92 ILLINOIS BIOLOGICAL MONOGRAPHS [92 



on September 12 1 was able to get only 5 after several hours of search over 

 a considerably more extended area. By adults are meant, of course, those 

 individuals which have passed through one winter and have spawned once. 

 Therefore it must be concluded that the old fish die at the age of about 

 fifteen to seventeen months, breeding but once and leaving it at that to 

 the young to carry on the species. 



SUMMARY OF RESULTS ON LABIDESTHES 



1. Labidesthes sicculus is the only fresh water representative of the 

 marine family Atherinidae found in Wisconsin waters. 



2. The fish are found only at the surface of the water, and under no 

 conditions will they descend below a meter from the surface. 



3. The spawning habits are described and data are presented which 

 show that the deposition of eggs begins when the water reaches a tempera- 

 ture of 20°C, with the optimum temperature of 23°C. 



4. The eggs hatch in approximately eight days at 23°C. 



5. The embryological development is unknown. 



6. On hatching, the young fish migrate at once away from the shore, 

 and assume a position over the deep water, living at all times within a 

 few centimeters of the surface film while the adults remain permanently 

 inshore. 



7. After hatching, the young grow very rapidly, and complete data 

 for the growth of the species are presented. 



8. When the young reach an average length of 1.4 cm, they migrate 

 into the shallow water each night and before sun-up they migrate back 

 again to their pelagic habitat over deep water. 



9. Data are presented which show that this inshore movement is co- 

 incident with a drop in temperature of the surface water over the deep 

 water, as well as with a change in the hydrogen-ion concentration of the 

 water at that stratum. 



10. Evidence is presented which shows that the shoreward migration 

 is in the direction of increasing light intensity, indicating a positive photo- 

 tropism. 



11. The fish are constantly active during the day; at night, if it be dark, 

 the fish remain quiescent over the shallows. 



12. If the night be light, due to a nearly full moon, the fish show ex- 

 treme activity, leaping out of the water and displaying unparalleled 

 vivacity. 



13. This activity can be induced on a dark night by introducing a 

 spot light on the water, proving the species to be positively phototropic. 



14. Further proof of this phototropism is presented by the fact that 

 the young can occasionally be lead entirely across the lake, a distance of 

 nearly a mile, as they follow a light; at all times they can be led to a point 



