380 



FISHEEY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



temperature, in pH, and in dissolved gases between 

 the two channels were created experimentally. 

 The influence of these differences in the physical 

 and chemical characteristics of water upon the 

 orientation of the migrating fish was measured 

 by the number of fish choosing each channel. 



The experimental trough was 21 inches wide, 

 10 inches deep, and IS feet long. It was open at 

 both ends so that when it was alined with the 

 direction of stream flow and partially submeiged, 

 the water flowed freely through it. The two 

 channels in the upstream end of the trough were 

 10 feet long and 10 inches wide. A 10-inch chan- 

 nel width was chosen because it was approximately 

 equal to the length of the fish and would allow 

 enough room for normal swimming movements 

 and turning. The experimental trough could be 

 raised or lowered in the water in order to create 

 any desired depth of flow regardless of the natural 

 fluctuation in the water level of the stream. The 

 flow of water through the trough was maintained 

 at a depth of 6 to 8 inches, while the velocity of 

 water through the trough varied, depending on 

 stream conditions, from a minimum of 1 foot a 

 second to a maximum of 2 feet a second. 



The trough was stained a dull mahogany to 

 provide a dark background for the fish so that 

 their behavior would be as natural as possible. 

 Exploratory tests the previous year had shown 

 that a light background made the fish extremely 

 nervous and excitable when they were in the shal- 

 low water and confinement of the experimental 

 trough. The trough was also placed in the stream 

 several weeks before tests were begun to reduce 

 the possibility of odors, or other factors that might 

 be unfamiliar or objectionable to the fish. 



To prevent light inequalities between the two 

 channels resulting from shadows, the experimental 

 trough was shaded from direct sunlight by a can- 

 vas canopy (fig. 3). 



At the downstream end of the trough a wire- 

 screen gate was installed to control the entrance 

 of the fish into the experimental trough. The 

 necessity for such a device became apparent during 

 preliminary exploratory tests in which the en- 

 trance of the fish was unrestricted. Wlien several 

 fish entered at the same time, they frequently ex- 

 hibited a schooling tendency and all followed the 

 choice of the leader. Their behavior was such that 

 it seemed probable that only one decision was 



FiGUKE 3. — General view of experimental station, Herring 

 River, Bourneclale, Mass., 1950. 



A. Entrance for upstream migrants. 



B. Bypass exit for downstream migi-ants. 



C. Laboratory for chemical determinations. 



D. Light-control arrangement. 



actually made for the entire school. There were 

 instances of schools splitting, with all those on the 

 left side entering the left channel, and all those 

 on the right side entering the right channel, sug- 

 gesting that perhaps in these cases spatial rela- 

 tions alone were involved. There were many 

 variations in such group behavior and they were 

 difficult to interpret in a quantitative way. To 

 avoid the problem completely, an entrance gate 

 was designed. Through this gate (figs. 4 and 5) 

 the fish were allowed to enter, one at a time, and 

 only after the previous one had made its decision 

 and was completely out of the trough. 



The entrance gate also served to center the fish 

 so that as it entered the experimental trough it 

 was subjected to a mixture of the waters of both 

 channels. Thus, the fish started in the center of 

 a strong transverse gradient (fig. 2) and almost 

 any lateral movement resulted in its being sub- 

 jected to water of a different quality. 



PROCEDURE 



As the fish progressed upstream in the trough 

 to the point where it had to choose between the 

 two channels (the "point of decision" in fig. 2), it 

 usually moved from one side of the trough to the 

 other, alternately approaching each channel until 

 it finally entered one. In those tests in which 

 both channels were completely unobstructed the 

 choice was recorded when the fish had completed 



