67] STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 67 



shallower the water is. One would therefore expect the eggs that drift into 

 shallow water to hatch earlier than those that settle in deeper water, as 

 they receive more degree-hours of heat per day. Such conclusions are 

 borne out by the facts as shown in table 8. 



Table 8 

 EFFECT OF DEPTH OF WATER UPON THE HATCHING 

 PERIOD OF EGGS OF LABIDESTHES SICCULUS 



THE YOUNG 



Immediately after hatching the young silversides wiggle themselves 

 to the surface. The term "wiggle" is used advisedly as best describing the 

 action of the young fish: the activity consists of much lateral movement 

 with relatively little forward progression. The activity of the fish begins 

 before it leaves the egg, and is responsible for the rupture of the egg mem- 

 branes and the subsequent liberation of the fish. Once the young reach the 

 surface it is a long time before they see the bottom of the lake again, for 

 they remain permanently just under the surface of the water for the 

 following weeks, and never forsake the upper few centimeters under any 

 circumstances. Their relation to the surface may be stated in these terms: 

 the younger the fish the closer to the surface film of the water do they 

 remain. Newly hatched Labidesthes frequent the upper three centimeters 

 or less of their habitat. 



As they hatch, then, the young work to the surface, where they congre- 

 gate in rapidly increasing numbers. Inasmuch as the eggs laid by a single 

 female in the course of her breeding activities do not scatter very far — 

 there is no current in the lake sufficient to cause a wide dispersal — all hatch 

 at very nearly the same time. Thus are built up at the surface schools of 

 young silversides varying in component numbers from thirty to perhaps 

 two hundred. The individual fish are constantly active, swimming around 

 in a course conspicuous for its irregularities. Yet there is a certain co- 

 ordination within the school: if the school is going ahead, all the members 

 are going in a course almost exactly parallel to each other; if the school 

 veers to the right, all the members shift in the same direction and almost 

 at the same instant. While the course of the school is therefore very 

 irregular and the progress in any one direction slow and uncertain because 

 of the many turnings and twistings of the path of progression, the school 



