ORIENTATION OF MIGRATING ANADROMOUS FISHES 



391 



Table 16. — Orientative influence of the visual factor 



T.Mii.K 17. — Influence of sex on the response to COz ami 

 tcwpcriiture 



' Heat added, 2° C. 



ill liydriiiilic conditions between the two chiinnels. 

 Tlio oxporimeiits were done with relatively small 

 iHiinl)ers of fisii iuid Ihe (le<>ree of the response 

 should not tic iiitfr])i('ted too literall}'. However, 

 tlie e.\])(>riniciils are jireseiited here bcause they 

 do iiidicMic tliat ciiaracteristics of flow, snch as 

 velocity and turliulence, can have a direct ional 

 influenci' ii[)on migrating fish. They also indi- 

 cate tliat, under some circumstances at least, 

 visual factors are capable of influencing fish 

 orientation. 



THE 3 : 1 RATIO OF THE RESPONSE 



The explanation for the persistence of the ap- 

 proximately :'> : 1 ratio in the response of the tish to 

 temperature ditferences and to CO2 differences is 

 not readily apparent. The absence of a response 

 much closer to a 100 percent response under such 

 controlled conditions would seem to indicate that 

 only a proportion of the fish were influenced by 

 the orienting factor. If, for example, half of the 

 fish were influenced by the testing factor and the 

 other half entered the channels at random, the 

 resulting ratio would be 3 : 1. 



A possible sexual variation in the response of 

 the fish was considered. A trap was placed at the 

 head of each channel and after a series of CO- 

 tests had been run, the fish in each trap were 

 examined for sex. This procedure was later re- 

 peated with a series of temperature tests. The 

 data (table IT) indicate that the sex of the fish 

 has no effect upon its response to differences in 

 CO. and temperature. 



The possibility of individual variation in the 

 sensitivity of the fish to temperature differences 

 and to CO:; differences was examined by gradually 

 increasing the differences between the two chan- 

 nels. Had there been significant individual vari- 

 ation in sensitivity, the response would have be- 

 come greater as the difference between channels 

 gradually exceeded the thresholds of more and 



more fish. This, however, did not occur. The 

 r('S|)onse remained ajiproximatelj' 3:1 even at the 

 niaximum attainable dilleiences (>7.00 p. p. m. 

 free CO, and 3.0° C). 



The possibility was also considered that some in- 

 dividuals might be completely insensible to the 

 ilifferences in temperatuie and CO, which were 

 being used in the experiments. An experiment 

 was set up in which lish that had i)reviously been 

 tested were again subjected to the same choice. 

 A trap was placed at the head of each channel 

 and a series of CO, tests were made. The trapped 

 fish were then brought back to the entrance of the 

 experimental trough and the CO, tests repeated. 

 The data (table IS) show that the response of 

 the fish that had i)reviously entered the channel 

 with the higher CO_. was a])pniximately 3: 1 in 

 favor of the channel with the lower CO,, and those 

 fish which had previously entered the channel with 

 tiie lower CO-, also exhibited a 3:1 response in 

 favor of the channel with the lower CO2. 



Such evidence strongly suggests that the 3 : 1 

 latio is not due to the failure of particular in- 

 dividuals or particular groups of individuals to 

 respond to the orienting factor. It seems more 

 jirobable that the explanation lies in the behavior 

 patterns inherent in all the fish. If, for example, 

 every fish responded to an orienting factor only 

 half the time and acted at landom the other half, 

 the result would also lie a '■'> : \ ratio. Further 



