24 



sifliulationB resulted in populations being greater than 5,000 fish in 

 2089. The exponential diffusion model also demonstrated markedly better 

 forecasts using data through 1994. The probability of the escapement 

 reaching 1 fish by 2089 decreased from over 99% to 35% using data from 

 1980-1994, and to 53% using data from 1975-1994. The saiiie reduction in 

 probability of extinction, when extinction is defined as 30 fish, was 

 estimated to be 68% and 82%; respectively. 



The addition of the 1991-1994 escapements considerably improves the 

 assessment of the status of the Snake River fall chinook salmon stock. 

 The 1993 escapement of 742 Chinook salmon was the largest since 1975 and 

 the 1991-1994 4-year average escapement of 512 fish was the largest 4- 

 year average since 1977-1980. These recent large escapements, compared 

 to the previous 10 years, are evidence that the trends in stock 

 abundance are not nearly as bleak as previously thought. These data do 

 not support changing the status of Snake River fall Chinook from 

 threatened status to endangered. 



Probability of Paraistence with Respect to Survival 



The Biological Requirements Work Group (BRWG) formed as a result of the 

 IDFC V NMFS litigation settlement negotiations has been modeling Snake 

 River fall Chinook under various alternative hydro management scenarios. 

 The State and Tribal Fishery Agencies Analytic Team (STFA), a component 

 of the BRWG, submitted a report entitled Preliminary Summary of fall 

 Chinook Model Results for 1995 Biological Opinion to NMFS on February 

 10, 1995, which provides modeling results obtained as of that date (STFA 

 1995). These results include estimates of the probability of 

 persistence of Snake River fall Chinook with respect to survival. The 

 BRWG defined 300 spawners as a critical minimum escapement level for 

 Snake River fall Chinook. The STFA identified 70% as a 24 year or 

 shprt-term probability level and 90% as a 100 year or long-term 

 probability level that is equivalent to the historic probabilities used 

 in the spring/summer Chinook assessment; these values provide useful 

 benchmarks in understanding STFA fall Chinook modeling results. 



The STFA estimated that the proportion of yearly escapements at or above 

 300 for a 24 year period was 0.38 and 0.39 with depensation and without 

 depensation; respectively, based upon recent runs and recent hydro 

 management conditions (BY 80-88). The STFA estimated that the 

 proportion of yearly escapements at or above 300 for a 100 year period 

 was 0.09 with or without depensation based on recent runs and recent 

 hydro management conditions (BY 80-88). In other words, the proportions 

 of Snake River fall chinook escapements exceeding the threshold value of 

 300 is projected to be under 40% in the short-term and under 10% in 

 long-term if hydro management conditions remained as they were during 

 the 19808. These short-term and long-term probabilities are 

 substantially less than the 70% and 90% STFA suggested benchmark levels, 

 indicating that the stock was in jeopardy in the 1980s and would remain 

 in jeopardy under these 1980 type hydro management conditions. 



The STFA conducted similar modeling for Snake River fall Chinook for a 

 variety of other hydro system management scenarios under discussion in 

 the IDFO V HMFS litigation settlement negotiations because management of 

 the hydro system is currently being altered to benefit listed salmon 

 8p>ecies and clearly, the hydro system will not be allowed to continue to 

 operate under the 1980-1988 base level conditions. The hydro system 

 management scenario, as outlined in the 1994-1998 biological opinion, 

 was used as one of the hydro system management scenarios. Depending 

 upon depensation and a variety of assumptions concerning transport 

 benefits and predator control effectiveness, the STFA estimated that the 

 proportion of yearly escapements at or above 300 for a 24 year period 

 ranged from 0.44 to 0.95 based on the 1994-1998 hydro biological opinion 

 with 4 of the 8 (50%) model cases exceeding the short term benchmark 

 level of 70%. Comparative probabilities for these 8 model runs over a 

 100 year period resulted in probabilities ranging from 0.11 to 0.96 with 

 2 of the 8 (25%) model cases exceeding the long term benchmark of 90%. 



