of the tags at the left and center ladders for the 4 

 years. While only 64 to 86 percent of the fish 

 released below the dam were observed at the count- 

 ing boards, we cannot say with certainty that these 

 missing tags indicated mortalities caused by the 

 dam. 



These are some possible reasons for the apparent 

 tag losses : 



1. Counters did not identify tagged fish crossing 

 the counting boards. 



2. Tagging harmed the fish. 



3. Fish lost the tags. 



4. Tagged fish refused to re-pass the dam. 



5. The dam itself caused mortalities. 



The counters did overlook tagged salmon. On 

 a few occasions when the forebay trap was fished, 

 we captured tagged fish the counters had not 

 reported. In some experiments, we recovered 

 more tags upstream than the counters reported at 



3 200 



DATE OF TAGGING 



Figure 10. — Recoveries of tagged sockeye upstream ex- 

 pressed as percentages of the number tagged at each 

 bank below, compared with water flow, 1955. 



1 Undetermined tap observations at Rock Island not identified as bar, 

 spaghetti, or streamer are omitted. 



• Steelhead and those tapped fish removed at Rock Island for other experi- 

 ments were omitted (ruin this column. 



3 Only soekcyc are included in the upstream returns in 1955 because of the 

 unequal recovery effort spent on chlnooks tapped with the two kinds of taps. 



40r 

 o 30 



UJ 



cr 



£ 20 



o 



o 



uj I O 



K 



£ o 



UJ 



O 



K 40 

 uj 



Q. 



30 

 20 

 10 



LEFT BANK 



RIGHT BANK 



10 20 30 10 20 



JULY AUGUST 



DATE OF TAGGING 



Figure 11. — Recoveries of tagged sockeye upstream ex- 

 pressed as percentages of the number tagged at each 

 bank below, compared with water flow, 1956. 



:•;;,(, 



U.S. FISH AND WILDLIFE SERVICE 



