. hexapterus and G. macro- 

 cephalus showed significant diel dif- 

 ferences in larval density, with high- 

 est densities occurring at night 

 (Table 4). None of the dominant 

 taxa, however, showed a significant 

 interaction between time and depth 

 strata. 



Length distributions 



The distribution of larval lengths by 

 time of day and depth showed no 

 consistent pattern among the three 

 most abundant species (Fig. 5). Al- 

 though time and time-depth interac- 

 tions were significant (all P<0.03) 

 factors in explaining the variation in 

 mean length of H. elassodon and 

 Bathymaster spp., none of the fac- 

 tors was significant for A. hexap- 

 terus. Examining only the strata 

 where more than two lengths were 

 available, we found that the small- 

 est larvae of both Bathymaster spp. 

 and A. hexapterus were caught in 

 the surface stratum at night but in 

 deeper strata during daylight hours 

 (Fig. 5). However, H. elassodon 

 showed an increase in mean length 

 with depth during daylight hours 

 and the reverse pattern at night 

 (Fig. 5). Hippoglossoides elassodon 

 was the only taxon to show a signifi- 

 cant difference in length distribu- 

 tions between night and day collec- 

 tions (Kolmogorov-Smirnov Test; 

 Z=3.881; P=0.001). Although the 

 lack of larger larvae in daytime col- 

 lections might suggest some daytime 

 gear avoidance by this species 



