236 
SEASONAL DISTRIBUTION OF CHYDORUS. 
AVERAGE NUMBER PER M.*—continued. 
Year July Aug. Sept. Oct. Nov. | “Dec. 
SOD a inact lcrttimenre, eee 95 O 461 100 16 56 
USO Shem cysnonste uate 91 103 164 38 203 448 
TSO Os voahonsewoe snc ek. aes 64 104 78 160 800 277 
OME Mecmeas eter ea iars oe 213 40 407 650 64 115 
LSB Fis kie.n bee a sponges tees 50 0) 30 60 28 Wii 
VSO ORF Renee eae neds ——— == Se ———— _ | ——— | =—— 
ASV CLA Crease cient yehs Biome 103 49 228 202 2ae. 214 
No. of occurrences..... 11 7 13 12 10 14 
Percentage of occur- 
MEMES aS oodsoas ode 61 33 81 Hil 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 
