sturgeon fishermen. Putrefied salmon flesh is at search stations. If these were typical recover- 

 regarded by many sturgeon fishermen as superior able carcasses, a correction for the resulting bias 

 bait, and one of the most productive locations could readily be made by the method suggested 

 on the Columbia River for sturgeon fishing and by Paulik and Robson (1969). However, carcasses 

 for finding putrefied salmon is just below Bonne- removed by fishermen probably have a higher 

 ville Dam. probability of floating than typical recoverable 



Beacon Rock Moorage, 6.4 km. below the dam, carcasses, so that a bias correction is not possible, 

 is the principal base of operations for sturgeon In any event, the bias is very slight and does not 

 fishermen in the area. Lee Motley, a moorage significantly affect our conclusions, 

 operator, recorded carcasses found by his em- The removal of carcasses by scavengers was 



ployees and customers near the dam during the also considered. Scavengers near Bonneville Dam 

 spring and summer of 1955. His data are as fol- that feed on dead salmon both before and after 

 lows: they float include sturgeon, squawfish, gulls, 



crows, raccoons, and skunks. Sometimes almost 

 every floater was accompanied by one gull or more, 

 but at other times, only a few were accompanied 

 by gulls. The effect (if any) of gull scavenging on 

 the length of time carcasses remain at the surface 

 was not determined. Floaters that go aground 

 probably disappear more quickly than those that 

 remain afloat because they are more accessible 

 Of the carcasses found by sturgeon fishermen to terrestrial or avian scavengers. Gulls and crows 

 in July, a relatively large nurhber were found were frequently seen feeding on dead salmon that 

 during the sampling period, July 7 to 17 (23 had drifted ashore. Nocturnal feeding by raccoons 

 untagged and three tagged chinook salmon and and skunks, which are numerous in the area, may 

 24 sockeye salmon). This record probably includes be even more important in the rate of disappear- 

 most of the carcasses removed by fishermen in ance of carcasses along the riverbanks. 

 the study area. Hence, scavengers reduce to an unknown extent 



The numbers of carcasses found by sturgeon the number of dead salmon available to be ob- 

 fishermen increased steadily during the summer. served. Whatever effect scavengers might have 

 The relatively large number of carcasses of both had, there was no evidence that it was different 

 chinook and sockeye salmon in July is of particular for tagged and untagged carcasses in the search 

 significance in relation to mortality at the dam. area. Therefore, scavenging was assumed to have 

 Many of these were not floating but were freshly no significant effect on the estimate of mortality, 

 killed fish that had collected on a shallow sub- Floaters originating above the dam could also 



merged bar near the upper end of Hamilton Island, affect our mortality estimate, but we believe that 

 close to the spillway. Each year Lee Motley re- few, if any, such chinook salmon floaters drifted 

 trieves many fish on this bar. into the recovery area during the estimate period. 



Some carcasses recovered by sturgeon fisher- The best and most direct way to evaluate the 

 men probably would have been found at our regu- possibility of recruitment of floaters originating 

 lar search stations had they not been intercepted. above the dam into the search area below the 

 The fishermen returned the three tagged carcasses dam would be to observe the river from above the 

 to the river because they knew of our study and dam. We looked for floaters on two dates from the 

 did not want to interfere with it; two tagged Bridge of The Gods, which spans the river 4.8 km. 

 chinook salmon floaters were subsequently re- above the dam, where the entire surface can easily 

 covered at station 6 (fig. 3). The removal of 23 be seen. No chinook salmon floaters were ob- 

 untagged chinook salmon carcasses by moorage served passing under the bridge on July 10, 1955, 

 personnel during the period of the experiment whereas seven were sighted below the dam. Again, 

 results in a slightly smaller estimate of mortality on July 3, 1956, observers were stationed at sta- 

 than would have been obtained if a few of the tion 5 (Cape Horn), and on the Bridge of The 

 removed untagged carcasses had been recovered Gods. In 6 hours and 35 minutes no chinook 



CHINOOK SALMON MORTALITY IN COLUMBIA RIVER NEAR BONNEVILLE DAM 



475 



