Lindley and Mohr: Impact of population manipulations of Morone saxatilis on Oncorhynchus tshawytscha 



323 



augmentation is constrained by the ESA because 

 of the potential impact on winter-run chinook 

 salmon. Striped bass prey upon a wide variety 

 of invertebrates and fish and are predominately 

 piscivorous from age 2. In the Sacramento River 

 system, juvenile chinook salmon compose a vari- 

 able portion of the diet depending on season and 

 location (Stevens, 1966; Thomas, 1967). By rear- 

 ing juvenile striped bass captured at the water 

 diversion fish screens in net pens and releasing 

 them after one or two years, it is thought that 

 the adult striped bass population could be stabi- 

 lized at 3 million adults. Without augmentation, 

 the population is expected to decline to about 

 500,000 adults. For a striped bass augmentation 

 program to be in compliance with the ESA, the 

 level of mortality on winter-run chinook salmon 

 must be specified and the impact of this mortal- 

 ity must not appreciably reduce the likelihood 

 of winter-run chinook salmon survival and 

 recovery. 



The California Department of Fish and Game 

 estimates annually the abundance of striped 

 bass; estimates have been reported by Kohlhorst 

 ( 1999 ). Adult striped bass are captured with gill 

 and fyke nets during the spring spawning mi- 

 gration and tagged with disc tags. Tags are re- 

 covered in summer and fall creel surveys and in 

 subsequent springtime tagging operations. The 

 field methods and estimation procedure, based 

 on the Peterson estimator, are described by Ste- 

 vens (1977). The estimate includes animals that 

 are 3 or more years old, although striped bass 

 begin feeding on juvenile salmon during their 

 second year of life. We assume that the adult 

 striped bass abundance estimate is a reason- 

 able index of the total striped bass population 

 capable of preying on juvenile chinook salmon. 

 Note that an abundance index is sufficient be- 

 cause the predation parameter estimate will 

 scale accordingly, i.e. the product of the striped 

 bass abundance index and the predation rate 

 parameter is unitless, as explained below. 



Winter-run chinook salmon 

 population model 



1965 



1970 



1975 



1980 1985 



Year 



1990 



1995 



2000 



Figure 2 



Estimated number of winter-run adult spawning chinook salmon pass- 

 ing the Red Bluff Diversion Dam. 



1965 



1970 



1975 



1980 1985 



Year 



1990 



1995 



2000 



In this section, we develop a probability model 

 for winter-run chinook salmon spawning escape- 

 ment. The model includes several potentially 

 important factors influencing winter-run chi- 

 nook salmon population growth: predation by 

 striped bass, initiation of conservation measures in 1989. 

 possibly density-dependent reproduction, and lognormal 

 variability in reproduction (so-called process variation). 

 Because winter-run chinook salmon juveniles are a minor 

 prey item in the striped bass diet, owing to the rarity of 

 winter-run chinook salmon in relation to other chinook 

 populations, we do not model the striped bass population 

 dynamics but rather treat the striped bass population-size 



Figure 3 



Peterson mark-recapture estimates of adult striped bass population 

 size in the Sacramento-San Joaquin rivers and estuary. 



observations as an input to the winter-run chinook salmon 

 population model. 



Winter-run chinook salmon adults spawn mostly at age 

 3 and to a lesser extent at age 4 (some males return at age 

 2, but we did not include them in the analysis on the pre- 

 sumption that 2-year-old males contribute little to popula- 

 tion growth). The number of adult spawning fish in year t 

 is the sum of 3- and 4-year-old spawning fish: 



