322 



Fishery Bulletin 101(2) 



The ultimate goal is to assess whether the proposed plan 

 might pose a significant increase in risk of extinction. We 

 took a Bayesian approach in order that uncertainty in 

 parameter estimates could be incorporated into extinction 

 risk predictions (Ludwig, 1996). If significant risk cannot 

 be ruled out, managers might reduce the scope of proposed 

 striped bass stocking and collect data to better constrain 

 the predation rate so that appropriate mitigation can be 

 implemented. Although not the primary focus of this paper, 

 our results also serve as a population viability assessment 

 (PVA) for winter- run chinook salmon that can be compared 

 to the recent winter-run chinook salmon PVA reported by 

 Botsford and Brittnacher ( 1998). 



Methods 



Background and data 



Winter-run chinook salmon Sacramento River winter-run 

 chinook salmon are genetically distinct from other chinook 

 salmon populations (Kim et al., 1999; Banks et al., 2000) 

 and have a unique life history pattern that is a blend of the 

 stream- and ocean-type life histories. Spawning fish leave 

 the ocean in winter, mature in freshwater, and spawn in 

 headwater areas from April through September (Healey, 

 1991). Juveniles enter the ocean the following spring. Up to 

 200,000 winter-run chinook salmon may have once spawned 

 in the Sacramento River headwaters (Fisher, 1994). The 

 completion of Shasta Dam in 1944 blocked access to the 

 entire historic winter-run chinook salmon spawning range 

 but created suitable spawning conditions for some distance 

 ( = 100 km) downstream of the dam (Moffett, 1949) (Fig. 1). 

 In 1967, Red Bluff Diversion Dam (RBDD) was installed 

 about 110 km downstream from Shasta Dam. Installation of 

 RBDD apparently created passage problems for both adult 

 and juvenile chinook salmon and the winter-run chinook 

 salmon population has declined dramatically since comple- 

 tion of RBDD; fewer than 100 adults returned to spawn in 

 1980 (Fig. 2). Additional factors contributing to the decline 

 of winter-run chinook salmon include high summer water 

 temperatures, water diversions, habitat modification and 

 degradation, fishing, hydropower operations, toxic spills, 

 and predation by native and introduced animals, includ- 

 ing striped bass (NMFS'). Winter-run chinook salmon were 

 listed as threatened under the U.S. Endangered Species Act 

 (ESA) in 1989 and as endangered in 1994. 



The California Department of Fish and Game counts re- 

 turning winter-run chinook salmon as they pass over fish 

 ladders on RBDD; counts have been reported by Myers 

 et al. ( 1998). Fish are determined to be adult (age 3 or 4) or 

 "jack" (age 2, usually male), but are not othei^wise routinely 

 aged or sexed. From 1967 to 1985, nearly complete counts 

 of winter-run chinook salmon were made. Since 1985, the 

 dam flashboards have been removed for much of the year to 



Upper Sacramento River 



McCloud River 



Shasta Dam 

 Keswick Dam" 



' NMFS iNalional Marine Fisheries Service). 1997. NMFS 

 proposed recovery plan for the Sacramento River winter-run 

 chinook. Southwest Region, 501 West Ocean Blvd. l.nnn Hcoch, 

 ('A908()'2-4213. 



Figure 1 



Sacramento River, tributaries, and dams. Current spawning 

 range of winter chinook salmon spawning is between Keswick 

 Dam and Red Bluff Diversion Dam. 



improve passage of juvenile and adult winter-run chinook 

 salmon. During this period, winter-run chinook salmon 

 spawning escapement ( spawning population size ) has been 

 estimated by expanding fish ladder counts made when the 

 flashboards are in place. It is estimated that about 15*7? of 

 the run is now counted, but the actual fraction observed 

 in any given year is unknown. Population estimates made 

 since 1985 therefore contain measurement error. 



Striped bass Striped bass were intentionally introduced 

 to the Sacramento River in 1879, supported a commercial 

 fishery in the early twentieth century, and now support 

 a popular sport fishery (Kohlhorst, 1999). Over the last 

 30 years, the striped bass population has declined from 

 around 2.2 million adults to fewer than 1 million adults 

 (Fig. 3). The striped bass decline has been attributed to 

 entrainment of striped bass larvae by the large State 

 and Federal water diversion facilities in the Sacramento 

 River delta (Stevens et al., 1985) and ecosystem changes 

 that have reduced the carrying capacity for subyearling 

 striped bass (Kimmerer et al., 2000). The State of Califor- 

 nia has a legal obligation to mitigate the negative effects 

 of State water diversions on striped bass, but striped bass 



