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Fishery Bulletin 104(3) 



location in the water column for some period of time, 

 perhaps adjusting to the presence of the tag or to life in 

 the sea (or to both). Furthermore. Reddin et al. (2004) 

 noted that this period of temperature stability could be 

 also due in some instances to the fish being near shore 

 in shallow water where large scale vertical movements 

 and their concomitant changes in temperature were not 

 possible. This period of restricted vertical activity seemed 

 to cease as the fish made its return migration towards 

 freshwater. Our study indicates that in freshwater the 

 water temperature recorded at the trap corresponds close- 

 ly to the temperature recorded by the DST on the fish 

 on its departure from the river and at its return. DST 

 temperatures in freshwater follow the diurnal rhythm of 

 warming and cooling of the river. In the sea, there is a 

 period of stable temperatures with a diurnal rhythm that 

 follows the daily warming and cooling. Next, the smolts 

 began a period of fairly rapid descents and ascents in 

 the water column during which temperatures rose and 

 fell much more quickly then during the natural daily 

 cycle. Also, night temperatures were warmer than day 

 temperatures, in contrast to the daily warming and cool- 

 ing cycle of the sea, indicating that smolts were higher 

 in the water column at night then during daylight hours. 

 The differences in temperatures between day and night 

 may reflect avoidance of avian predators as suggested by 

 Reddin et al. (2004) and Montevecchi et al. (2002). This 

 finding is similar to that shown for adult chum salmon by 

 Friedland et al. (2001) but contrasts with that reported 

 by Shelton et al. (1997) who reported that no catches of 

 postsmolts occurred at night during surface trawling in 

 the northeast Atlantic. Because the records of trawling at 

 night are sparse, we recommend that more effort should 

 be put into night trawling in light of the information 

 from our DST-tagged postsmolts. 



In terms of our objectives, we have shown that Atlan- 

 tic salmon smolts can carry DSTs, and that a sufficient 

 number of tags with useful data on thermal ecology can 

 be successfully recovered. The return rates were low and 

 validated the preliminary use of the less expensive DSTs 

 but were high enough that these experiments could be re- 

 peated elsewhere. The use of more expensive DSTs with 

 geoposition and more environmental sensors will require 

 methods to recover them at sea if sufficient tags are to 

 be available to provide meaningful results. The thermal 

 habitat used by Atlantic salmon is shown; postsmolts are 

 found in water with temperatures ranging from 8° to 

 about 15°C in the spring. Lastly, although we show from 

 the water temperature records that salmon postsmolts 

 are frequently found near the surface, it is also evident 

 that they make frequent deep dives of short duration. We 

 hypothesize that these deep dives may be directly related 

 to feeding or to evasion from predators (or to both), as 

 has been observed for kelts by Reddin et al. (2004). 



Acknowledgments 



We acknowledge the staff at the counting fence on Camp- 

 bellton River, who assisted with tag application and 



data recording. The enthusiasm of Garnet Clarke and 

 Roger Johnson who provided several helpful ideas on 

 tag application is greatly appreciated. The comments 

 provided by R. Poole and two anonymous reviewers are 

 gratefully acknowledged. 



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