ZOOPLANKTON IN METABOLISM OF LAKES 



91 



but 1ms demonstrated its ability to eatcli 

 most of the larger forms in sj)ite of the 

 eomparatively small opening. 



The remaining sonree of error, due to 

 sampling, fraetioning and counting, is prob- 

 ably not great and, with ordinary care and 

 an occasional recheck, can be reduced to 

 a mininunn. Rather elaborate statistical 

 studies of errors in plankton collection and 

 enumeration have been undertaken by 

 Ricker (1937a) and other Canadian lim- 

 nologists. These studies, Avliile they are of 

 great interest, seem to be of less practical 

 value to the limnologist. Sampling of a 

 body of water is not assumed to be an exact 

 science, and this has well been illustrated 

 in some remarks by Allen (1938) in com- 

 menting on the accuracy' of certain plank- 

 ton catches in the Gulf of California. He 

 states, ' ' Under such considerations it should 

 be apparent that the rigid accuracy more 

 or less erroneously attributed to all scien- 

 tific activity is actually unattainable and 

 that the investigator must be content with 

 something less than the popular ideal. As 

 a matter of fact, this condition is not as 

 bad as it seems or as it may be made to 

 appear to be. The life of the investigator 

 liimself is not a result of rigid accuracy. 

 It is a series of compromises, readjustments, 

 and approximations. In fact, all life is like 

 that to a greater or lesser degree and even 

 inanimate Nature is not free from appear- 

 ances of uncertainty or confusion. And, if 

 a determination of the characteristics of a 

 population in the ocean could be made with 

 rigid accuracy for a given instant, the terms 

 expressing this accuracy might be widely 

 misleading if applied to the population 

 present an hour or a day later. ... To really 

 know Nature one must sacrifice something 

 of mathematical accuracy at times and 

 depend upon a series of approximations 

 (which, paradoxically, will lead to a nearer 

 approach to accuracy than would be pos- 

 sible by the use of extreme refinements)." 



The Utilization of Plankton by Fish 



The examination of stomach contents has 

 shown that many fish utilize plankton as an 

 important element in their diet. It has gen- 



ei-ally beeu believed that fish simply strained 

 a great quantity of water and separated out 

 whatever was in it. Some recent work, how- 

 ever, has shown that in some fish, at least, 

 this is not true and that there is an actual 

 selection of food b\- tiie fish. Battle et al. 

 (1936) found that the herring captures each 

 individual plankter. This can also be 

 noticed in goldfish when Daphnia are put in 

 the aciuariiun. Every plankter is hunted 

 down by individual ra[)ture. Ricker (1937) 

 found tliat the socke.ye salmon seemed able 

 to distinguish between individual plankters 

 and even shoAved individual preferences. 

 He confined 10 fish in the same limited 

 environment and found that seven of them 

 took almost all Daphnia, three took Cyclops 

 as well, while one of the three took only 

 Cyclops! Ricker points out that individual 

 preferences for different organisms may 

 cast some doubt on the reliability of average 

 food consumed in series containing a feAV 

 stomachs only. The kind of food eaten Avas 

 also found to vary with the season ; in May 

 and June, when the fish were small, Cyclops 

 and Bosmina were fed upon. Later in the 

 summer the food was mainly Daphnia, and 

 in winter Cyclops and occasionally consid- 

 erable numbers of Bosmina were taken. The 

 sockeye salmon continued to feed mainly on 

 plankton to the third year. 



Proble:\is for Future Investigation 



During the course of this discussion a 

 number of lines of investigation, in which 

 much more work can be done, have been 

 pointed out. In summary these are as fol- 

 lows : further study of zooplankton organ- 

 isms and their distribution; pioneer quanti- 

 tative work on the plankton of little known 

 regions, particularly in the tropics and in 

 northern regions, more work on seasonal 

 distribution of plankton and organic matter, 

 especially in unknown regions; further 

 study of seasonal changes in the diurnal 

 migrations of zooplankton and the develop- 

 ment of a "perfect" collecting device for 

 macro-plankton. 



Eefeeences Cited 



Allen, W. E. 1938. Tlie TiMupleton Crocker Ex- 

 Xiedition to tlie Gulf of California in 1935 — The 



