144 University of California Publications in Zoology. [Vol.8 



while salinity determinations for 15-20 fathoms were not made, 

 sity it is possible, as pointed out on page 139, that the salinity is 

 usually between 33.605 and 33.648. 



Our data, then, seem to justify the truth of the above assump- 

 tion. To prove it, however, would require carefully planned col- 

 lecting with all conditions of light-intensity in each depth 

 recorded, as by an automatic light meter, systematic closing net 

 hauls with corresponding temperature observations in each depth, 

 and salinity determinations corresponding to each haul in each 

 depth. 



Notwithstanding that these more accurate methods of collect- 

 ing were not employed, from what data we have the following 

 conclusions may be drawn : 



1. The region of 15-20 fathoms is the center from which the 

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

 greatest number of optimum conditions favorable to this species 

 are found. 



2. It migrates to the surface at sundown and sunrise because 

 the conditions of light intensity at these times are similar to those 

 occurring in 15-20 fathoms during the greater part of the morn- 

 ing and afternoon. 



3. Other things equal, it remains on the surface in greater 

 numbers when the temperature of the water is between 15?9 and 

 17? 5, because this is approximately the normal temperature occur- 

 ring in 15-20 fathoms. 



4. It remains on the surface in greater numbers when the 

 salinity of the water is between 33.605 and 33.648 because this 

 approximates the normal salinity in 15-20 fathoms. This, at 

 present, is barely suggested by our data; it still remains to be 

 proven. 



5. The animals leave the surface at night because the light 

 incentive which caused them to migrate upward is absent, and 

 they presumably return to the region of 15-20 fathoms where 

 optimum temperature and other conditions occur. 



6. Probably light has more pronounced effect on vertical dis- 

 tribution than temperature or salinity because its variations are 

 more regular and periodic. 



