248 BULLETIN OF THE BUREAU OF FISHERIES. 



3. Four fish communities are provisionally recognized in Douglas Lake in mid- 

 summer — the community of young fishes, the stony-shoal community, the vegetation 

 community, and the deep-water community (p. 239-246). 



4. The community of young fishes is characteristic of the shoals. On the sand 

 shoals it consists of perch {Perca flavescens), spot-tailed minnows (Notropis hudsonius), 

 and suckers {Catostomus commersonii), all in schools together and all about 2 inches 

 long in late August. On the stony shoals, young blunt-nosed minnows [Pimephales 

 notaius) may be added to these schools (p. 241). 



5. All the members of the young-fish community feed exclusively on plankton 

 Crustacea (p. 241). 



6. The occurrence of young fish of several species (a) in large schools and (b) on 

 open shoals are conditions which, when they occur together, favor escape from preda- 

 tory enemies. It is held to be the presence of such enemies that keeps the members 

 of the young-fish community together and on the shoals (p. 241). 



7. The community of young fishes is temporary. Before their second season its 

 members forsake the shoals (p. 241). 



8. The stony-shoal community consists of the young and adults of three species 

 which lay their eggs beneath stones — the blunt-nosed minnow {Pimephales notaius), the 

 johnny darter {Boleosoma nigrum), and the miller's thumb {Cottus iclalops). With these 

 is associated a small crayfish (Cambarus propinquus Girard) (p. 242). 



9. The factor which holds the members of the stony-shoal community together 

 is the presence of stones or other similar objects which afford the conditions necessary 

 for breeding (p. 242). 



10. The stony-shoal community is permanent, except as it may be interfered with 

 by winter conditions (p. 242). 



11. The vegetation community consists of 10 species which, with one exception, 

 are unknown except in or very near vegetation (p. 242). 



12. The occurrence together of the members of the vegetation community is not 

 attributed to a single factor, but to two or more factors, of which food, protection, and 

 breeding conditions are specified (p. 243). 



13. All members of the vegetation community find their food in the vegetation; 

 in addition seven of them find there probably necessary protection, and six find in con- 

 nection with vegetation their usual breeding conditions (p. 243). 



14. The deep-water community consists of four or five species — the common sucker 

 {Catostomus commersonii), the pike {Esox lucius),thc^ small-mouthed black bass {Microp- 

 terus salmoides), the burbot {Lota maculosa). All of these occur near the bottom in 

 deep water outside vegetation, although they may also penetrate vegetation and invade 

 shallow water (p. 245). 



15. The members of the deep-water community obtain their food in the deeper 

 water and about vegetation. Their size is held to enable them to leave the vegetation, 

 since by it each species is in some degree protected from enemies (p. 245). 



16. The species of fishes found in Douglas Lake give preference in Illinois (Forbes 

 and Richardson) to those conditions of water and bottom that are available to them 

 in Douglas Lake (p. 246, 247). 



17. The fishes of Douglas Lake are distributed over the continent in those districts 

 in which the conditions available to them in Douglas Lake occur (p. 247). 



