LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
417 
is of little value as food for fishes. Most fishes feed upon littoral crustaceans, 
mosquito larvae, worms, and other shore or bottom animals. Therefore, according 
to Schiemenz and Zander, the estimation of the fitness of a lake or pond for fish 
culture ought to be made on the basis of an examination of shore and bottom fauna 
rather than on the study of plankton. The examination of the composition of 
plankton is useless for estimating the productive capacity of a lake, and only the 
determination of nitrates dissolved in water is of importance for that purpose. 
Schiemenz (1902-1905) pointed out that if the fishes in a pond change their food 
and become plankton eaters it indicates that the conditions in the pond are unfavora- 
ble for fish culture. With regard to the nutrition of carp Schiemenz discovered some 
very interesting facts. In carp ponds the number feeding on shore and bottom 
organisms progressively increases from midsummer to fall. The ratio between the 
plankton-eating carp and those consuming bottom animals and plants was, in 
July, 1 : 2; in August, 1:5; and in October, 1 : 13. Schiemenz’s interesting obser- 
vations have been much criticized. His opponents have pointed out that he gives 
no information concerning the average productivity of the ponds in which the 
observations were made and says nothing about the character of the ponds in ques- 
tion, and that therefore a different conclusion might be arrived at if all the factors 
were taken into consideration. 
The estimation of the productive capacity of a lake, based on the determination 
of the total amount of plankton, does not mean that the plankton is regarded as the 
principal food of fishes, mussels, and other edible or useful animals. From numerous 
investigations made in America and in Europe — Pearse (1921), Schiemenz (1902- 
1905) , Walter (1905) , Arnold (1902) , and Geineman (1902) — we know that the food of 
most of the commercial fishes consists of other fishes or shore and bottom inhabitants. 
There are but few fishes living on a pure plankton diet, and even these plankton 
eaters do not consume plankton without regard to its composition. Thus Hofer 
(1896) found that 75 per cent of the total amount of food of the whitefish in Bodensee 
Lake was composed of a planktonic cladoceran, Bytotrephes longimanus. The white- 
fish in this lake evidently are able to choose their food among the small planktonic 
crustaceans, and Hofer affirms that their gathering at different depths depends on 
the. vertical distribution of Bytotrephes. Regardless of whether this interpretation 
is justified or not it seems certain that the fishes occur in greater abundance at the 
depths where Bytotrephes is most numerous. 
Similar observations made in various countries show that there are many fishes 
that feed on definite plankton crustaceans. For example, Osmerus eperlanus, in the 
Russian lakes, feeds in summer on Leptodora and Hyalodaphnia and on Bosmina 
and Cyclops in winter. Coregonus shinsii, in Neuenburgersee Lake, according to 
Fuhrman’s (1905) observations, consumes exclusively Bytotrephes longimanus. 
Juday (1907) described the food of a trout in Twin Lakes, Colo., as consisting of an 
immense quantity of water fleas — 4,500 Daphniawere found in the stomach of asmall 
specimen only 30 cm. long. 
Some fishes are known to change their diet with the season. For instance, 
Alburnus lucidus in summer feeds almost exclusively on Cladocera, although in win- 
ter its food consists mainly of diatoms (Arnold, 1902). Some fishes {Abramis brama 
and Leuciscus rutilus ) become plankton eaters when there is a lack of food on the 
bottom or near the shores (Arnold; Schiemenz, 1. c.). 
