240 BULLETIN OF THE BUREAU OF FISHERIES. 



communities of fish primarily with reference to their origin and interrelations. Bv 

 community is here meant no more than a group of species or individuals whose mem- 

 bers are regularly found together for a longer or shorter time. No precise use of the 

 concept in its relation to habitat is attempted. Thus, the stony-shoal habitat har- 

 bors two communities, one of which, that of the young fishes, is more characteristic 

 of sandy shoals. To attempt greater accuracy at the present time would be mislead- 

 ing. Although it seems best to treat communities of fish rather than the fish of a 

 habitat, it is most convenient to use for most of these communities the names of the 

 habitats in which they are best developed. 



THE COMMUNITY OF YOUNG FISHES. 



On the sand shoals are found mixed schools of young perch, spot-tailed minnows, 

 and suckers, all about 2 inches long in late summer. These schools doubtless vary in 

 the proportion of their constituents. Our only record gives perch 203, spot-tailed 

 minnows 237, suckers 5; but the suckers are often seen to make up a larger per- 

 centage of the whole. On the stony shoals there are added to these schools many 

 young blunt-nosed minnows, distinguishable from the other fish of the school bv the 

 jerky mode of swimming and by other field characters already described. What follows 

 refers to the schools on the sand shoals, but would presumably answer as well for those 

 on the stony shoals. Perch are known to spawn in the spring. In the hatchery ponds 

 at Mill Creek, Mich., Mr. Dwight Lydell reports to me that they hang their purse-Uke 

 masses of eggs on aquatic plants, brush, and the like. Abbott (1878) found them 

 spawning on gravel amongst vegetation. H. M. Smith (1907) gives a similar account, 

 and also finds the egg masses floating. I have found the floating egg masses in Saginaw 

 Bay, Mich. Their spawning grounds in Douglas Lake are unknown, but from them 

 the young perch must make their way to the shoals. Suckers (see under that species) 

 are not known to spawn except in running water on gravel bottom. Presumably 

 those of Douglas Lake spawn in such situations in Maple River and Bessie Creek, and 

 the young fish make their way along shore to the shoals. The breeding habits of the 

 spot-tailed minnow are unknown. From two or perhaps three different breeding 

 grounds the young fish forming the communities under discussion reach the sand shoals 

 and finally live together there in schools through the summer. The schools move 

 about over the sand shoals, engaged now in feeding, now in fleeing from enemies. If 

 the water is quiet, they approach the shore; as it becomes rougher they pass toward 

 the lakeward edge of the shoal and are lost to sight. They may then enter the vegeta- 

 tion, but as this harbors many enemies it is more probable that they avoid it. All 

 three species feed on the microcrustacea of the plankton, which at this season are 

 extremely abundant. The total daily consumption of the whole school referred to 

 above must reach many millions. Since the alimentary canals of the voung fish are at 

 all times crammed, there appears to be enough food for all, so that the fish are not in 

 competition with each other. 



While the schools of yoimg fishes are on the sand shoals they are often pursued by 

 young black bass about 3 inches long, small-mouthed and large-mouthed. The water 

 on the shoals is not obstructed by vegetation, so that the stalking bass are plainly 

 visible to their prey which attempt to escape the rushes of the bass by short rapid flights, 



