Reddin et al : Diurnal and nocturnal temperatures for Salmo solar 



425 



indicated that Atlantic salmon kelts tagged with DSTs 

 were found where temperatures ranged from a low near 

 0° to over 25°C, although most of their time was spent 

 in water of 5° to 15°C. This finding was similar to that 

 for adults by Sturlaugsson' and Karlsson et al.'" The 

 present study indicates that salmon postsmolts are 

 located mostly near the surface in water temperatures 

 ranging from 8° to about 15"C. Because the tempera- 

 tures experienced by kelt are somewhat warmer then 

 those used in correlation studies of salmon growth and 

 survival, it may be that our results will encourage other 

 researchers to review the temperatures they use in 

 such studies. 



Prey items in the diet of adult Atlantic salmon are 

 fairly well known revealing that Atlantic salmon are 

 opportunistic feeders, feeding on whatever is abundant 

 in the area (Reddin, 1988; Hislop and Shelton, 1993). 

 Although food resources are still unknown for some 

 regions, the prey species of smolts and postsmolts at 

 sea has recently become an important area for research 

 in the northeast Atlantic (Andreassen et al., 2001; Sal- 

 minen et al., 2001; Rikardsen et al., 2004). Rikardsen 

 et al.'s (2004) extensive analysis of postsmolt diet in 

 eight fjords in Norway revealed extensive feeding on 

 pelagic larval fish species and crustaceans, as well as 

 substantial geographic and annual variation in prey 

 diversity and feeding intensity. Furthermore, Levings 

 (1994) summarized the diet of smolts and postsmolts in 

 the estuarine and near-shore environments from publi- 

 cations by Power and Shooner (1966), Dutil and Coutu 

 (1988), and Hvidsten et al.^* which shows postsmolts 

 feeding on gammarid amphipods and intertidal and 

 land-based insects. Although diet information exists 

 for postsmolts, the feeding mechanisms of postsmolts 

 are unknown because there are few, if any, tracking 

 studies in estuaries and the coastal areas. However, 

 the new data on postsmolts early in their marine life 

 that we have collected using DSTs can help us infer this 

 information and the diet studies, although not specific 

 to Newfoundland waters, indicate that salmon prey 

 on pelagic fish larvae and crustacean species, some of 

 which inhabit the deeper waters of the water column. 

 Thus, it would appear that postsmolts in the present 

 study and the kelts in a previous study (Reddin et al., 

 2004) were diving from surface waters to greater depths 

 to obtain prey. The temperature profiles from the DSTs, 

 compared to oceanographic data available from other 

 studies (Colbourne'^; Colbourne and Fitzpatrick'-'), in- 

 dicate that these dives may be anywhere from 25 to 

 50 m in depth. 



1^ Karlsson, L., E. Ikonen, H. Westerberg, and J. 

 Sturlaugsson. 1996. Use of data storage tags to study 

 the spawning migration of Baltic salmon (Satmo salar L.) in 

 the Gulf of Bothnia. ICES CM. (council meeting) 1996/M: 

 9, 15 p. 



18 Hvidsten, N. A., B. O. Johnson, and C. D. Levings. 1993. Be- 

 haviour and feeding of emigrating salmon smolts in 

 Trondheimfjord. Res. Rep. no. 164, 17 p. [In Norwegian.] 

 Norwegian Institute for Nature Research, Trondheim, 

 Norway, Tungasletta 2, 7485 Trondheim, Norway. 



There is a tendency for postsmolts to be caught in the 

 upper part of the water column, as evidenced by trawl- 

 ing for postsmolts at sea (Shelton et al., 1997; Holm et 

 al., 2000; Rikardsen et al., 2004), by acoustic tracking 

 of postmolts in fjords (Holm et al., 2000; Moore et al.-"), 

 and by net catches in surface waters (Dutil and Coutu, 

 1988; Reddin and Short, 1991; Thorisson and Sturlaugs- 

 son-i). The temperature profiles from our DST-tagged 

 postsmolts indicate that although most of their time 

 is spent near the surface (and nearer the surface at 

 night), salmon postsmolts undergo deeper dives, prob- 

 ably in search of prey. Deeper diving activities have 

 been reported previously for salmon kelts also from 

 Campbellton River by Reddin et al. (2004). The present 

 study demonstrates that postsmolts also show the same 

 type of behavior although the frequency of dives varies 

 somewhat from fish to fish. Holm'-- et al. (2000) noted 

 that salmon postsmolts caught in pelagic trawls in the 

 Norwegian Sea were all caught during surface trawls; 

 none were caught at deeper depths. Westerberg (1982) 

 during coastal tracking studies noted that salmon made 

 dives of short duration to greater depths; his results are 

 similar to the results shown in our study. 



Wada and Ueno (1999) listed three hypotheses to 

 explain diving behaviour in Pacific salmon, viz. the 

 salmon are making orientation for homing migration, 

 for feeding, and for controlling body temperature. Red- 

 din et al. (2004) suggested a fourth hypothesis: that the 

 salmon are diving to avoid predators. Our conclusion is 

 that the deep diving activities of salmon postsmolts (to 

 as deep as 50 m) recorded by the DSTs, because of their 

 frequent nature, are probably related to prey seeking 

 and feeding. Reddin et al. (2004) further pointed out 

 that there may be an energetic advantage for salmon to 

 seek prey in cooler, deeper waters where prey are more 

 abundant and then to return to warmer surface waters 

 where their food can be digested more rapidly. 



Ecological information can also be discerned from 

 the temperature patterns provided by the DSTs. Ogura 

 and Ishida (1995), Wada and Ueno (1999), Walker et al. 

 (2000) and Reddin et al. (2004) all noted a period of more 

 stable temperatures experienced at both the beginning 

 and at the end of the temperature time series from the 

 DSTs and concluded that the fish remained at the same 



IS Colbourne, E. B., and C. Fitzpatrick. 2003. Physical 

 oceanographic conditions in NAFO subareas 2 and 3 on 

 the Newfoundland and Labrador Shelf during 2002. NAFO 

 (North Atlantic Fisheries Organization) SCR Doc. 03/14, 

 57 p. Northwest Atlantic Fisheries Organization. P.O. Box 

 638, Dartmouth, Nova Scotia, Canada B2Y 3Y9. 



20 Moore, A., I. C. Russell, M. Ives, E. C. E. Potter, and C. 

 P. Waring. 1998. The riverine, estuarine and coastal 

 migratory behaviour of wild Atlantic salmon (Salmo salar 

 L.) smolts. ICES CM 1998/N: 16, 11 p. 



-1 Thorisson, K. and J. Sturlaugsson. 1995. Postsmolt of 

 ranched Atlantic salmon (Salmo salar L.) in Iceland: IV. 

 Competitors and predators. ICES CM. 1995/M: 12, 9 p. 



" Holm, M., I. Huse, E. Waatevik, K. B. Doving, J. Aure. 

 1982. Behaviour of Atlantic salmon smolts during the sea- 

 ward migration. I. Preliminary report on ultrasonic track- 

 ing in a Norwegian fjord system. ICES CM 1982/M:7, 10 p. 



