394 



FISHERY BXTLiLETIN OF THE FISH AND WILDLIFE SERVICE 



lation by means of symmetrically placed receptors 

 and turn toward (or away from) the maximmn 

 stimulation. If this was the method of orienta- 

 tion, the orientative influence of gradients would 

 be limited to those gradients great enough so that 

 the differences between points a few inches apart 

 (the distance between paired receptors) are above 

 the tlu'eshold of sensory perception of fish. Grad- 

 ients as high as this do actually exist in nature but 

 usually only as n narrow zone between two hodies 

 of water which are just beginning to mix. 



A second metliod \>\ wliicli a lish might orient 

 in a gradient is similar to the ''trial'' juethod of 

 orientation characterized by Jennings (1906) as 

 "selection from among the conditions produced 

 by various movements." He points out that in 

 this type of behavior, "Each stimulus causes as a 

 rule not merely a single definite action that may 

 be called a reflex, but a series of "trial" movements, 

 of the most diverse character, and including at 

 times practically all the movements of which the 

 animal is capable." This type of orientation 

 Fraenkel and Gunn (1940) have labeled "klino- 

 taxis," and defined as "a directed orientation 

 made possible by means of regular deviations and 

 involving comparison of intensities at successive 

 points in time," and they pointed out examples of 

 such behavior in many invertebrates. In this 

 method of orientation the fish, subjecting itself to 

 varied conditions by its active movements, would 

 select the most favorable condition or direction. 

 The method involves both movement and com- 

 parison of intensities of stimulation that are suc- 

 cessive in time. 



The behavior of the fish as they selected a chan- 

 nel in the experimental trough at Bournedale 

 strongly suggested that this latter method of ori- 

 entation is the one used. As the fish entered the 

 trough they usually swam from one side to the 

 other, approaching first one channel and then the 

 other. When once the fish had left the narrow 

 zone in which the water was mixing (fig. 2) it was 

 no longer subjected to a gradient. Its sensory 

 receptors were all subjected to the same intensity 

 of stimulation and, therefore, the current was the 

 only orientative influence dii'ecting the niovement 

 of the fish upstream, unless memory was involved; 

 Yet the fish repeatedly swam from one side of the 

 trough to the other and f recjuently even after en- 

 tering one channel, they turned back and entered 

 the other. 



Orientation by means of a comparison of in- 

 tensities that are successive in time would make 

 it possible for fish to become oriented in much 

 smaller gradients than would be necessary if orien- 

 tation was by a simultaneous comparison of in- 

 tensities. The minimum gradient in wliich a lish 

 could orient would depend on the speed of the 

 fish and the time interval of its sensory memory. 



Tlie experiments at Bournedale have demon- 

 strated that one group of migrating anadromous 

 fishes will orient with reference to differences in 

 certain physical and chemical water characteris- 

 tics created artificially. It is logical to suppose 

 that the fish will also be oriented by similar dif- 

 ferences occurring along their migration routes 

 in nature. 



SUMMARY 



The iDurpose of this investigation was to exam- 

 ine experimentally the influence of certain physi- 

 cal and chemical water characteristics upon the 

 orientation of one type of migrating anadromous 

 fish. The migrating fish were presented witli a 

 choice between two channels that carried water 

 with different characteristics. The orientative in- 

 fluence of the water properties in question was 

 measured by the number of fish selecting each 

 channel. The reactions of more than 8,000 lish 

 of the genus Poinolohus — alewife, P. pseudoharen- 

 gus (Wilson), and glut herring, P. aestivalis 

 (Mitchill) — were tested as the fish migrated wp- 

 stream through the Herring River at Bournedale, 

 Mass., toward their spawning area, in the springs 

 of 1949 and 1950. 



The fish were not removed from the stream or 

 handled in any way. Each was tested individ- 

 ually and tested only once (the few exceptions are 

 noted). The findings of this investigation were: 



1. Presented with a choice of waters having dif- 

 ferent temperatures, 77 percent of the fish entered 

 the channel with the warmer water when the tem- 

 perature difference continuously exceeded 0.5° C. 

 The temperature differences examined ranged 

 from 0.4° to 3.0° C. Water temperatures during 

 experiments varied from 11.1° to 22.3° C. 



2. The response of the fish to temperature dif- 

 ferences near the threshold difference decreased 

 as the temperature level of the water increased. 



3. Presented with a choice of waters having 

 different amounts of free CO2, 72 percent of the 

 fish entered the channel with water of a lower 



