36 NORTHERN FISHES 



In some cases raising the water level a few feet seems to be the best 

 remedy. A rise of only a few feet in the water level of many of our lakes 

 now suffering from low-oxygen conditions would provide a sufficient 

 volume of water to hold the necessarj^ oxygen supply through the winter. 

 This procedure is particularly effective with the lakes, such as Big 

 Kandiyohi Lake in Minnesota, which fell many feet during the drought 

 years of 1932-35 and have never regained anything like their former 

 levels. If the levels of these lakes can ever be restored and maintained, 

 oxygen may once more be retained in sufficient quantities to hold the 

 fishes over the winter. 



Remedying oxygen depletion is very difficult. There is no definite 

 amount of oxygen that can be said to be absolutely necessary to sustain 

 fish life. Moreover, fishes differ greatly in their requirements. Unfortu- 

 nately most game fishes require more oxygen than many rough fishes 

 and hence often die while the rough fishes survive. When the oxygen is 

 about one-third of the normal amount present in the summer, the 

 fishes are in danger. At times fishes may survive on much less, where- 

 as at other times they may die when this point is reached. These 

 differences are probably due largely to the condition of the fishes and to 

 the temperature and other conditions of the water. Often in emergency 

 cases where the oxygen is rapidly disappearing there is no immediate or 

 practical remedy. A careful study of the oxygen supply in various lakes 

 during the late winter will determine when the supply is approaching 

 the critical point. If no means are available for application of some 

 method of aeration, the only procedure left is to rescue or salvage as 

 many fish as possible before it is too late. 



In many lakes the spawning conditions can be improved. Conditions 

 for natural propagation should be maintained at the highest possible 

 levels in order to maintain the natural production of fishes. Keeping up 

 the natural production is much cheaper than artificial propagation and 

 stocking. For gravel-nesting species gravel beds should be constructed 

 in lakes with muddy shores. In lakes with very soft mud bottoms boxes 

 of sand may be placed for bass nests. The production of bluntnose 

 minnows, one of our most valuable forage fishes, may be greatly in- 

 creased by placing spawning boards in shallow water, because these 

 fishes deposit their eggs on the underside of submerged objects. The 

 protection of spawning beds for sunfishes, bass, and crappies from dis- 

 turbance during the season when the adult fishes are on the nests is 

 important. Several bays containing good spawning beds will produce 

 as many fry as the best rearing pond. 



The control of rough fishes is one of the important steps in lake im- 

 provement. Rough fishes such as carp and dogfishes may compete 

 >vith the game fishes or may destroy conditions necessary for such fishes. 

 In some states carp are used as forage fishes because they multiply 

 rapidly and produce a large amount of food for game fishes. However, 



