A STUDY OF TWIN LAKES, COLORADO. 



161 



were so few in number and covered such a brief period of time each year that they give 

 onlya fragmentary notion of the plankton lifeof the lakes. Likewise the vertical haul 

 method is by no means a satisfactory one and both of these factors must be taken into 

 consideration in the results given below. The following table shows the number of 

 thousands of individuals per square meter of surface. With the exception of a few 

 figures for the rotifers, these numbers are averages of either two or three hauls. 

 The rotifers were not counted in all the catches, so thai in a few instances the 

 numbers given for them represenl only the individuals of a -ingle catch. 



Quantity of plankton in Twin Lakes, as shown by series of vertical hauls during summt 

 [Average number of t lousanda of individuals per square meter.] 



DT8TRIB1 Tiox. 



With respect to vertical distribution, Anurea cochlearis, Notholca longispina, 

 and Asplanchna were confined almost exclusively ti> the upper L0 meters. Anurea 

 aeuleata and Triarthra longiseta were found almost entirely below a depth of In 

 meters, rarely occurring in the upper 10 meters. Polyarthra was rather evenly 

 distributed throughout the entire depth of both lake-. 



There was no diurnal movement of Diaptomus, Oyclqps,&nd Nauplii. Daphnia 

 hyalina showed a diurnal movement of half a meter in July and 1 meter in August. 



The phytoplankton, as stated above, composed a comparatively small portion of 

 the total plankton, and the few forms remained practically uniform in quantity dur- 

 ing the two periods of these observations. The Crustacea made up by far the 

 greatest hulk of the total plankton. 



FISHES. 



Catostomus commersonii Lacepede. Common Sucker: White Sucker. 



This sucker is very abundant in the lakes. At times immense numbers were 

 seen swimming at the surface with the dorsal tin and a part of the back projecting 

 above the water. This habit was noted when the water was quiet, either early in the 

 morning or in the evening. As food fish, the sucker does not stand very high in the 

 estimation of local fishermen, consequently little attention is paid to it except as bait 

 for trout. A few are caught in Lake Creek by means of grab hook-. Some of the 

 trout feed rather extensively on young sucker-. 



B. B. F. 1'.I06— 11 



