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



DISCUSSION OF VERTICAL DISTRIBUTION OF THE SAN DIEGO 

 CHAETOGNATHA 



In any accurate quantitative consideration of the vertical 

 distribution of a species, all factors affecting the abundance of 

 specimens obtained must be taken into account. Conditions of 

 weather, currents, temperature, and salinity of the water, light, 

 and season are some of these factors. In addition, each net used 

 in collecting filters the water in a unique manner, and even the 

 quantity of plankton obtained in two successive hauls with the 

 same net usually varies. These net factors as well as the environ- 

 mental factors affect the number of specimens obtained and 

 their exact effect ought to be ascertained. 



This problem of quantitative estimates would be much 

 simpler if our engines and cables had permitted an earlier use 

 of vertical closing nets. Being hauled a known distance at a 

 known velocity and against gravitation, the pressure of water 

 on such nets may be calculated and their filtering capacities 

 determined. Furthermore the nets may be rapidly hauled so 

 that the environmental conditions at each depth might be con- 

 sidered constant. These nets have, however, been used for only 

 a few days during February, ^larch, and April of 1910, and 

 while the data obtained are valuable they are not sufficient to 

 permit definite conclusions concerning the vertical distribution 

 of the various species of this region. 



Since we are compelled to depend mainly upon data obtained 

 with surface nets and horizontal closing nets, it is almost impos- 

 sible to estimate the eff'ects of net and environmental factors on 

 the number of specimens obtained in each haul. Obviously the 

 pressure of water is greater on the lower portions of the net, and 

 consequently each area of net surface filters the water at a differ- 

 ent rate. The distance the net is hauled and the number, direc- 

 tion, and velocity of currents encountered have a marked effect 

 on the quantity of plankton obtained, and our data supply prac- 

 tically no information regarding these factors. Furthermore, 

 ten to twenty minutes is consumed in hauling, so it is question- 

 able whether or not the environmental conditions of the water 

 can be regarded as constant. For these reasons the amount of 



