76 ILLINOIS BIOLOGICAL MONOGRAPHS [76 



silversides along seventy-five feet of the north shore of Oconomowoc lake 

 at night, from July 10 to September 24, 1923. 



Once the fact of this nocturnal-diurnal migration was established, the 

 next step was to accumulate the facts concerning it: the time at which it 

 occurred both in the morning and at night; the conditions existing when 

 the migration did not occur as well as when it did occur; a correlation 

 between light conditions and migration; chemical conditions of the water 

 in possible relation to the movement, and various other phases of the 

 problem. These will now be discussed. 



The fact that young Labidesthes sicculus proceed after hatching to a 

 pelagic habitat over deep water, while the adults select a shoal environ- 

 ment was noted by Hubbs (1921), who dismisses the subject with the single 

 sentence: "As these young fishes rapidly grew during the summer, they 

 showed less aversion to shoal waters." It is evident, therefore, that Hubbs 

 has missed entirely the tremendously interesting fact of the nocturnal 

 migration. 



In an effort to analyze the change in preference from a suprapelagic 

 habitat to a shoal environment as the fish ages, Hubbs offers a series of 

 possible causes, and rejects them all in turn. It is well to summarize his 

 conclusions briefly, 1) Temperature: this is cast out as a deciding factor 

 as the difference in temperature of the water inhabited by the young over 

 deep water and the adult over shallow water is too small to be effective and 

 is wholly inconclusive. 2) Light: since both adults and immature live 

 very near the surface the light intensity for each would be essentially the 

 same. 3) Oxygen: the amount of oxygen is so nearly the same in both 

 habitats and the range overlaps to such an extent that this is eliminated 

 as the causing factor. 4) Food: is eliminated on the grounds that food is 

 abundant where the species is not found. 5) Protection: is eliminated on 

 the ground that the young were found to "avoid shoals even when other 

 fish were not in evidence" nor "can the seclusion from enemies explain the 

 invasion of the shoals by these young silversides in the late summer, nor 

 the exclusive presence there of the adults." 6) Reaction: Hubbs reaches 

 the conclusion that the young have a negative reaction to large objects, 

 hence seek the uniform environment over deep water, and bases his con- 

 clusions partially on the fact that the fish "swam away from the boat" 

 as it approached. Had Hubbs hit upon the night movements of the fish, 

 this conclusion would have had to be given up. 



The discovery by the writer of this nocturnal diurnal migration puts 

 an entirely different aspect on the problem, which becomes not what causes 

 the young to assume a different habitat from the adult during the early 

 summer and causes them to come back again to the shallow late in the 

 summer? but: what are the factors at work that result in the production of 

 this daily migratory phenomenon? 



