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Fishery Bulletin 91 [I), 1993 



Fish 4 



Salinity 



Fish 5 



Salinity 



 Available H Experienced 



 Available Q Experienced 



13 15 17 19 21 23 25 27 29 



Salinity (o/oo) 

 Temperature 



13 15 17 19 21 23 25 27 29 31 

 Salinity (o/oo) 



Temperature 



 Available H Experienced 



I Available □ Experienced 



20 21 22 23 



Temperature (C) 



10 11 12 13 14 15 16 17 18 19 20 21 22 23 

 Temperature {C) 



Figure 4 



Histograms of experienced and available temperature and salinity distributions for Fish 

 4 and 5. 



fluenced by species- or stock-specific preferences for 

 particular temperature or salinity regimes, light lev- 

 els, positions in the water column (relative to the sur- 

 face or bottom), or by the need to locate orientating 

 clues. Our observations of tracked chinook making ver- 

 tical movements through large ranges of salinity and 

 temperature indicate that they are tolerant of rapid, 

 short-term changes in these features over several min- 

 utes, but utilized intermediate salinities and interme- 

 diate-to-warm temperatures while swimming in mid- 

 water. The presence of a lag time in equilibrating 

 internal and external ion concentrations and tempera- 

 tures could explain their ability to tolerate rapid 

 changes in these factors because adult chinook salmon 

 have been observed to require 30-60 min to equilibrate 

 their internal temperature to ambient water tempera- 

 ture when moved between 19° and 9°C, depending on 

 direction of the transfer and size of the fish (Berman 

 &Quinnl991). 



Vertical movements which determine long-term in- 

 ternal temperature may be affected by energetic fac- 

 tors related to stock origin. Stocks such as upriver 

 brights which have extensive in-river homing migra- 

 tions might be expected to take advantage of the cooler 

 water near the bottom of the estuary. Brett & Glass 

 (1973) reported that oxygen consumption for a 5 kg 

 sockeye salmon would be about 20-40% less in 10° 

 than 20° C water, depending on whether the fish was 

 at rest or active. Assuming a similar relationship for 

 chinook salmon, most fish characterized as tules in 



the present study were not min- 

 imizing their energy-expenditure 

 rate while swimming in mid- 

 water, as they did not prefer the 

 coolest water. Indeed, several of- 

 ten occupied relatively warm wa- 

 ter (Fig. 5, Table 3). In contrast 

 to the dark- and dusky-skinned 

 fish which swam primarily in 

 midwater, the three bright fish 

 (Fish 1,3,4) potentially from up- 

 river stocks swam for substan- 

 tial periods near the bottom and 

 may have been attempting to 

 minimize their energy expendi- 

 tures by utilizing the coolest wa- 

 ters available to them in the wa- 

 ter column. 



In contrast, a fish's preferred 

 mean external salinity may be 

 affected by its current physiologi- 

 cal status and the degree to 

 which the osmoregulatory system 

 has switched its direction of ac- 

 tive ion transport. Vertical salin- 

 ity and temperature gradients are often correlated in 

 estuarine environments, and the maturity level of a 

 tracked fish is unknown; therefore, determining the 

 degree to which salinity or temperature affect vertical 

 movements is confounded in field experiments. 



Fish orienting to olfactory stimuli (Hasler & Scholz 

 1983) might be expected to move up and down through 

 the halocline (Westerberg 1982, 1984), and fish tracked 

 in the Columbia River estuary were observed at depths 

 containing large vertical gradients of salinity and tem- 

 perature. Olfaction is an important component in hom- 

 ing by salmonids in rivers and streams (Hasler & 

 Scholz 1983), but it is unclear to what extent and how 

 olfaction is utilized for orientation in coastal and es- 

 tuarine waters. Westerberg (1984) hypothesized that 

 salmonids might derive information from the current 

 shear at haloclines separating water layers containing 

 different concentrations of natal river olfactants. Track- 

 ing data on Atlantic salmon Salmo salar by Westerberg 

 (1982) and Doving et al. (1985) supported this hypoth- 

 esis, demonstrating characteristic and regular dives to 

 the halocline one or two times per hour. Data for Pa- 

 cific salmon and steelhead trout in coastal waters 

 (Ichihara & Nakamura 1982, Quinn & terHart 1987, 

 Soeda et al. 1987, Quinn et al. 1989, Ruggerone et al. 

 1990) show a less clear pattern of vertical movements 

 relative to the thermocline than that reported by 

 Westerberg (1982) and Doving et al. (1985). Results of 

 sockeye tracking (Quinn & terHart 1987, Quinn et al. 

 1989) in both mixed and stratified waters demonstrated 



