MAYNARD ET AL.: HAWAIIAN MESOPELAGIC MICRONEKTON 



for one deep tow (no. 183). This is about nine times 

 the mean abundance of micronektonic 

 euphausiids, but only one-fifth of the mean 

 euphausiid biomass. 



To establish a rough index of the contribution of 

 neritic and meroplanktonic animals to the pelagic 

 catch for future comparison, the standing stock of 

 stomatopod larvae (otherwise included in the 

 zooplankton group (Table 5)) was compared to the 

 total micronekton stock (Table 6). The stomatopod 

 concentration is large relative to shallow-day 

 micronekton catches but quite small with respect 

 to shallow-night and especially to deep-tow 

 catches. 



Table 6.-Mean standing stock of stomatopod larvae expressed 

 as percent of total micronekton standing stock per 100 m- of 

 ocean surface, number of organisms and wet-weight biomass. 



Item 



0-1,200 m 



Day 

 0-400 m 



Night 

 0-400 m 



Number (%) 

 Biomass (%) 



4.3 

 0.6 



58.3 

 12.9 



11.4 

 2.9 



Diel Vertical Migration 



Because there were no significant differences 

 between standing stock estimates of deep-day and 

 deep-night tows, we concluded that any trawl 

 avoidance was not due to diel factors and that 

 increased shallow-night catches were primarily 



the result of vertical migration. Thus, the amount 

 of vertically migrating micronekton was the 

 difference between the shallow-day stock and the 

 shallow-night stock (Tables 4, 5, 7). The percent 

 migrating was then computed by dividing the 

 amount migrating by the amount of micronekton 

 deeper than 400 m during the day (deep-tow 

 standing stock minus shallow-day stock). Percent- 

 ages larger than 100 are considered sampling ar- 

 tifacts. 



Of the total micronekton which resided deeper 

 than 400 m during the day, 43% of the individuals 

 with 47% of the biomass migrated into the upper 

 400 m at night (Table 7). This dramatic diel change 

 in the catch rates of shallow tows is illustrated in 

 Figures 4 and 5. The most numerous migrators 

 were crustaceans, but most of the biomass was 

 fishes. Between day and night tows the difference 

 in composition of the 0- to 400-m layer of fauna is 

 quite pronounced (Figure 3b, c). 



The average weight per organism in each group 

 for the 0- to 1,200-m deep water column was com- 

 puted by dividing the mean group biomass in Ta- 

 ble 5 by the mean number of organisms in the 

 group shown in Table 4. The results are presented 

 in Table 8 along with computations of the average 

 biomass of individual migrators and non-migra- 

 tors, based on the data in Table 7 and assuming 

 that the non-migrator standing stock was the 



Table 7.— Mean standing stock of vertically migrating micronekton, grams 

 wet-weight biomass and number of organisms per 100 m^of ocean surface 

 between and 1,200 m, groups ranked by biomass and abundance. Percent 

 migrating represents the portion of the group residing deeper than 400 m 

 during the day which migrated into the 0- to 400-m layer at night. 



iO-400-m day stock > 0-400-m night stock. 



731 



