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The Journal of Experimental Biology 205, 3553-3560 (2002)
Copyright © 2002 The Company of Biologists Limited

Effects of an advanced temperature cycle on smolt development and endocrinology indicate that temperature is not a zeitgeber for smolting in Atlantic salmon

Stephen D. McCormick^1,*, J. Mark Shrimpton^1,, Shunsuke Moriyama² and Björn Thrandur Björnsson³

¹ USGS, Leetown Science Center, Conte Anadromous Fish Research Center, Turners Falls, MA 01376, USA
² Laboratory of Molecular Endocrinology, School of Fisheries, Kitasato University, Sanriku, Iwate, Japan
³ Fish Endocrinology Laboratory, Department of Zoology, Göteborg University, Göteborg, Sweden
Present address: Biology Program, University of Northern British Columbia, Prince George, British Columbia, Canada, V2N 4Z9

^* Author for correspondence (e-mail: stephen_mccormick{at}usgs.gov)

Accepted 8 August 2002

Atlantic salmon (Salmo salar) juveniles were reared under simulated conditions of normal photoperiod (LDN) or short days (LD 9:15) and ambient temperature (AMB: normal temperature increases in April) or an advanced temperature cycle (ADV: temperature increases in February). Under both photoperiod conditions, the timing of increased and peak levels of gill Na⁺,K⁺-ATPase activity were not altered by temperature, although the rate of increase was initially greater under ADV. ADV/LD 9:15 resulted in peak gill Na⁺,K⁺-ATPase activity that was half of that seen under normal photoperiod and temperature conditions. Plasma growth hormone (GH) levels increased threefold in late March under ADV/LDN, but not under ADV/LD 9:15, indicating that there is a photoperiod-dependent effect of temperature on levels of this hormone. Plasma insulin-like growth factor I (IGF-I) increased in spring in all groups, with increases occurring significantly earlier in the ADV/LDN group. In each photoperiod condition, the advanced temperature cycle resulted in large decreases in plasma thyroxine (T₄) levels in March, which subsequently recovered, whereas plasma 3,5,3'-triiodo-L-thyronine (T₃) levels were not substantially affected by either photoperiod or temperature. There was no consistent pattern of change in plasma cortisol levels. The results do not provide support for the role of temperature as a zeitgeber, but do indicate that temperature has a role in the timing of smolting by affecting the rate of development and interacting with the photoperiod.

Key words: temperature, photoperiod, smolting, Atlantic salmon, Salmo salar, zeitgeber, gill Na⁺,K⁺-ATPase, growth hormone, insulin-like growth factor I, thyroid hormone

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