606 



tf. S. BUREAU OF FISHERIES 



summer of 1930. Collections of the plankton and of the bottom and 

 \veed-dwellin<^ organisms were made at 2- week intervals throughout 

 this time. At the same time determinations were made of the chemi- 

 cal conditions. 



The observations revealed that the sloughs differ widely in the 

 production of natural food, that the bottom and weed populations 

 are apparently composed of few species but large numbers of indi- 

 viduals, and that the character of the bottom of a slough can be 

 used in predicting the quantity and density of the food it contains. 

 Gas analyses of samples taken in winter beneath the ice showed that 

 the oxygen supply of the sloughs investigated was exhausted by the 

 end of January. For this reason it will be impossible to hold fish 



Figure 8.- 



-Limnological invrstisations in the sloughs of the Upper Mississippi Wild 

 Life and Fish Kefiise 



in the sloughs during the winter and holding ponds for the brood 

 fish must be provided elsewhere. Complete analysis of the data 

 accumulated in these studies will not be possible for several months. 



A few of the sloughs have no direct connection with the river, 

 but the great majority have one or more natural channels which 

 connect with the river except at low stages. These channels must 

 be closed by screens when the sloughs are used for the propagation 

 of bass or other game fishes. It is also necessary to remove the larger 

 aquatic plants and the bushes lining the shore so that the entire 

 slough can be seined. 



The first serious attempt to propagate largemouth bass in the 

 sloughs was made during the season of 1930. A number of sloughs, 

 which had been selected for the purpose, were cleared; and screens 

 constructed across any connection with the river. After the enemy 

 fish had been removed the sloughs were stocked with forage minnows 



