Examination of fallback activity is of 

 particular interest in this discussion since 

 there is a suggestion that the phenomenon 

 may be associated in some way with capacity, 

 i.e., an unusually high fallback frequency 

 may be taken as an indication that the fish- 

 way pools have become excessively crowded 

 and can no longer accommodate all fish en- 

 tering. 



Observations in the Bonneville facili- 

 ty during the past 2 years have shown that 

 moderate fallback occurred even at times 

 when crowding could not possibly have been 

 a contributing factor. This leads us to 

 believe that fish may move "to and fro" in 

 a f ishway as they do in other areas of their 

 natural environment. The point in question 

 then becomes one of differentiating between 

 normal and unusual fallback activity. 



A summary of fallback observations 

 made during the five 1957 fishway capacity 

 tests is presented in table 5. By far the 

 greatest number of fallbacks occurred in 

 the downstream or entry area of the fishway, 

 and there was a marked decline in fallbacks 

 as the fishway was ascended. Of all fall- 

 backs, 91 percent occurred at the lower three 

 weirs of the fishway. A similar distribu- 

 tion of fallbacks was indicated in 1956. 

 This suggests that there may be an initial 



period of learning or adaptation of fishway 

 conditions. Once the fish have Jiscended 

 several pools, orientation may become more 

 complete and there may be much less inclina- 

 tion to drift back from pool to pool in the 

 succeeding upstream areas of the fishway. 



To assess the magnitude of fallback 

 activity in each test, the total number of 

 fallbacks was converted to a percentage of 

 the total entry. The resultant values are 

 of particular interest. In tests 1, 2, and 

 5, fallback percentages were virtually iden- 

 tical despite the fact that there was a con- 

 siderable difference in the number of fish 

 entering the fishway in each test. Signifi- 

 cantly, these values were almost three times 

 as high as those shown for tests 3 and 4. 

 An interesting feature of this comparison 

 is that the brail-type release was employed 

 in tests 1, 2, and 5, while the free release 

 was used in tests 3 and 4. Clearly, the 

 use of the brail appears to have been a con- 

 tributing factor in markedly increaising 

 fallback activity. 



Graphic comparison of fallback fre- 

 quency in the five 1957 trials is shown in 

 figure 8. Thus far in the experiments there 

 is no suggestion that fallbacks increased 

 in proportion as the number of fish entering 

 the fishway increased. 



Table 5. — Total numbers entering fishway and fallbacks at each 

 weiri' during five 60-minute capacity tests, 1957. 



[Note: 



A "brail type" release was employed in tests 1, 

 2, and 5. No brail was used in the release 

 during tests 3 cind 4.] 



Totals 



320 110 72 26 10 13 



551 



1/ There were 7 weirs in the fishway but a finger trap was 

 installed on the last weir (No. 7) to prevent fallbacks. 



2/ Estimate in part. 



11 



