FISHERY BULLETIN: VOL. 74, NO. 4 



complex interaction of: 1) the recruitment and 

 mixing of species characteristic of the water 

 masses that compose the California Current, 2) the 

 daily vertical movements of euphausiids, 3) the 

 ability of most species to avoid the sampling gear, 

 and, to some extent, 4) their contagious dispersion. 

 Consequently only the more obvious patterns have 

 been noted. 



The vertical distributions of adults and 

 juveniles in the upper 500-700 m are summarized 

 in Table 3 and compared with data from the 

 southern part of the California Current. As 

 previously mentioned, 7 of the 20 species collected 

 appear to be diel migrants. Distances of 300 m or 

 more were traversed by four species of Euphau- 

 sia. Portions of other populations such as Nema- 

 tobrachion flexipei^, T. longipes, and T. spinifera 

 may migrate up to 200 m. 



The larval phases of most species live in the 

 upper 150 m. Tessarabrachion oculatus and Stylo- 

 cheiron spp. larvae were found more often below 

 the thermocline. The young of Euphausia spp. 

 tended to occupy and migrate through the same 

 depths as the older stages. In nearly all instances, 

 differences in density between day and night 

 catches of larvae were small. 



The nonmigrating species included Thysanoessa 

 gregaria, Tessarabrachion oc id at us, S. maximum, 

 S. affine, and S. longicorne. The first three species 

 were usually scattered throughout a broad vertical 

 range. The other two species, S. affine and S. 



longicorne, were vertically segregated and oc- 

 curred within much narrower depth intervals. The 

 different strata occupied by these two nonmigrat- 

 ing species was also observed in other regions by 

 Brinton (1967), Baker (1970), and Youngbluth 

 (1975). 



DISCUSSION 



Differences in the distribution patterns of many 

 species of zooplankton have been associated with 

 their response to environmental gradients, par- 

 ticularly temperature and illumination (Harris 

 1953; Lewis 1954; Banse 1964; Boden and Kampa 

 1967). In this study, the causative factors 

 influencing vertical and horizontal distributions 

 are difficult to elucidate. It is clear, however, that 

 the thermocline was an upper distribution bound- 

 ary for several species, e.g., T. oculatus, E. 

 gibboides, S. affine, S. longicorne, and 5. maximum. 

 In the southern part of the California Current, the 

 upper range of these species was also restricted by 

 the thermocline (Brinton 1967). Studies on the 

 tolerance of E. pacifica to changes in temperature 

 and salinity suggest that other unknown factors 

 probably regulate its distribution in the California 

 Current (Gilfillan 1972a, b). 



Recently Isaacs et al. (1974) have proposed that 

 "by responding to light intensity, most vertically 

 migrating marine creatures are directed to food. 

 ... In areas of low standing crops of phytoplank- 



Table 3.-Comparisons of diel changes in the vertical distributions of adult and 

 juvenile euphausiids. Depth ranges (m) are 10% and 90% levels. 



934 



