Table 4, — Maximum number of fish present in fishway and mean 

 passage time (minutes) required to ascend fishway 

 in each of five fishway capacity trials, 1957. 



the water, sexual maturity, and spawning 

 locale of the migrants may be some of the 

 underlying causes. 



Differences in the performance of 

 various species of migrating fish have also 

 been observed. Experiments in June 1956 

 give insight into the variability among the 

 performances of several species of salmo- 

 nids. The following average passage times 

 (minutes) were required to ascend a 6-pool, 

 l-on-16-slope fishway with a width of 11.5 

 feet and a mean pool depth of 6.4 feet: 

 chipook salmon 14.3, blueback salmon 7.6, 

 and steelhead 5.9. Although specific pas- 

 sage rates have not been obtained for non- 

 salmonids (carp, suckers, squawfish, £uid 

 chubs), our observations indicate these 

 fish do not ascend fishways as rapidly as 

 the salmonids. 



Variation in light intensity may also 

 govern fish behavior in fishways. Long 

 (1959) has noted that salmonids (principal- 

 ly steelhead) passed through a darkened 

 fishway significantly faster than through 

 an identical well-lighted structure. 



Aside from the foregoing conditions, 

 all of which may influence the capacity of 

 a fishway, it is important that we assess 

 the effect of numbers of fish present in 

 the fishway (fish density) on the rate 

 of ascent. Do fish travel at comparable 

 speeds regardless of the number of fish in 

 a fishway at a given time, or is there some 

 limiting density at which movement is encum- 

 bered sufficiently to reduce the rate of 

 ascent, so that we may expect an increasing 

 accumulation of fish in the fishway? Even- 

 tually there may then be a point at which 

 maximum entry can no longer be sustained 

 because of limited space within the fishway 

 pools. This, in effect, would indicate that 

 the capacity of a fishway had been exceeded. 



A comparison of the maximum 

 number of fish present in the fishway 

 during each trial and the corresponding 

 mean passage time and rate of ascent 

 cire presented in table 4. The mean 

 passage time for a 6-pool ascent is 

 expressed as a rate in terms of pools 

 ascended per hour. An arbitrary meas- 

 ure of fish density in terms of the 

 observed maximum number of fish in the 

 fishway during each 60-minute test was 

 used. 



A direct comparison of the rate 

 of ascent as related to density is not pos- 

 sible for all five trials because of inher- 

 ent differences in performances which may 

 be attributable to season and species, but 

 three trials conducted on June 25 may be 

 compared. Temperature and turbidity of the 

 water remained relatively constant through- 

 out the day. Although species composition 

 varied slightly, the differences were not 

 considered sufficient to appreciably affect 

 the passage times. The tests in question 

 began successively at 9 a.m. , 1 p.m. , Eind 

 3 p.m. and continued for 60-minute periods. 

 On the basis of previous work,—' the dif- 

 ference in time of day was not considered a 

 factor likely to affect rate of ascent. 



Proceeding, then, on the assumption 

 that a general uniformity prevailed in the 

 three experiments, one variable that might 

 have affected rate of accent was the number 

 of fish present in the fishway during each 

 test. Inspection of this relation in the 

 June 25 tests (table 4) suggests that crowd- 

 ing apparently did not impede the rate of 

 ascent. Indeed, ascent in the test with 

 the largest number of fish (No. 3) was 

 slightly faster than that in the other two 

 trials (Nos. 4 and 5). It may be added 

 that the minor differences in rate could 

 have been related to incidental variation 

 in the species composition (table 1) rather 

 than to the number of fish involved in the 

 respective tests. 



Fallbacks 



The term "fallback", as used here, 

 applies to fish which enter a pool and then 

 drift or swim back into the pool below. 



6y Gauley, Joseph R., axid Clark S. Thompson. Further 

 studies on fishway slope and its effect on the rate of pas- 

 sage of salmonids. Manuscript in prepaiation. 



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