The role of seasonally altering photoperiod in regulating physiology in Atlantic cod (Gadus morhua). Part II. Somatic growth

Research to date has not clearly defined the role of photoperiod in regulating somatic growth in Atlantic cod (Gadus morhua). The present study followed individual growth performance, plasma insulin-like growth factor-I, and relative liver size in response to a range of experimental photoperiod treatments where populations were transferred from an ambient photoperiod regime to continuous illumination at strategic times during the first 2 years of life. While this work demonstrated that application of continuous illumination could directly stimulate somatic growth, this growth stimulation is transitory with no accumulative effect of prolonged exposure to continuous illumination. Importantly, it was apparent that the photic inhibition of maturation realized a far more significant growth stimulation, and in populations where this occurred, sexually dimorphic growth patterns became apparent. Plasma insulin-like growth factor-I displays a seasonal rhythm correlated with ambient temperature under natural photic conditions and in certain circumstances could be used as an accurate predictor of growth rate. Overall, this work further refines the guidelines on photoperiod management during commercial ongrowing of the species to help realize the maximum economic potential of cod aquaculture.