A Statistical Model for the Survival of Chinook Salmon Smolts Outmigrating Through the Lower

U.S. Fish and Wildlife Service provided nancial support for Newman and California Urban Water Agencies provided nancial support for Rice. A statistical model was constructed for the survival of juvenile chinook salmon smolts outmigrating through the lower portions of the Sacramento river system. Coded-wire-tagged (CWT) chinook salmon smolts were released at various locations within the river system between the years 1979 and 1995. Recoveries of these juvenile salmon in a lower river trawl shery and later recoveries of adults from samples of ocean catches provided the basic data. Variation in recoveries, over and above di erences in release numbers and shing e ort, was modeled with a generalized linear model having many covariates. The most in uential covariates included environmental factors largely outside the control of man such aswater temperature, water salinity, and river ow. Two covariates, more readily manipulated, were of particular interest: the position of a water diversion gate (open or closed) separating the mainstem from the central delta and the relative fraction of water exported for irrigation and urban consumption. Of these two, only gate position suggested a strong e ect. When th