This thesis examines the growth response of Atlantic salmon post-smolt (Salmo salar) in a factorial experiment with three temperatures and two light regimes. The aim of this study was to investigate under laboratory conditions the interaction between photoperiod and temperature in order to make recommendations on the use of additional cage light under low temperatures in Northern Norway. The experimental part of the study was conducted at the High Technology Centre in Bergen in the period from October 15th 2013 until March 17th 2014. 1140 post-smolt (96 g SE ± 3.1) were distributed in six groups, and exposed to 4.3 (4), 6.5 (6) and 9.3 (9) °C, and either natural light regime of Tromsø (LDN, N 69° 40`) or LDN 24:0. Each group consisted of two replicate tanks for a total of 12 tanks. Subsets of 20 fish in each replicate, approximately 240 fish in total, were individually tagged to follow individual growth responses. Growth was measured as increase in weight and fork length from the start of the experiment to four time points including the end of the experiment at day 145. Feed intake was monitored during the last 4 weeks of the trial period. Blood glucose, Na+, HCO3-, CO2 partial pressure, dorsal fin area, heart weight, liver weight and gill tissue were also sampled or measured in order to identify physiological and welfare effects of photoperiod and temperature treatments. Samples for measurement of filet quality were also taken (by Dr. Bjørn Roth, NOFIMA Stavanger) and are partly presented in this thesis. The fish exposed to low temperature and natural light regime (4LDN) had a significantly lower growth (26 % less in overall SGR) than the 4LL group, corresponding to the effect of approx. 1.2 °C temperature increase. Fish in the 6 °C and 9 °C groups did not show any significant growth benifit of continuous light (LL). Compared to the 4LDN group, the 4LL group showed overall higher condition factor, higher total feed conversion efficiency, lower levels of blood Na+ and lower hepato-somatic and cardio-somatic indexes. A negative correlation between growth rate and filet hardness was observed, but no direct correlation between temperature and light was shown
