254 



Fishery Bulletin 100(2) 



about 1°C warmer than long-term means, although well 

 within the tolerance limits for the species at these lat- 

 itu(ies (Baker et al., 1995). This temperature has been 

 shown to allow growth rates of about Vi body weight/day 

 in juvenile chinook salmon under simulated field feeding 

 conditions (Weatherley and Gill, 1995). 



Water flows and temperature are two primary environ- 

 mental variables affecting the distribution and develop- 

 ment of fishes; thus it is reasonable to assume that the 

 timing of the emigration in 1997 was a function of that 

 year's hydrology. Flow and temperature were believed to 

 influence the migration and distribution of juvenile chi- 

 nook salmon in the Sacramento and San Joaquin Rivers 

 (Kjelson et al., 1982) and other systems (Cramer and 

 Lichatowich, 1978; Healey 1980b; Wallace and Collins, 

 1997). In fact, higher water temperatures of the Central 

 Valley drainage may be a factor in the early emigration 

 and physiological development seen in California chinook 

 salmon in contrast to northerly populations (Healey, 1991). 

 Mean water temperature between 1988 and 1997 at the 

 estuary entrance (km 68) was 19.9°C in June and 21.7°C 

 in July,-'' within the range of predicted W7t mortality esti- 

 mated with a fitted model (Baker et al., 1995). Tempera- 

 tures within the Central Bay through July, however, re- 

 mained below 18°C. 



The strongest El Niiio of the century began in 1997 and 

 may have influenced the physiology and ecology of salm- 

 on in the coastal waters of the Gulf of the Farallones. Wa- 

 ter temperatures in the gulf were 0.5°C above the mean 



^ Baylosis, J. I., J. L. Edmunds, J. L. Cole, and J. E. Cloern. 

 1998. Studies of the San Francisco Bay, California estuarine 

 ecosystem: regional monitoring program results, 1997. LI. S. 

 Geological Survey Open-File Report 98-168, 194 p. lAvailable 

 from U. S. Geological Survey, 345 Middlefield Road, Menlo Park, 

 CA 94025.1 



May Jul 



1997 



Date 



Figure 7 



Freshwater outflow into the San Francisco Estuary from 

 the Sacramento River-San Joaquin River Delta in 1997 

 compared with 1984-98 mean. Outflow data are mean 

 daily cubic feet per second (cfs) from California Depart- 

 ment of Water Resources, Sacramento, CA. 



for May and June, 1989-97. Although temperatures were 

 within the preferred range for chinook salmon in the ocean 

 (Weatherley and Gill, 1995), altered oceanographic condi- 

 tions may have changed the composition of the prey com- 

 munity, affecting the feeding of juveniles. 



Among Pacific salmon, chinook salmon display the great- 

 est variability of life history patterns, including time spent 

 in fresh and brackish water before ocean entry, growth 

 rates, the proportion maturing each year, and fecundity 

 (Healey, 1991). Populations from the Central Valley of Cal- 

 ifornia are at the extreme southern end of the species' dis- 



