392 



FISHEBY BULLETESr OF THE FISH AND WILDLIFE SERVICE 



experiments may be necessary to throw some light 

 on tlie nature of the ratio and to learn whether 

 the phenomenon has any significance beyond the 

 restricted circumstances of this experimental 

 method. 



DISCUSSION 



Wlien the results of the preceding experiments 

 are compared with the findings of other investi- 

 gators several interesting possibilities are sug- 

 gested. One possibility is that the response of the 

 fish to temperature differences might be a family 

 characteristic. The two species of fish, Pomolohuii 

 pseud oharengus (Wilson) and Pomolobu.s aesti- 

 valis (Mitchill), which at Bournedale showed a 

 consistent preference for warmer water, are mem- 

 bers of the family Clupeidae. Shelford and 

 Powers (1915) found that herring, Clufea 

 paUasi, fry preferred warmer water. The ol)ser- 

 vations of Roule (1933) indicated tliat migi-ating 

 shad, Paralosa nilotica rhodanensis and Alosa 

 aJosa, which also belong to the family Clupeidae, 

 selected water of higher temperature. 



The threshold of the response to temperature 

 differences shown by the two species of fish at 

 Bournedale (0.5° C.) agrees very closely with that 

 reported by Shelford and Powers (1915) for 

 herring fry. This threshold may seem high com- 

 pared to the minimum effective thermal stimulus 

 range for certain fresh-water fishes (differences 

 of 0.03° to 0.07° C.) reported by Bull (1936). 

 However, it must be remembered that this temper- 

 ature diffei-ence represents the minimum tempera- 

 ture difference which will provoke an uncondi- 

 tioned orientative response in the fish. The 

 minimum temperature difference which can be 

 perceived by the fish is probably much less. 



Powers and Hickman (1928) presented evidence 

 to show that rivers draining lakes usually had 

 lower CO2 tensions than other rivers (average 

 difference 0.5 mm. Hg) . Powers (1939) contended 

 that by means of these differences in CO2 tension 

 migrating fish could select certain types of streams. 

 The results of the CO2 experiments at Bournedale 

 would fit well into his argmnent. The alewife 

 and the glut herring both showed a strong prefei'- 

 ence for water of lower CO2 tension (threshold 

 difference 0.1 mm. Hg). These fishes usually 

 spawn in small ponds and shallow lakes and the 

 choice of water of lower CO2 content would result 



in the selection of streams leading to lakes or 

 ponds. 



The fundamental nature of the response of the 

 fish to differences in CO2 and to differences in 

 temperature is indicated by the manner in which 

 CO2 and temperature were able to dominate com- 

 peting orienting factors in the exploratory tests 

 shown on page 390. The low threshold values for 

 these responses also suggest their probable 

 importance. 



The experiments examining the orientative in- 

 fluence of i>\l (p. 3S8) confirm the conclusion of 

 Powers (1930) that /)H was largely ineffective as 

 a factor influencing the behavior of aquatic ani- 

 mals. Whether the response of the fish in the 

 experiments at Bournedale was to differences in 

 free CO2 or to tlie associated difforences in HCO.-, 

 was not actually established although the thres- 

 hold values involved suggest that the response was 

 to free CO2. If the response was to the free CO2 

 the influence of COo differences is limited to waters 

 with a pH of less than 8.4. This fact might be 

 used to advantage in an experiment to determine 

 to which of the two factors the fish are responding. 

 Although the main efforts of this investigation 

 were directed toward examining the orientative 

 influence of CO2 and temperature upon the mi- 

 grating fish, this was not meant to imply that 

 these are the only major influences which might 

 be concerned in other situations. The influence 

 of differences in dissolved O2 may be of great im- 

 portance when lower O2 values are concerned, 

 which was not the case at Bournedale. Under 

 .special conditions flow characteristics such as ve- 

 locity and turbulence may play an important part 

 in directing the fish. The role of olfactory mem- 

 ory needs to be further explored. The influence 

 of factors affecting the orientation of the fish in- 

 directly (e. g., by controlling the depth of swim- 

 ming) must be considered. It was largely to 

 stress the fact that many factors may be con- 

 cerned in fish orientation that the exploratory 

 tests involving velocity, turbulence, and visual 

 factors were included in this report. 



Perhaps one of the most important considera- 

 tions to which the foregoing experiments call at- 

 tention is that not only are there many factors 

 which may have a directional influence upon the 

 migrating fish but also that they must all be con- 

 sidered together. The experiments examining 



