380 



Birge — The Crustacea of the Plankton. 



January; but the population fell off more rapidly in the latter 

 part of that winter, and there was no very noticeable difference 

 in March. 



Table XXVIII. — Average percentile distribution for the winter — Jan- 

 uary, February, March. 



1895. 

 1896. 



Pee cent, in each 3 m. level. 



Aver- 

 age No. 



123,000 

 237,000* 



0-3m. 



18.1 

 34.1 



3-6. 



19.3 

 15.7 



6-9. 



13.7 

 H.8 



9-13. 



12. 

 10. 



13-15. 



15.8 

 10.3 



15-18. 



20.3 

 13.6 



* Chydorus omitted on account of its rapid decrease in late winter. 



Spring — April and May. 



Tables B, C, Appendix. 



The distribution of the Crustacea during the first half of Apri 

 is on the whole fairly equal in the different levels of the lake,, 

 but with irregularities which mark it as an accidental distribu- 

 tion. The ice breaks up in the first days of April, and the lake 

 is consequently exposed to the action of the wind. The tem- 

 perature is fairly uniform at all depths, and the algae hardly 

 begin rapid multiplication much before the middle of April. 

 The water at this time has a more active circulation than 

 at any other, as is shown by the presence in the net of numer- 

 ous particles of vegetable debris from the soft mud at the bot- 

 tom of the lake. 



During this time Cyclops begins its rapid increase towards 

 the spring maximum, if the multiplication has not already begun 

 under the ice. Its swarms of young are in the upper strata of 

 the water. It may be laid down as a general rule that large 

 numbers of young of any species of Crustacea appear first in the 

 upper levels of the water, and the animals later pass toward 

 the middle of the lake; and later still, occupy the water toward 

 the bottom. It may be said, therefore, in general, that the 

 presence in the upper water of a very high percentage of the 

 catch of any species indicates the beginning of a period of re- 



