REACTIONS OF FISHES TO SALTS 247 



point brought out by the conductivity measurements was that 

 the water, after flowing in at the ends of the tank for 15 min- 

 utes or less, often showed a pihng up of the salt at a point about 

 two-thirds of the way to the tap water end, i.e., a little past 

 the middle. This piling up was brought out graphically by the 

 use of colored salt solutions, which showed a more intense color 

 at this point for a short time. Later the deepening in color 

 disappeared, and tests showed the gradient to be continuous 

 from one end of the tank to the other. 



Before the fact of the piling up of the salt was discovered, it 

 was noted that the fishes often gave a negative reaction to this 

 part of the tank. With the demonstration of the increased salt 

 concentration at the point in question, and the fact that the 

 increase disappeared after the flow at the ends had been on for 

 about 30 minutes, most of the experiments were delayed until 

 sufficient time had elapsed for the adjustment to take place; 

 any marked reaction of the fishes at the point of higher concen- 

 tration, was noted and recorded. That the gradient as shown 

 in figure 1 is a typical gradient is supported by the fact that 

 Shelford and Powers ('15) figure a similar gradient which they 

 obtained between sea-water and fresh water, in their work with 

 marine fishes. In the following gradient experiments, attention 

 should be called to the fact that the reactions whether positive 

 or negative are seldom 100 per cent reactions. In other words, 

 the fishes are nearly always positive to some concentration of 

 the salt in question. It seems that for most fresh water fishes 

 there exists an optimum salt concentration somewhere between 

 a O.OIN and that of the tap water. This fact is brought out in 

 the experiments with a majority of the salts. 



The species of fishes used principally have been the black 

 bullhead (Ameiurus melas), blue gills (Lepomis pallidus), rock 

 bass (Amblopites rupestris), green spotted sun fish (Lepomis 

 cyanellus), white crappie (Pomoxis annularis), pumpkin seed 

 (Eupomotus gibbosus), and small mouth black bass (Micropterus 

 dolomieu). Numerous experiments have also been run with 

 various species of Cyprinid minnows. 



