jm;()Gi;kss in iwolock ai. i.\(iriiiiKS, 1929 



1109 



vatioiis arc bciii*^" coiitimicd (lir()ii<j,li llic winter so tlicrc will soon be 

 available detailed iiifoi-mat ion I'cjjfardinjj: condititioiis in the slon^jfhs 

 tlii'ou<;hout the yeai-. The limnolo^ical woik includes the collection 

 of sani])les of the plankton and bottom fauna and detei'niinations 

 of dissolved oxyg'en. free and fixe(| carbon dioxide, and alkalinity at 

 2-\veek intervals. 



Studies of the plankton and bottom oreanisms have shown that 

 the sl()u<>hs are very rich in fish food and are apparently capable of 

 .supportin<ir a much larger fish population than is present usually. 

 However, Mr. Surber has found that in most of the sloughs the 

 supply of dissolved oxygen becomes ra})idly depleted after they 

 fieeze over, and that by the middle of December the oxygen con- 

 tent is below 0.5 parts per million. In several instances fish half 

 dead from asphyxiation came up to the holes that had been chipped 

 in the ice for making the collections. It is evident th.at uiidei- huch 



FiciKE S. — Cli'iining slouuh for icnriiif;- I):is« flii;ifi lings In Upper Mississippi River 

 Wild Life iuul Fish Rpfuge 



conditions very few fish can survive in the sloughs through the win- 

 ter. Doubtless it is owing to this fact that sloughs which are over- 

 flowed only at infrequent intervals usually contain very few fish 

 even though there may be an abundance of food available. These 

 studies indicate that, in general, the small slough fishes eat the food 

 that is most available in tlieir particular environment and exercise 

 little choice or selection. There are, however, numerous exceptions 

 to this rule, especially among the minnows. 



COXTROL OF AQUATIC VEGETATIOX 



One of the greatest problems in connection with fish cultural work 

 in the sloughs is the control of aquatic vegetation. Unless the plants 

 are kept within bounds the sloughs become choked with a rank 

 growth of both emergent and submerged ac^uatics which absoluteh' 



