158 University of California Puhlications in Zoology. [Vol.8 



4 and 6 a.m., in 15-20 fathoms between 8 and 10 a.m., in 25-85 

 fathoms between noon and 2 p.m., and in 40-75 fathoms between 

 10 a.m. and noon. 



5. In all depths above 25 fathoms the species attains maxi- 

 mum abundance twice during the day, one maximum occurring 

 between 6 and 10 a.m., and the other between 6 and 8 p.m. 

 Below 25 fathoms only one maximum occurs in each depth, and 

 that one between 10 a.m. and 2 p.m. 



6. The species attains its morning maximum on the surface 

 within an hour after sunrise and its evening maximum within 

 an hour after sunset. 



7. Other conditions constant, the species is most frequent and 

 abundant on the surface when the temperature of the water is 

 between 15?9 andl7?5. 



8. It also tends to remain on the surface in greater numbers 

 when the salinity is between 33.605 and 33.648. 



9. No significant correlation between season and variations 

 in distribution could be detected. 



10. No significant correlation between locality, or distance 

 from the coast, and variations in distribution were found. 



From these facts we concluded : 



1. It is probable that the region of 15-20 fathoms is the center 

 from which the species migrates or, in other words, it is the 

 depth where the greatest number of optimum conditions favor- 

 able to the species are found. 



2. It migrates to that depth in which the intensity of light 

 is similar to that occurring throughout a large part of the day in 

 15-20 fathoms, and it remains in greater numbers in that depth 

 when the temperature and salinity of the water, and probably 

 other unknown conditions, are similar to those typical of 15-20 

 fathoms. 



3. This explains why the species occurs in maximum number 

 on the surface during morning and evening twilight, why the 

 evening maximum is the largest, why it leaves the surface so 

 quickly after it becomes dark, why it is more abundant on the 



