236 
SEASONAL DISTRIBUTION OF CHYDORUS. 
AVERAGE NUMBER PER M.3—continued. 







Year July Aug. Sept. Oct. Nov. Dec: 
TOY oe WE a pee try see meer ve 95 0 461 100 16 56 
SOS eee Ye Scene Slee wen bea by asters 91 103 164 38 203 448 
LS OGM EN te de eee 64 104 78 160 800 277 
US Oiaerene oe eee DAS 40 407 650 64 iil 
{SORE Ae es otaeemer eas 50 0 30 60 28 iL fD 
TRS OO pee te eros ae ey tee Parte —— ——. —— —_—_——_ | ———_ | —— 
INVETAC CH ey sioecitiers oro 103 49 228 202 222 214 
No. of occurrences..... 11 7 13 12 10 14 
Percentage of occur- 
TENCES HPT rete cee 61 33 81 71 63 82 







occurring in average temperatures, for these months, of 60.5° and 
68.3°, while the minor autumnal development appears in September— 
October at 74.2° and 57.6° respectively, and the December pulse, if 
indeed it be a separate and independent pulse, is at the low tempera- 
ture of 35.2°. The December movement may be simply the result 
of the more stable conditions which attend the appearance of the 
ice-sheet on the approach of winter. 
An analysis of the course of the seasonal distribution of Chydorus 
in channel waters, as given in Table I. and in statistics of other 
years, indicates the following seasonal regimen. In January—Feb- 
ruary, at minimum temperatures, the occurrences are irregular 
(75 and 40 per cent.) and the numbers small (average, 167 and 53 
per m.*), while in March, with rising temperatures, occurrences are 
more numerous (100 per cent.) and numbers rise to 668 per m.* In 
April-May a high percentage of occurrences (82 and 90 per cent.) 
continues, and they mount rapidly to the maximum record of the 
year, which in our statistics varies from 4,088 in 1895 to 32,800 in 
1897. This vernal pulse reaches its maximum in our records on 
April 29 in 1895, at 64°, and in 1896 on the same day, at 70°; on 
ge 0 VS 
» a — LL 
