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DRAFT 



likely more susceptible to predation due to passage related injuries or disorientation. It 

 is also likely that the gulls in this study were scavenging dead smolts below the facility. 



"Wood (1987) noted that mergansers may limit salmon production in 

 nursery areas in British Columbia. He estimated that young merganser 

 ducklings can consume almost one half pound ofchinook salmon fiy per 

 day. Thus, a brood often ducklings could consume between four and 

 five pounds offish daily during the summer rearing period. " 



The authors reference for Wood (1987) does not correspond to the information 

 presented. Wood actually presented two studies in the referenced journal; the 

 information presented app^s to be derived from Part n, not Part I as cited. 



Wood (1987a) concluded that merganser predation on seaward migrating salmonids was 

 not likely to affect salmonid populations: 



"Juvenile salmon are more vulnerable to predation for a period of days, 

 or at most weeks, during their downstream migration. A predatory 

 species must be very abundant to inflict appreciable mortality ...the 

 overall mortality rate due to mergansers was depensatory-mergansers 

 were simply swamped by the output from spawning channels and 

 hatcheries." 



"The mortality rate due to mergansers is very unlikely to have exceeded 

 8% for any particular stock during downstream migration.." 



"Probably no single species of avian predator is capable of inflicting 

 compensatory mortality on juvenile salmonids during their seaward 

 migration... it follows that predation by all fish eating birds, acting in 

 concert must also be depensatory and that salmon populations cannot be 

 regulated by avian predation during their seaward migration." 



The actual study the authors cite (Wood, 1987b), discussed merganser predation on 

 stream-resident populations of salmonids. A major conclusion of this study was not 

 reported: 



"It is not clear whether mortality due to merganser broods has any effect 

 on the eventual size of smolt migrations in Vancouver Island streams. 

 Because merganser ducklings kiU salmonid fry during summer months, 

 there is ample opportunity. . . for compensation to occur if overwinter 

 survival is further limited by food or habitat..." 



An analysis of the applicabili^ of this study to Oregon streams should consider whether 

 the abundance of mergansers is similar to that observed in the British Columbia study 

 area. 



A-42 



