122 American Fisheries Society. 



stantial addition to the menu. The entomostraca which eat the 

 algae directly in the open water and the insect larvae which 

 feed on them as they die and sink to the bottom, constitute the 

 main direct contribution of the open lake to the food of the fish. 

 There are fish, like the gizzard shad, which feed directly on 

 algae, but such fish are few and most fish get their food in form 

 of animals. We are therefore much concerned with those 

 creatures which serve as intermediaries between algae and fish. 



We have been able to make rough determinations of the 

 quantity of entomostraca in the plankton. In Lake Mendota the 

 eaters in the plankton — Crustacea and rotifers — make on an aver- 

 age about one-eighteenth of the total plankton. Such an aver- 

 age is, of course, subject to wide variation, as both animals and 

 plants come on in waves ; but it is rarely the case that the eaters 

 find in the water less than a dozen times their weight of food. 

 This seems a liberal provision. If a "beef critter," for instance, 

 weighing 1,000 pounds had a constantly renewed stock of green 

 food amounting to 10,000 pounds or 20,000 pounds, he would 

 seem to be amply supplied. But the algae are not concentrated 

 into a sort of sheet or carpet like the grass; the animal must 

 strain them or pick them out of the water. No fresh water 

 animal has a better straining apparatus than has Daphnia, but 

 when I tell you that in Lake Mendota a Daphnia must extract the 

 algae from 60,000 times her own weight of water in order to ob- 

 tain her own weight of food you will see that life for her in- 

 volves no small amount of work. 



The same statement may be made of other plankton animals. 

 In lakes like Green Lake, the quantity of plankton is smaller 

 and the Crustacea are in general smaller and fewer per unit of 

 volume of water. The animals of the plankton seem everywhere 

 to be as great in quantity as the plants will support. 



In general the same statement may be made of the weight 

 of Crustacea per unit of area of a lake, that was made of the total 

 plankton. In lakes of very various depths the number of pounds 

 per acre is not widely diflferent, and the deep lake has no ob- 

 servable advantage over the shallower one. In the deeper water 

 (20-24 m.) of Lake Mendota the plankton Crustacea and rotifers 

 may yield a standing crop of something over 20 pounds of dry 

 organic material per acre, or over 200 pounds of fresh meat, 

 "on the hoof," as it were. This average is subject to much varia- 



