Effect of photoperiod on plasma thyroxine hormone level of mirror carp (Cyprinus carpio) raised at a low water temperature in a controlled environment

Abstract

The objective of this study was to examine the effects of various lighting regimes on the plasma thyroxin hormone (T4) level of mirror carp (Cyprinus carpio). The carp were kept at the low temperature of 9°C to eliminate any influence of water temperature on feed intake, growth, and the hormone level. Treatments were 8 h light:16 h dark, 12 h light:12 h dark, and 16 h light:8 h dark. Plasma thyroxin levels were measured every four weeks for 12 weeks. The levels were significantly higher (p<0.05) in the groups exposed to 8 or 12 h light than in the group exposed to 16 h. The T4 levels significantly dropped with time in all photoperiods.