I 



may at times be limited by the available food supply. 

 The types and abundance of food organisms must suf- 

 fice for the larval fishes at the proper seasons and later 

 stages throughout the year. The Crustacea of the mac- 

 roplankton fully meet these requirements by reproduc- 

 ing in great abundance through a large part of the year, 

 and especially at times when the newly hatched fish 

 fry require them. These fry, almost without excep- 

 tion, feed practically exclusively upon Crustacea, and 

 such fish as ciscoes, minnows, and darters continue to 

 feed largely upon them as adults. Other species change 

 their diet as they become older, but directly or indi- 

 rectly all are dependent on this vital link in the food 

 chain. Hence the plankton Crustacea occupy a crit- 

 ical position in the life of all animals that inhabit the 

 lake, and especially in the economy of fish propaga- 

 tion. 



COMPONENTS AND AMOUNT OF PLANKTON 



The bulk of the macroplankton is made up of 8 

 copepods, 7 cladocerans, 1 mysidacean, and a few in- 

 sect larvae. The copepods include 3 species oiCy- 

 clops , 3 of Diaptomus , and 1 each of Epischura and 

 Limnocalanus. The cladocerans include 2 species 

 of Daphnia and 1 each of Bosmina , Diaphan osoma, 

 Holopedium , Leptodora , and Sida. The other species 

 are all included in the list beginning on page 163, but 

 do not occur in sufficient numbers to form an appre- 

 ciable percentage of the total bulk. 



The cladocerans are probably of more value as 

 fish food than the other Crustacea. Daphnia pulex is 

 both larger and more numerous than any of the cope- 

 pods, and forms on an average considerably more than 

 half the entire macroplankton in the littoral and lacus - 

 trie zones. It is not as abundant in the marginal zone 

 but is supplemented there by large numbers of several 

 other cladocerans that are absent in the other 2 zones. 

 Limnocalanus and Epischura , the 2 largest copepods, 

 each have as great food value as Daphnia , but their 

 distribution is much more limited and erratic, and 

 their percentages in the plankton are frequently very 

 low. Mysis and Pontoporeia are of course much larger 

 than either the copepods or cladocerans, but as they 

 are usually found adjacent to the bottom in the deeper 

 portions of the lake their average food value is thereby 

 considerably reduced. 



Some investigators express the amount of the 

 plankton in terms of weight, some in volume, and a 



third group in the number of individuals of each sep- 

 arate species. In the present survey a combination 

 of the last two methods has been employed. The 

 total bulk of each catch was measured in cubic centi- 

 meters, and the percentage of each species was com- 

 puted by an actual count of the number of individuals 

 present in a measured sample (2 cc. ) of the catch. 

 Other samples were then examined without counting 

 to make certain that no species escaped detection. 



This method of dealing with the plankton has es- 

 tablished several things with reference to its amount and 

 the relative value of its several components. One of 

 the first facts came to light as a result of careful com- 

 parison of the macroplankton components in the tows 

 made simultaneously with the foot and meter nets. The 

 2 nets were towed at the same rate and for the same 

 length of time and were drawn through the water close 

 together. Obviously the bulk of plankton in the meter 

 net would be much greater than that in the foot net, 

 but the percentages of the species in the 2 nets were 

 expected to be the same. On the contrary, repeated 

 comparisons showed striking differences, especially 

 among the copepods. Limnocalanus and Epischura 

 never appeared in the foot -net samples in anything 

 like the relative percentages found in the meter -net 

 samples. Indeed they were often entirely absent from 

 the former but present in the latter in percentages that 

 sometimes reached two figures. Obviously these larger 

 and more active copepods were able to escape the 

 smaller net. Such an admission, however, invalidates 

 the foot -net samples as indicators of either the amount 

 of the plankton or the percentage composition of its 

 components. Hence, in the present survey all the data 

 on macroplankton of the littoral and lacustric zones 

 have been taken from the meter -net catches. And in 

 the marginal zone where the foot net was necfessarily 

 used, the total amount of the macroplankton and the 

 percentages of the species were not recorded since they 

 would possess no quantitative accuracy. The species 

 found in these marginal tows were recorded simply as 

 abundant, common, few, or scarce. Necessary vari- 

 ations in the rate and length of the tows also made 

 quantitative comparisons of marginal macroplankton 

 impossible. 



A second fact, very completely proven, is that 

 no species of the macroplankton is uniformly dis- 

 tributed throughout either the lake as a whole or the 

 particular zone in which the species is found. Indeed 

 the exact opposite is true and every species exhibits 



146 



