26 Urwv&rsity of California Publications in Zoology [ v " 1 - 2 - 



The Clogging of the Net 



The clogging of the net is undoubtedly an important factor affect- 

 ing the catch and it was also quite variable under 1913 conditions 

 here. Hence some designation of its probable condition is very desira- 

 ble. In spite of this it was finally decided to ignore it for 1913 at 

 least. This is because too few filter paper, or other supplementary, 

 catches have been made to give adequate ground for estimation. 



VOLUMETRIC DATA 



There is not a great deal to say on this topic as yet. The main 

 points are distinctly shown by plate 1 and table 6. The two most 

 interesting points, in the light of such investigations elsewhere, are 

 that only Station I shows a very distinct vernal pulse and that the 

 autumnal pulses are most prominent at all stations. This statement 

 needs some qualification since there was a higher maximum shown at 

 Station I in March than in the fall. This vernal pulse was, however, 

 so very abruptly developed, and the autumnal so very gradually, that 

 it seems natural to assign the greater importance to the latter. At 

 neither of the other stations does the vernal pulse compare in magni- 

 tude with the autumnal. Indeed, at Station III, there is no well 

 marked, vernal pulse. 



It is worthy of note that volumes appear least variable at Station 

 I and most so at Station II. It might, at first thought, seem that 

 this was owing to the uniformity of food supply at Station I, caused 

 by the constant inflow of sewage throughout the year. Closer exami- 

 nation of the records suggests, however, that variation in speed of 

 currents in the river, together with the dilution due to flood waters 

 is more potent. This estimate of the importance of the current as a 

 factor is supported by collections made in the San Joaquin River 

 near Fresno, California, in August, 1916. Although this was the 

 season for maximum occurrence of plankton at Stockton, not enough 

 was taken by the net at Fresno to be measurable volumetrically with 

 any accuracy. Since the current at Fresno is about as rapid in the 

 dry season as at Stockton in flood season, it seems certain that it has 

 a profound influence. Still the importance of the uniform food sup- 

 ply at Station I must not be minimized. 



