ORTF.NT.\TinX OF MIGR.VTIXr, .\X.\DROMOUS FISHES 



389 



T.vnt.E 9. — Experinicntu icith CO-, tabiihtted accortliiir/ to 

 diffe rones in pH, May 10'i9 



Tviii.K 12. — Measun mnnts of free CO, taken during the 

 tents sltmrn in taljlc Jl 



[Samples titrated with N'aOH] 



' K;iilBe, iill ti.U to pH U.S. 



Taui.e 10. — Ejcix rinicnis iritli XaOII tnhiiloted accordinii 

 to differences in pH, May H, 19^9 



> Unmodified stream water, pH 6.6 to pH 6.8; low pH; high COj content. 

 2 High pH: low COj content. 



modified water. Although CO2 was not measured 

 in this test it seems very probable that the dif- 

 ference in free CO2 which would be associated with 

 tlie large diiference in pYi would exceed the 

 threshold 0.3 p. p. m. 



After several acids and bases were tried, an ex- 

 periment was undertaken using KjHCPOj) as the 

 modifying agent. The data collected (table 11) 

 reveal no indication of a response on the part of 

 the fish although differences in /'H were present. 

 These differences in pH were of the same magni- 

 tude as those in many CO2 tests in which the fish 

 responded. The accompanying table 12 of free 

 CO2 measurements made while the tests were in 

 progress shows that during the tests differences of 

 free CO2 between channels were very small, gen- 

 erally less than the precision of measurement. 



T-'iiiLE 11. — Orientative influence of pE as shoun in tests 

 made June 10-12, 1950 



(Differences in pH include differences ±0.1 pH unit] 



On the basis of the evidence froiii lliis e.xpeii- 

 iiieiit. it wdiild scciu reasonable to conclude that 

 tlie response shown by tlie migrating fish in (lie 

 ])revioiis tests was a response to differences in ('(X 

 ratlicr ihiiii a I'esponse to diff'erences in pH. 



EXPERIMENTS USING NITROGEN AND OXYGEN 



Nitrogen 



During the tests with CO2, particularly those 

 tests in which the CO2 had been added to only one 

 of the two channels, the possibility had to be con- 

 sitlered that the i)hysical presence of many bub- 

 bles in the modified channel might be influencing 

 tlie choice of the fish. To eliminate this possi- 

 bility nitrogen was used as a control. The gaseous 

 nitrogen was bubbled into the water of one of the 

 liiannels in the same manner and at the same rate 

 as the CO2 in the previous experiments. The ad- 

 dition of the N2 produced no measurable differ- 

 ence ill the amount of dissolved O2. The data 

 (table 13) reveal that the fish failed to show any 

 response to the nitrogen or to the presence of the 

 many bubbles produced. The nitrogen was then 

 used as a control in a series of CO2 tests. The 

 results f)f these tests show the same response to CO2 

 that was shown in i)revioiis tests, again indicating 

 tliat tlie presence or absence of the bubbles had no 

 influence upon the choice of the fish. 



Oxygen 



To investigate the orientative influence of O,, 

 the migrating fish were presented with a choice of 

 waters which contained (btl'erent amounts of dis- 

 solved O2. 1 he water of one of the channels was 

 modified by the addition of gaseous O.. Tlie ex- 

 periment, however, was severely limited by the 

 fact that the water of the stream was already more 

 than biO percent saturated. The data in table l."> 

 show that the fish did not respond to a relatively 

 small difference in O2 luuler these conditions. The 



