75] STUDY OF SOUTHERN WISCONSIN FISHES— CAEN 75 



collection, these young averaging 1.4 cms in length — evidently early- 

 hatched young of the year. Not one was found in the day seinings. Here- 

 after, with occasional exceptions, young Labidesthes were found every 

 night in the shallow water, and were conspicuously absent during the day 

 from the same region. The one factor which kept the fish from the shallows 

 at night was the condition following occasional heavy rains when the 

 shallow water was in a state of great turbidity. Attention was therefore 

 centered on this migration phenomenon and its progress investigated 

 from various angles. Beginning with July 21, an increase in the number 

 of silversides found each night was noted, and this increase reached its 

 climax on August 6, when the astonishing total of 621 young silversides 

 was collected in a single drawing of the net. Investigation was immediately 

 started at other points along the shore to determine if this was merely a 

 local condition, with the following results: 



Table 15 



DISTRIBUTION OF YOUNG LABIDESTHES SICCULUS IN OCONOMOWOC 



LAKE ON THE NIGHT OF AUGUST 6 IN RELATION TO TYPE 



OF BOTTOM SELECTED 



From this table two conclusions can be drawn: 1) that the condition noted 

 was not a local exception but a general rule, that throughout the lake there 

 was a great inshore migration of young silversides; and 2) that these young 

 fish showed a preference to a gravel bottom relatively free from plant 

 growth, that the increasing abundance of vegetation reduced markedly 

 the number of young inhabiting the location. A mechanical error may 

 creep into these figures because of the increasing difficulty of drawing a 

 seine through heavy weeds, but as the silversides is strictly a surface 

 inhabitant, this error is reduced to a minimum as the surface water is 

 covered with reasonable efficiency even in the presence of heavy bottom 

 vegetation. The next morning the same areas were re-seined, with the 

 result that only 38 young were found. Of this number 21 were evidently 

 injured, and probably were cripples left over from the night seining — for 

 it must be said that every effort was made to replace the fish as quickly as 

 possible after they were caught, and with the least possible injury. Since, 

 therefore, the fish have returned to the deep water again in the daytime, 

 a third conclusion can be drawn from the facts: 3) that this migration is 

 a nocturnal-diurnal phenomenon. Figure 14 shows the abundance of the 



