VERTICAL DISTRIBUTION AND DEVELOPMENT OF LARVAL FISHES 

 IN THE NORTH PACIFIC CENTRAL GYRE DURING SUMMER 



Valerie J. Loeb' 



ABSTRACT 



Abundance data are presented on the mesopelagic fish larvae taken in 60 opening/closing bongo net 

 samples from the North Pacific central gyre in late summer. Vertical abundance and size-depth 

 distributions are described for 43 species of gonostomatids and myctophids and two stemoptychid 

 genera. Developmental stages at which these fishes leave the surface layers (either moving deeper or 

 begirming extensive vertical migration) are estimated from sizes of captured larvae. 



Over 96% of the estimated larval water column abundance occurred within the upper 100 m. 

 Maximum abundance and diversity were at 25-50 m, possibly related to the bottom of the seasonal 

 mixed layer. Most of the abundant species had distinct depths of maximum abundance within one of the 

 25 m depth intervals sampled and demonstrated changes in size composition with depth. Different 

 larval distributional patterns were found within and between the Gonostomatidae, Stemoptychidae, 

 and Myctophidae. Larvae of the two myctophid subfamilies had significantly different overall vertical 

 distribution patterns; Myctophinae larvae were more deeply distributed than Lampanyctinae larvae. 

 The myctophids exhibited two patterns of ontogenetic migration: one group of species remains in the 

 surface layers until transformation; the other leaves the surface layers in early stages of photophore 

 development. 



Mesopelagic fish species dominate the fish fauna 

 in oceanic regimes, both in terms of numbers of 

 species and numbers of individuals. The adults are 

 important components of oceanic communities. 

 The vertically migrating and more active species 

 are known predators upon other nekton and upon 

 zooplankton (Pearcy and Laurs 1966; Legand and 

 Rivaton 1969; Merrett and Roe 1974). We know 

 much about the depth distributions and diurnal 

 migrations of adult fish species; comparatively lit- 

 tle is known of the vertical distributions of their 

 early life stages. 



The larvae of most mesopelagic fish species are 

 found within the upper several hundred meters of 

 the water column ( Ahlstrom 1969) where they are 

 part of the zooplankton. This larval fish fraction of 

 zooplankton assemblages is called the ichthyo- 

 plankton. Ahlstrom's (1959) study of vertical dis- 

 tributions of larval fishes in the California Cur- 

 rent included some mesopelagic species. He found 

 that the majority of the species occurred within 

 the mixed layer and upper thermocline and that 

 each species had a characteristic depth distri- 

 bution; these depth distributions, however, varied 

 with the highly variable (10-90 m) mixed layer 

 depth. 



'Scripps Institution of Oceanography, University of Califor- 

 nia, San Diego, La Jolla, CA 92093. 



Larval fishes grow and develop within the upper 

 levels until some point of development when the 

 individuals leave the plankton and adopt 

 juvenile-adult roles. Changes in depth distribu- 

 tion with larval development and the stage! s) of 

 development at which the young leave the upper 

 levels and either descend to juvenile depths or 

 begin extensive vertical migrations have not pre- 

 viously been reported. 



The North Pacific central gyre is an excellent 

 area in which to examine the vertical distribution 

 of ichthyoplankton. Physically the upper several 

 hundred meters are horizontally monotonous and 

 vertically well stratified (McGowan and Hayward 

 1978; Gregg et al. 1973). In contrast to the Califor- 

 nia Current, the summertime mixed layer depth 

 (ca. 40 m) is quite constant. The ichthyoplankton 

 is composed of a diverse and rather equitably dis- 

 tributed assemblage of mesopelagic fish species. 

 Overall species composition and relative abun- 

 dance relations of larvae taken in integrating 

 0-300 m Isaacs-Kidd plankton trawl samples are 

 similar from tow to tow within and between sum- 

 mers (Loeb 1979b); repeated patterns of species 

 composition and abundance relations also occur in 

 replicated bongo samples taken within the same 

 depth interval (Loeb 1979a). 



In this study I present catch information on a 

 large number of larval fish species taken in 60 



Manuscnpt accepted Mav 1979 



FISHERY BULLETIN: Vol 77, NO 4.1979. 



777 



