the rate of recovery rose rapidly. We then re- 

 leased our entire supply of carcasses over a 2-day 

 period, June 30 and July 1, so that our estimate 

 of mortality would include a period during which 

 large numbers of salmon were dying. The counts 

 of chinook salmon migrating over the dam and 

 the riverflows were also near their summer maxi- 

 mums. Thus, large numbers of live fish were pres- 

 ent, coinciding with inimical high flows. 



Because of our experience in 1954 with disinte- 

 gration of carcasses in the extremely turbulent 

 flow in the spillway rollback, we released the 

 tagged carcasses in 1955 into the spillway channel 

 from a boat downstream from the rollback. In 

 that area high velocities and extreme turbulences 

 shunt migrating salmon toward the fish ladder 

 entrances at either end of the spillway (figs. 1 

 and 2). At the release points, which were dis- 

 tributed across the river, one man kept the boat 

 in position under power while a second pitched 

 the tagged carcasses overboard — 1,068 were re- 

 leased in the spillway channel (183 m. below the 

 dam) and 101 in the powerhouse channel (64 m. 

 below the dam). 



The 2-day release period coincided with a tem- 

 porary slackening of chinook salmon passage over 

 the dam; the count dropped from a peak of 4,912 

 fish on June 23 to slightly more than 1,000 per 

 day near the end of June. At this time, many 

 live fish were below the dam and were liable to be 

 killed, as shown by a second peak count of 3,434 

 on July 4 and large numbers of chinook salmon 

 floaters through the first half of July (fig. 6). 



The search by boats for floaters downstream 

 from Bonneville Dam was not begun until July 5 

 because we knew from previous experiments on 

 the floating characteristics of chinook salmon car- 

 casses that the tagged carcasses would not float 

 in fewer than 5 days at the existing water tem- 

 perature of 13.9° to 14.4° C. (see appendix). 

 Daily searches for floaters continued until July 22. 

 Table 4 gives the release and recovery data on 

 which we base our estimate of the number of 

 recoverable chinook salmon carcasses from mor- 

 talities at the dam from June 21 through July 10, 

 1955. No tagged carcasses were recovered from 

 July 18 through July 22. Table 4 includes only 

 data actually used in computing the estimate. 



The level of recovery effort throughout the 

 recovery period was essentially constant — an ob- 

 server in a boat recovered floaters for 8 hours a 



day at each of the seven stations below the dam 

 (fig. 3). The only exception was July 16, when no 

 searches were made. This was the 16th day after 

 the releases on June 30 and the 15th day after the 

 releases on July 1; for this reason estimates of 

 fractions of recoverable carcasses available for 

 recovery a given number of days after release 

 (discussed on p. 16) are slightly biased. 



The absence of recovery efforc on July 16 also 

 affects the estimate of Yi, the number of recover- 

 able untagged carcasses in the river. However, 

 trial calculations made by using plausible values 

 for the numbers of tagged and untagged carcasses 

 that might have been recovered on July 16 sug- 

 gest that the likely error is slight. For example, if 

 we assume that one tagged carcass and six un- 

 tagged carcasses would have been recovered — a 

 probable event in the light of the data of table 

 4 — our estimate of Yi (given on p. 17) would 

 change by only 1.8 percent. 



Table 4. — Data for computing mortality of the summer 

 chinook salmon run near Bonneville Dam, 1955 



(Only data actually used in computations are included) 



Tagged floaters 



recovered from 



Tagged releases of Untagged 



carcasses floaters 



Date ladders released June 30 July 1 recovered 



Chinook salmon 



counted at 



Bonneville Dam 



ladders 



Number Number Number Number 



June: 



21.. 



22.. 



23.. 



24.. 



25.. 



26.. 



27... 



28... 



29... 



30... 

 July; 

 1... 

 2... 



4. 



6.. 



6.. 



7.. 



8.. 



9.. 

 10.. 

 11.. 

 12.. 

 13.. 

 14.. 

 15.. 



3,567 

 4,798 

 4,912 

 3,890 

 3,868 

 2,220 

 2,343 

 1,680 

 1,291 

 1,185 



1,411 



1,713 

 1,667 

 3,434 

 2,729 

 2,576 

 1,940 

 1,339 

 1,241 

 962 



831 



8 

 15 

 13 



7 

 13 

 19 

 IS 

 16 



9 



Another minor problem was the removal of 

 untagged carcasses (either floating or stranded in 

 shallow water alongshore) from the river by 



474 



U.S. FISH AND WILDLIFE SERVICE 



