69] STUDY OF SOUTHERN WISCONSIN FISHES-CAEN 69 



On this basis one can figure as follows: 211 cm in 2 minutes; that is 6,330 

 cm per hour; and since this is early summer, one can figure at least fourteen 

 hours of activity, which gives 88,622 cm traveled in the course of a day, 

 by a fish that is 2.2 cm long. That is 40,283 times its length! 



Throughout the day these tiny, semi-transparent fish work their way 

 incessantly back and forth just under the surface of the water. Their small 

 size, together with their inconspicuous coloring and nearly transparent 

 bodies affords them a protection against wandering enemies, both in the 

 water and in the air. At the approach of a boat the school breaks up 

 immediately, the individuals darting like lightning in every direction 

 except down. When the school is broken up the tiny individual fish is 

 extremely difficult to see, while its zig-zag course makes it difficult to 

 capture even after it is discovered. The approach of a tern (Clidonias 

 nigra surinamensis or Sterna hirundo) sends the fish scattering wildly, 

 and a similar reaction follows the approach of a larger fish either from below 

 or from their own level. This indicates a keen alacrity on the part of the 

 silversides, which no doubt saves many of them from destruction, for 

 they have many enemies. 



With the coming of darkness, however, all this changes. The incessant 

 activity dies down with the failing light until darkness finds the fish entirely 

 inactive, floating motionless just under the surface. Thus they remain 

 throughout the night, drifting back and forth with the waves, or hanging 

 suspended under a glassy surface. Little co-ordination exists within the 

 school at night, for a spotlight flashed upon the school shows the fish point- 

 ing in all directions: there is no marked orientation. This is particularly 

 true on quiet nights. When the surface is agitated by wind, however, there 

 is a vague indication of orientation with a tendency on the part of the 

 individuals to head into the waves. The following table (10) shows the 

 results of a series of observations on orientation in relation to the pre- 

 vailing night wind. 



Table 10 



ORIENTATION AT NIGHT OF LABIDESTHES 

 SICCULUS IN RELATION TO WIND 









