PKOBLE.Mb Ui ribll \Mj ii.^lil.NLi 



lUu'i 



food; that is, if they were thriving and growing, iliuy winUd 

 resist attacks of saprolegnia or other disease germs. A variety 

 of plants and animals is essential in a halancctl {mukI it" it is 

 to snpply food continuously to all its inhal)itants. As with 

 similar pi'ohlems on land, the most necessary tiling is an 

 abundance of {)lants, to sui)ply food for snails, mussels, in- 

 sects, worms, Crustacea, and vegetable-feeding lishcs; ih.-n 

 mussels should be present in sullicient numbers to strain ont 

 any excess of floating 



alga? and fungi : and, 

 Hnally, there nuist be 

 enough carnivorous 

 forms to prevent exces- 

 sive nuiltiplication of 

 the vegetarians. Of 

 course this natural bal- 

 ance of lakes and ponds 

 is a more complex mat- 

 ter than tliat of our 

 aquaria, since these are 

 never required to pro- 

 duce all the foods of 

 the fishes. 



^^^ 



9 



Fi(i. 143. Tiuy (»t \\il<l-tn>ul eijfjfs, with in»>s- 

 (|uito net and moss in wliirh they were packed 



I'nilc'd States liiircau i«f FiNlicrirs 



Even good-.si/t 1 1 hikrs 

 may lose balance, and cer- 

 tain species may suiter. The white bass in Lake !\biidi'i;i, W'isounsin, in 

 the summer of ^889, died in sucli iiiinibfrs that windrows of them were 

 washed upon the shores, necessitating; thf rmioval of over *Jno wa^onloads 

 from the mile or so of beach in Madison. They iiad iM'comr overcn>wd»'d 

 and weakened by starvation. T>ake Louise, in Pt'imsylvania, was st<n-k«'<l 

 with black bass, and the rules of tliefishiniLri'lul* that ci mi trolled it re<piired 

 that all the fish caught be returned to the lake. In a few vt*ars the lake 

 had notiiing but black bass in it, and these were .so starved that the 

 fish w'ere almost all heads and mouths, with shrunken bodies. 'I'he case 

 was investigated by the United States P>ureau of Fisheries, which 

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



