84 ILLINOIS BIOLOGICAL MONOGRAPHS [84 



still eaten commonly by the adults, and since insect food is notably absent 

 over the deep water of the lake, it must be assumed that the change in 

 food habits is a secondary one, a result of the changing of the environment, 

 rather than a cause of it. It is a well known fact that the food habits of a 

 species undergo radical changes as the fish grow; this has been pointed 

 out by Forbes, Turner, Pearse, and others, but in no case has it been shown 

 that this change causes migration or in any way disturbs the distribution 

 of the species. However, the consideration of the cisco later furnishes an 

 example of just this type of phenomenon. The case of Labidesthes is 

 different, and it is necessary to consider the change in diet as characteris- 

 tically an accompanying phenomenon, not a causing one. 



There can be no doubt but that the habit of the young silversides in 

 assuming a pelagic environment over deep water is a very great protection 

 to the species. As they are for the most part the only inhabitants of these 

 waters, they escape the very keen competition existing in the shallow water. 

 Their very small size would make them ideal food for the dozens of species 

 of larger fish which feed in the shallows along the shore. As it is their ene- 

 mies are reduced to a minimum as long as they remain over the deep water. 

 In this habitat they are preyed upon by the following species: Clidonias 

 nigra surindmensis ; Sterna hirundo; Lepisosteous osseus; Leucichthys artedi 

 which occasionally comes to the surface and captures them; Micropterus 

 dolomieu; Micropterus salmoides. The last two named make occasional 

 raids into the region and capture a good many silver sides. 



However, once the species begins its shoreward movement, the number 

 of enemies increases tremendously. I have found remains of silversides 

 in the stomachs of the following animals which captured the fish while in 

 the shallow water: 



Micropterus dolomieu; Micropterus salmoides; Amiatus calvus; Esox 

 lucius; Amploplites rupestris; Lepomis cyanellus; Lepomis incisor; Eupo- 

 motis gibbosus; Roccus chrysops; Perca flavescens; Esox vermiculatus (Fox 

 river only) ; Leucichthys artedi (during spawning season only) ; Botaurus 

 lentiginosus ; Ixobrychus exilis; Ceryle alcyon; Mergus senator; Necturus 

 maculatus; Chelydra serpentina; Natrix sipidon; Cambarus spp. ; and 

 Mustela vison. 



Thus it becomes evident that the species is tempting fate when it 

 comes inshore, and that this temptation results in wholesale destruction 

 of the species is evidenced by the curve showing the abundance at night 

 along the shore (Fig. 14). Here is shown the tremendous decrease in 

 numbers that occurs between August 6 and September 6, when the silver- 

 sides are reduced to such an extent that they no longer offer the wholesale 

 food supply to their competitors. From these facts two conclusions can 

 be drawn: 1) that the pelagic habitat is a great protection for the young 

 as long as they remain in it; and 2) that the change from the deep to the 



