MacFarlane and Norton: Physiological ecology of Oncorhynchus tshawytscha 



255 



tribution and, like many populations at the edges of their 

 distribution, may differ in their early life history from 

 more centrally located stocks. Certainly, Central Valley 

 chinook salmon are known to have unique characteristics 

 such as unusually high fecundity, larger size at age (Heal- 

 ey, 1991), and large population size (PFMC). Lesser es- 

 tuarine dependence may be another unique trait of these 

 southerly populations. Our data suggest that juvenile chi- 

 nook salmon derive little benefit from their time spent in 

 the San Francisco Estuary. There was little growth, deplet- 

 ed condition, and no accumulation of lipid energy reserves 

 during the relatively limited time the fish spent transiting 

 the 65-km length of the estuary. Extensive use of estuar- 

 ies, especially by ocean-type chinook juveniles, seems more 

 typical for populations to the north. Rearing and rapid 

 growth have been characteristic of northerly stocks dur- 

 ing extended periods of residence (Reimers, 1973; Healey, 

 1982; Simenstad et al, 1982; Pearcy, 1992) but were not 

 evident in the San Francisco Estuary (Kjelson et al., 1982; 

 our study). 



Emigrating juvenile chinook salmon may not have al- 

 ways used the San Francisco Estuary as they do now. Ex- 

 tensive modification and degradation of the estuary by hu- 

 man activities in the past century or so may have altered 

 the life history pattern of Central Valley chinook salmon 

 populations. Loss of habitat, changes in prey communities, 

 water-flow alterations and reductions, etc. may have con- 

 tributed to less use of the estuary than previously. 



Because Central Valley juvenile chinook salmon exhibit 

 little estuarine dependence, they appear to benefit from 

 expedited ocean entry. The rapid growth seen in the Gulf 

 of the Farallones may confer potential for greater surviv- 

 al. Other chinook salmon studies have determined that 

 larger juveniles are more likely to survive in the ocean 

 (Higgs et al.. 1992), leading to higher spawning returns 

 (Burrows, 1969; Reimers, 1973). Faster growth and per- 

 haps greater survival of juvenile chinook salmon, once in 

 the ocean, may be a consequence of a typically favorable 

 and stable thermal environment and the abundant food 

 that results from the high biological productivity of this 

 upwelling-dominated ecosystem (Ainley, 1990; McGowan 

 et al., 1998 ). The abundant prey resources and lower meta- 

 bolic costs associated with lower temperatures and ease of 

 obtaining food may improve growth efficiency in the ocean 

 compared with that in the San Francisco Estuary. Reli- 

 ance on extended freshwater and estuarine residence prior 

 to ocean entry could reduce Central Valley chinook salm- 

 on stocks, especially during prolonged periods of drought, 

 which lower freshwater flows and raise temperatures. The 

 ecological and evolutionary propensity of emigrating juve- 

 niles to conform to a strong ocean-type life history, with 

 little estuarine dependency and a hastened ocean entry 

 may be another unique attribute of Central Valley chinook 

 salmon populations. 



Acknowledgments 



We thank Maxwell Eldridge and Brian Jarvis for their 

 expert assistance in field collections and laboratory analy- 



ses. We are grateful to David Woodbury, Keith Sakuma, 

 and Dale Roberts for providing additional salmon and 

 oceanographic data from their rockfish assessment cruise 

 in the Gulf of the Farallones in May and June 1997. The 

 officers and crew of the NOAA ship David Starr Jordan 

 are also acknowledged for their highly professional con- 

 duct, skill, and expertise during our cruises in the Gulf of 

 the Farallones. We thank the U.S. Fish and Wildlife Ser- 

 vice and California Department of Fish and Game, Stock- 

 ton, CA, for their assistance in collecting salmon at km 

 68 and for providing data from coded-wire-tagged fish. 

 Anonymous reviewers provided insightful comments that 

 improved the manuscript. 



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