MacFarlane and Norton Physiological ecology of Oncorhynchus tshawytscha 



253 



leaving the estuary- Further, Zaugg and McLain (1972) 

 showed that condition was either unchanged or declined 

 for several months after stabilization of higher Na*, K*- 

 ATPase levels in salmonids undergoing parr-smolt trans- 

 formation. The increase in A'-factor in fish captured in the 

 coastal waters of the Gulf of the Farallones argues against 

 smoltification as the main cause of lower condition in ju- 

 venile salmon leaving the estuary. 



Although total lipid concentrations were unchanged in 

 juveniles within the estuary, they declined greatly in salm- 

 on in the Gulf of the Farallones. Most of the decrease 

 was due to depletion of TAG. It appears that the rapid 

 growth of juveniles in the ocean was fueled by TAG energy 

 reserves acquired during downstream migration. Growth 

 seems to take precedence over maintenance of energy re- 

 serves. Increased body lipid (presumably TAG) at the time 

 of ocean entry has been shown to improve juvenile chinook 

 survival (Burrows, 1969; Higgs et al., 1992); thus a lack of 

 substantial TAG accumulation while in the estuary may 

 compromise early ocean survival should prey be scarce. 



There was little difference in protein concentrations in 

 juveniles from the estuary and the gulf This is not sur- 

 prising: protein appears to be conserved except during pe- 

 riods of stai-vation. Protein levels in juvenile salmonids 

 have been shown to be unaffected by ration, growth, en- 

 vironmental variation (Shearer et al., 1997; Edsall et al.. 

 1999), or locomotion (Alsop and Wood, 1997). 



As juvenile chinook salmon migrated through the San 

 Francisco Estuary, their feeding changed progressively 

 from invertebrates to fish larvae. Young salmon leaving 

 the freshwater delta had recently eaten primarily Co- 

 rophium. which seems also to be a major prey of juvenile 

 chinook in Oregon estuaries (Reimers, 1973; McCabe et 

 al., 1986). In 1979-80, juvenile chinook in the delta preyed 

 primarily on cladocerans and dipterans (Kjelson et al., 

 1982). Over the past several decades, the zooplankton com- 

 munity in the delta and estuary has changed dramatically 

 because of water diversions and introductions of many ex- 

 otic species (Kimmerer and Orsi, 1996; Cohen and Carl- 

 ton, 1998). The change in prey found in our study may be 

 a result of anthropogenic alterations in the ecosystem. 



There was greater diversity in feeding in the upper 

 bays — Suisun and San Pablo — where insects, copepods, 

 mysids, and cumaceans formed a major portion of young 

 chinook salmon diet. These embayments are bordered by 

 the last remaining extensive brackish and salt marshes 

 in the northern estuary. Additionally, Suisun Bay includes 

 the mixing zone of fresh and salt water, a well-known lo- 

 cale of high plankton abundance. Thus the high biological 

 productivity of these habitats could be expected to provide 

 diverse food items. The composition of the diet here was 

 similar to that found in more northerly estuaries (Healey. 

 1991) and provided further evidence that juvenile chinook 

 salmon feed on about the same prey types in these habi- 

 tats throughout their range. 



Chinook salmon are highly piscivorous. Fish lai-vae were 

 clearly the dominant prey in central San Francisco Bay 

 and in the Gulf of the Farallones. Typically young chinook 

 eat mostly fish larvae as they grow and enter marine en- 

 vironments (McCabe et al., 1986; Fisher and Pearcy, 1996; 



Date 



Figure 6 



Juvenile chinook salmon catch by midwater trawl at the 

 entrance to the San Francisco Estuary (km 68 1 in 1997 

 compared with 1995-98 mean. Catch per unit of effort 

 (CPUE) is presented as the number of salmon/1000 m^ 

 water. Data from U.S. Fish and Wildlife Service, Stock- 

 ton, CA. 



Landingham et al., 1998). In the coastal waters of the Gulf 

 of the Farallones, juvenile chinook salmon fed on early life 

 stages of euphausiids and decapods as well, but to a less- 

 er extent than on fish. Fish, euphausiids, and decapods 

 were the major prey in coastal ecosystems from Oregon to 

 southeastern Alaska (Brodeur and Pearcy, 1990; Landing- 

 ham etal., 1998). 



Once juvenile chinook entered the ocean, feeding inten- 

 sity appeared to increase. Whereas stomach contents were 

 0.5% of body weight in the estuary, in the gulf they were 

 0.9% ±0.1% of body weight. The rapid growth rate found 

 in young salmon shortly after ocean entry seemed to re- 

 sult from greater feeding activity in combination with the 

 utilization of lipid reserves to fuel the synthesis of formed 

 tissues. 



To understand the context of data on juvenile salmon 

 physiological ecology, it is helpful to know the relative 

 abundance of these fish and the environmental conditions 

 in 1997. Emigi-ating juvenile salmon abundance is moni- 

 tored at the estuary entrance (km 68) by a consortium of 

 California and federal research agencies ( Interagency Eco- 

 logical Program). Data for 1997 indicated that abundance 

 was lower and migration to the ocean occurred earlier 

 than the 1995-98 average (Fig. 6). Mean daily catch per 

 unit of effort ( 1 January to 30 June 1997) was 0.132 ±0.02 

 juveniles/I lO'Vm^ water), about 80% less than in 1996 and 

 1997 and 50% less than in 1998. Freshwater outflow from 

 the delta into the estuary in 1997 was categorized by 

 California Department of Water Resources as "above nor- 

 mal" and peak flows occurred in January, earlier than typ- 

 ical (Fig. 7). Hydrologic conditions in the estuary during 

 May and June 1997 were similar to long-term means, but 

 water temperatures and salinities were statistically high- 

 er (P<0.0001, Table 3). Water temperatures were 18.4°C, 



