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period of time (albeit at a decreasing rate) before increasing to 

 return to the pre- 1990 levels. Table 1 shows the expected 

 trajectory of abundance for spring/ summer and fall chinooK salmon 

 if the goal were just met. The suitability of the stability goal 

 was evaluated by assessing the genetic and demographic risks to 

 the listed species if the populations actually followed these 

 trajectories. The analysis is summarized here and described in 

 more detail in a memoranaum dated 23 December 1992 'Schiewe 

 1392b) . 



Genetic considerations 



a . Background 



The primary genetic concern with respect to low population size 

 is that severe bottlenecks in the population can cause inbreeding 

 and resulting declines in productivity. Examples of these kinds 

 of effects have been documented in a number cf stuaies of various 

 fish species. With somewhat larger (but still lowj population 

 sizes/ the danger is that, over time, the gradual erosion of 

 genetic variability will cause inbreeding depression and/or 

 reduce the population's ability to respond to future changes in 

 environmental conditions. 



For evaluating the effects of genetic bottlenecks (inbreeding and 

 loss of genetic variability) on a population, the key parameter 

 is the effective population size per generation (N e ) . In Pacific 

 salmon, the data that are typically available are some measure of 

 the number of adults returning per year. Recent research (Waples 

 1990) has shown that the effective number per generation in 

 Pacific salmon is equal to the harmonic mean effective number per 

 year (N b ) times the average age at reproduction. For Snake River 

 soring/ summer and fall chinook salmon, average age at 

 reoroduction is about 4-5 years, so if (for example) the 

 effective number per year were 100, the effective size per 

 generation would be about 400-500. 



There is at present no consensus regarding the minimum acceptable 

 effective size for a population, but most scientists would 

 probably agree that an N c of a few hundred per generation is 

 necessary for long-term maintenance of current levels of genetic 

 variability. Waples (1990) suggested that maintaining an N b of 

 100 or more per year is desirable for most Pacific salmon 

 populations. Smaller bottlenecks may not cause severe genetic 

 losses if they are of short duration (a few generations or less) . 

 Because N. is typically smaller than the number of spawners 

 (because of unequal sex ratio and unequal reproductive success 

 among individuals) , spawning escapements must be somewhat larger 

 than the targeted effective population size (perhaps twice or 

 three times as large) . 



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