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



This objection would be justified if the greatest number of hauls 

 in water varying from 15?9 to 17?5 had been made during morn- 

 ing and evening twilight, while those in water varying from 19? 6 

 to 21 ?5 had been made during intense daylight. However, as 

 frequency means the percentage of hauls in which the species 

 appears without reference to number of specimens obtained, its 

 parallelism with abundance indicates that the hauls were evenly 

 distributed. This is true because, if the species occurred at all, 

 uneven distribution of hauls with respect to light would affect an 

 increase or decrease in abundance but not in frequency. To 

 demonstrate that hauls were evenly distributed, the following 

 table is supplied, which includes only those hauls made during 

 daylight. 



TABLE 41 

 Surface Distribution of S. bipunctata between 6 a.m. and 6 p.m. 



These data show that, during daylight, the species is most 

 abundant and frequent in water having a temperature between 

 15?9 and 17?5. It further shows that the decrease in both fre- 

 quency and abundance is significantly greater than that found 

 in table 40. This is exactly what would be expected if intensity 

 of light affects the distribution of this species. In the former 

 table, where twilight hauls are included, the effect of higher 

 temperatures in decreasing the abundance of specimens is, to 

 some extent, counteracted by the effect of twilight in increasing 

 it. In table 41, however, only a minimum of twilight hauls are 

 included, and both the effects of more intense light and higher 

 temperatures tend to decrease the abundance. 



Still, this greater decrease in abundance and frequency 

 during daylight is possibly affected by an uneven distribution 

 of hauls throughout the various times of day. Our data show 

 that, while three of the four hauls in water having a tempera- 



