PROBLEMS OF FLSH AND FLS 



food; that is, if they were thriving and growing, they would 

 resist attacks of saprolegnia or other disease germs. A variety 

 of plants and animals is essential in a balanced pond if it is 

 to supply food continuously to all its inhabitants. As with 

 similar problems on land, the most necessary thing is an 

 abundance of plants, to supply food for snails, mussels, in- 

 sects, worms, Crustacea, and vegetable-feeding fishes; then 

 mussels should be present in sufficient numbers to strain out 

 any excess of floating 

 algiB and fungi : and, 

 finally, there must be 

 enough carnivorous 

 forms to prevent exces- 

 sive multiplication of 

 the vegetarians. Of 

 course this natural bal- 

 ance of lakes and ponds 

 is a more complex mat- 

 ter than that of our 

 aquaria, since these are 

 never required to pro- 

 duce all the foods of 

 HIP fi '1 P Fift. 143. Tray of wild-trout eggs, with mos- 



quito net and moss in which they were packed 



Even good-si/ed lakes T^ited States Bureau of Fisheries 



may lose balance, and cer- 

 tain species may suffer. The white bass in Lake Mendota, Wisconsin, in 

 the summer of 1SSS), died in such numbers that windrows of them were 

 washed upon the shores, necessitating the removal of over 200 wagonloads 

 from the mile or so of beach in Madison. They had become overcrowded 

 and weakened by starvation. Lake Louise, in Pennsylvania, was stocked 

 with black bass, and the rules of the fishing club that controlled it required 

 that all the fish caught be returned to the lake. In a few years the lake 

 had nothing but black bass in it, and these were so starved that the 

 fish were almost all heads and mouths, with shrunken bodies. The case 

 \vas investigated by the United States Bureau of Fisheries, which 

 advised fishing out the surplus black bass and transferring them to the 



