OHIKXTATIOX OK .MICKATING AXADHO.MOUS FISHES 



387 



RELATION OF TEMPERATURE RESPONSE TO 

 TEMPERATURE LEVEL 



Taltiilal ion of cxpciiiiiciilal data aiioidiii;^ to 

 stream temperature levels (table (J) revealed a 

 ireiieral teiHlenvv lor the response to temperature 

 (liti'erences between 0.5° and 1.0° C. to decrease as 

 I he temperature level of the stream increased. 

 When the temperature ditl'ereuces between chan- 

 nels were greater than l.n ("., the temperatuie 

 level of the stream had no discernible influence 

 u]><)n the response of the fish. 



Tabi.k (i. — Rvlittiiiii of tinii>rriiliirr rrxponxc t(i l( iiiin i uliin 

 Uvil 



' Temperature difference between chaimels, 0.5° to 1.0° C. 

 2 Temijerature ditlerence between channels, 1.1° to 3.0° C. 



Talvinjr into account once ajzain the eddyinii' con- 

 dition (see p. 385), the data indicate that the re- 

 sponses of the fish were atfected by the tempera- 

 ture level only vrhen temperature differences of 

 thre.sliold magnitudes were concerned. The evi- 

 dence suggests a possible relation between this 

 phenomenon and the type of threshold phenomena 

 described by Weber's law. Weber (1846) be- 

 lieved the threshold of difference to be propor- 

 tional to the intensity of stimulus. Although the 

 ratio of these two factors has since been shown 

 to lie variable, a tendency for the threshold of 

 difference to increase with an increase in intensity 

 of stimulus has been observed. Tlie experiments 

 !it Bournedale, planned with other purposes in 

 mind, did not produce the type of data necessary 

 to examine tliis particular aspect of the response 

 to temperature differences. However, it seems 

 very piobable that the decrease in the response of 



the fish (shown in group A, taiile (i) was due to 

 an increase in the threshold of the response to 

 li'inpiMat lire dillVrences as the leinpeiature Icxel of 

 ihe stream increased. 



ORIENTATIVE INFLUENCE OF CO2 



The migrating lish were jjresented with a choice 

 of waters having different amounts of free, or 

 uncombined. CO;. The difference in free CO2 

 between the waters of the two channels was estab- 

 lished by the direct addition of gaseous CO2 to the 

 water of one channel while the other remained 

 unmodified. During some of the tests this pro- 

 cedure was varied by the addition of the gaseous 

 CO2 to both channels but at different rates. 

 Throughout the tests the fish indicated a definite 

 preference for the water with the lower free CO; 

 content (table 7). 



Table 7. — Responsr.i to iliffcii nci x in ('()• hh xhoirii in 

 CJ-ijeriments, Mail I -III. I'.i'i'-K and Mnii 7^/i(»c 'J, JUiiO 



Free C02 difference between channels 



Number of 

 decisions 



>4.0 p. p. m. 

 1.4 p. p. m. 

 0.6 p. p. m_ 

 0.3 p. p. m.. 

 0.2 p. p. m. 



1.120 

 128 

 063 

 216 

 157 



Entered cbannel with— 



Lower 

 COj 



Percent 

 73 

 77 

 71 

 69 

 59 



Higher 

 COa 



Percrnt 



23 

 29 

 31 

 41 



Note. — Unmodified stream water varied from 0.7 p. p. in. free COi to 2.9 

 p. p. m. free CO: and pH 6.9 to pH 6.4. Stream temperatures 11.1° to 22.3° 

 C. COi differences listed include differences ±0.1 p. p. m. of listed dilTer- 

 ences except 0.3 which includes only -f-O.l p. p. m. 



"Whether the fish in the CO2 experiments were 

 responding to differences in free COj or to associ- 

 ated differences in HCO3 is not actually known. 

 The data (table 7) indicate that if the response 

 was to differences in the amount of free CO., the 

 threshold of the response lies below differences 

 of 0.3 1). p. m. If the response of the fish was to 

 differences in IICO.^i, then the threshold of the 

 response must be much lower. During most of 

 the tests in which the differences in free CO2 were 

 less than l.n p. p. 111.. the differences in IICO^^ 

 were not even measurable by the method of meas- 

 urement used (i. e., they were less than 0.1 p. p. m. 

 IICOj as CaCO:,). It seems more probable that 

 the response of the fisli was a response to free 

 CO2. 



The experiments do not indicate whether the 

 respon.se of the fish (assuming that the response 

 was to free CO2) was to differences in the amount 



