From the foregoing it may be concluded 

 that there must have been an accumulation of 

 slow fish in the lower pool of the fishway. 

 If there is a tendency for slow-moving fish 

 to accumulate in the lower pool(s) of the 

 fishway (further experimentation will be 

 necessary to verify this) then the restric- 

 tion on capacity will be determined by the 

 residence in a single pool and not on an 

 average residence for all pools. 



Whether the phenomenon was a natural 

 occurrence or the result of experimental 

 procedure remains to be determined. Should 

 the relationship persist in subsequent 

 experiments in which varying release tech- 

 niques are employed, it may be possible to 

 demonstrate that a natural accumulation of 

 slow fish develops in the lower section of 

 a fishway under crowded conditions, and 

 that resultant spatial restrictions will 

 undoubtedly influence passage (capacity) 

 through the fishway. If this be true, the 

 suggestion has been made that some of the 

 lower fishway pools could be constructed 

 considerably larger than succeeding upstream 

 pools to provide additional space for the 

 slow fish and at the same time allow suffi- 

 cient passage area for the other migrants 

 which proceed up the fishway more rapidly. 



Fallbacks 



In some of the initial capacity tests 

 involving the release of relatively small 

 numbers of fish, a few fish in each trial 

 were observed to enter a pool and then drift 

 back downstream into the pool below. Fish 

 exhibiting this particular behavior were 

 called "fallbacks". As the number of fish 

 introduced in each test was increased, the 

 number of fallbacks also increased. The 

 frequency leads us to speculate that the 

 phenomenon might be a possible indicator of 

 a capacity condition. 



first observed at the lowermost weir and 

 were subsequently noted at successive up- 

 stream weirs with respect to time (table 3, 

 Appendix). Initial fallback activity in 

 each pool aippeared to commence at about the 

 time considerjjjle numbers of fish had col- 

 lected in that pool (tables 3 and 4, Appen- 

 dix). 



However, aside from the actual number 

 of fish involved in a given test, other 

 factors may have influenced the frequency 

 of fallback activity. For instance, we 

 observed that streaming flows could very 

 well have been responsible for a consider- 

 able number of feillbacks since the fish 

 would orient to the surface area of the 

 pool rather than in the subsurface levels 

 as was generally the case when plunging 

 flows prevailed. This surface alignment of 

 fish during streaming flows increases the 

 possibility of drifting back downstream 

 over the weir crest and into the pool below. 



We also observed that steelhead 

 appeared to be more inclined to fallback 

 activity than the other salmonoids. Conse- 

 quently, the species composition in a given 

 test might well affect the number of fall- 

 backs. 



Fallbacks were also observed in other 

 experiments at the Bonneville facility in 

 which individual fish or small groups (30 or 

 less) of fish were being timed up the fish- 

 way. Here the implication was that fallback 

 activity was not the result of crowding but 

 due to some other factor. Continued study 

 on the nature of fallback activity will be 

 necessary to establish the import of this 

 phenomenon as it may be related to fishway 

 capacity. 



DISCUSSION 



In the September 7 trial, fallbacks 

 were recorded at each of the four lower 

 weirs with respect to time. The following 

 totals were observed during the 2-hour test 

 period: at weir 54-566 fish, weir 55-392, 

 weir 56-82, and weir 57-35. Total numbers 

 of fallbacks observed do not necessarily 

 imply the actual number of individual fish 

 involved since a single fish may have been 

 responsible for several fallback observa- 

 tions. Apparent in these observations is 

 the progresfive decline in fallbacks as the 

 fishway is ascended. Also, fallbacks were 



Certain limitations must be considered 

 when using results obtained under the pre- 

 sent experimental technique. The very 

 nature of the experiments requires that 

 large numbers of fish be readily available 

 for use in the tests. Since it was not 

 possible to supply sufficient numbers in- 

 stantaneously, the method required that 

 fish be collected and held until a presumed 

 ample supply was on hand for an experiment. 

 It is not known what effect the collection 

 period may have had on the eventual motiva- 

 tion of the fish once they were released. 



16 



