LOEB: VERTICAL DISTRIBUTION OF LARVAL FISHES 



metamorphic stages only; juvenile and adult in- 

 formation is provided separately. I was able to 

 identify most of the abundant larvae to species. 

 Some of the larvae, however, could not be iden- 

 tified with certainty. For these I assigned "proba- 

 ble" and "possible" species names. These designa- 

 tions were based on the accumulation of enough 

 larvae to establish developmental links to known 

 adult species (Loeb 1979b, >29,000 larval iden- 

 tifications) and a list of adult central gyre meso- 

 pelagic fish species and their relative abundances 

 (Barnett 1975). 



For analyses of abundance distributions with 

 depth, the larval fish catches from each sample 

 were converted to numbers per 1,000 m^. These 

 were averaged for each depth interval and then 

 summed to provide estimated water column abun- 

 dance. Individual species distributions are ex- 

 pressed as percent of their estimated water col- 

 umn abundance caught within each of the main 

 depth intervals sampled. The 100-225 m catches 

 were used in calculations only if they contained 

 species not present in 100-350 m or 350-600 m 

 samples. The 100-225 m and 100-350 m samples 

 were compared to assess whether larval abun- 

 dance was concentrated in the upper portion of the 

 larger depth range. 



Significance of differences in size composition 

 with depth were determined, where sample sizes 

 permitted, using the Kolmogorov-Smirnov test 

 (Conover 1971) on cumulative size-frequency dis- 

 tributions of 0.5 mm (SL) categories of the total 

 larvae taken (all samples combined) within each 

 depth interval. A one-tailed probability of the 

 maximum difference between cumulative size- 

 frequency distributions in two depth strata s0.05 

 was deemed "significant." Rejection of the null 

 hypothesis of no difference indicates that one of 

 the size distributions being compared is sig- 

 nificantly larger than the other. The results of 

 these tests are in no case altered by the exclusion 

 of larvae from the "day" samples in their calcula- 

 tion. 



Descriptions of the developmental stages of 

 myctophid larvae in the plankton include addi- 

 tional information obtained from six other central 

 gyre cruises in the vicinity of lat. 28° N, long. 155° 

 W. These cruises utilized Isaacs-Kidd plankton 

 trawls (IKPT) fished obliquely from the surface to 

 about 300 m (Loeb 1979b). Those data (24,500 

 identified larvae) provide a broader range of larval 

 sizes and developmental stages, and development 

 from eairly larvae to metamorphosis (transforma- 



tion) has been traced for many species. I used this 

 information as an aid for estimating levels of de- 

 velopment reached while larvae are still in the 

 upper water column, prior to descent to deeper 

 juvenile-adult depths. 



RESULTS 



I identified a total of 5,448 larvae (Table 2). 

 These included 94 generic and species identifica- 

 tions from 36 families, and one ordinal grouping. 

 Three families (Gonostomatidae, Sternop- 

 tychidae, and Myctophidae) together contributed 

 91% of the individuals and 50% of the species. 



Larvae were taken throughout the 600 m depth 

 range (Figure la); however, over 97% of the esti- 

 mated water column abundance was in the upper 

 100 m. Only 13 of the 95 kinds of larvae appeared 

 to have maximum abundance below 100 m. 

 Maximum larval abundance (and diversity, or 

 number of species) occurred within the 25-50 m 

 interval (Figure la); the bottom of the summer 

 mixed layer (ca. 40 m) is within this interval (Fig- 

 ure lb). Total larval abundance (Figure la) as well 

 as individual species abundances were highly var- 

 iable from tow to tow within each interval. Despite 

 this variability, most species demonstrated a 

 definite peak of abundance (generally >60% of 

 their estimated water column abundance) and 



MEAN AND RANGE 

 LARVAE PER l.OOOm^ 





 25 

 50 

 75 

 100. 



500 



1000 



^225 

 Q 



350- 



600 J 



•(10) 



'(10) 



-(10) 

 "(10) 



(6) 



(8) 



(a) 



FIGURE 1.— Vertical distribution of ichthyoplankton in relation 

 to lat« summer thermal structure in the North Pacific central 

 gyre, (a) Mean and range of total numbers of larvae per 1,000 m^ 

 caught in replicate samples within each depth interval (bracket- 

 ed values are numbers of replicate samples), (b) Temperature 

 profile of upper water column during Climax I, based on average 

 values fi-om 10 day and 7 night STD lowerings. 



779 



