LOEB: VERTICAL DISTRIBUTION OF LARVAL FISHES 



Most of the smallest C. alba larvae apparently 

 occur near the bottom of the mixed layer, and 

 move first up and then downward with increasing 

 size and development; the most advanced stage 

 attained in 0-100 m is prometamorphic. A rapid 

 descent may then occur, indicated by the neair 

 absence of any individuals in 100-225 m and 100- 

 350 m samples. Photophore completion and 

 metamorphosis probably occur at depths >350 m, 

 in agreement with Ahlstrom's (1974) report that 

 only the white photophore stage oiCyclothone spp. 

 is found above 200 m, and that more advanced 

 stages occur deeper in the water column. 



Kobayashi (1973) gives a 300-1,000 m depth 

 range for adult Pacific C. alba. He found that the 

 individuals occurring in the range of maximum 

 abundance (400-600 m) were smaller than those 

 occurring shallower or deeper; intermediate-sized 

 adults were shallower and largest adults were 

 deeper. This size distribution of the adults paral- 

 lels that found herein for the larvae, although 

 shifted well downwards in the water column. 



Cyclothone sp. A is probably the larval form of C. 

 pseudopallida, and is the only other larval Cyclo- 

 thone species found in abundance in the central 

 gyre (Loeb 1979b). Fifty-nine of the sixty larvae 

 caught were from the upper 50 m, and maximum 

 abundance was at 0-25 m (Table 3). Median stan- 

 dard lengths increased with depth but, due to the 

 small sample sizes, significance of differences in 

 size-frequency distributions could not be tested. 

 No metamorphic stages were taken either in the 

 stratified tows or among the 365 Cyclothone sp. A 

 larvae taken in 0-300 m IKPT samples during 

 other gyre cruises. Most of esirly larval develop- 

 ment may occur at 0-50 m, with a subsequent 

 rapid descent to the juvenile-adult depth ranges 

 (500-900 m; Kobayashi 1973). 



VINCIGUERRIA SPP. —Vinciguerria nimbaria 

 was the second most abundant species caught {9% 

 of all larvae). On a year-round basis it is the most 

 abundant larval fish species taken in the gyre 

 (Loeb 1979b). The larvae occurred in samples 

 from to 350 m (Figure 3b), but were consistently 

 present ( 29 out of 30 samples ) only between 25 and 

 75 m. Ninety percent of the estimated water col- 

 umn abundance was between 25 and 75 m ( 74% at 

 25-50 m). Abundances in replicate tows within the 

 25-50 m depth interval were significantly greater 

 (P<0.01) than in any other interval (Table 3). 



Samples from 25 to 100 m contained a wide 

 remge of larval sizes. However, median standard 



length increased with depth (Table 3 ) and cumula- 

 tive size-frequency curves from 25-50 m and 50-75 

 m (Figure 5) were significantly different from each 

 other. The proportion of metamorphosing indi- 

 viduals increased below 50 m (Table 4). These 

 included the prometamorphic (white photophore), 

 midmetamorphic (rapid body shape change), and 

 postmetamorphic (photophore completion and 

 body pigmentation) stages described by Ahlstrom 

 and Counts (1958). All V. nimbaria present in 

 0-25 m samples were early larvae, as were most in 

 the 25-50 m samples (only 3% from 25-50 m were 

 pro- or midmetamorphic). In contrast, 75% from 

 50-75 m were in metamorphic stages. Size dis- 

 tribution at 75-100 m was essentially bimodal: 

 40% of the larvae were very small (3.5-6.0 mm) 

 and 50% were metamorphic (10.5-17.5 mm). No 

 juveniles or adults were taken. 



Vinciguerria poweriae was much less abundant, 

 and had a deeper distribution than did its con- 

 gener; it occurred from 50 to 350 m (Table 3), with 

 maximum abundance at 75-100 m. There was a 

 trend for increased size with depth (Table 3). Only 

 four metamorphosing individuals were caught. 



100 



60 80 100 120 140 



STANDARD LENGTH (mm) 

 Figure 5. — Cumulative size-frequency curves for Vinciguerria 

 nimbaria larvae by 25 m depth interval ( 10 samples per interval ) 

 within the upper 100 m of the North Pacific central gyre during 

 summer. 



Table 4. — Abundance of metamorphic stages of Vinciguerna 

 nimbaria by depth during late summer in the North Pacific 

 central gyre. 



785 



