Photoperiod regulates the timing of sexual maturation, spawning, sex steroid and thyroid hormone profiles in the Atlantic cod (Gadus morhua)

Atlantic cod of both sexes were held under four different photoperiod regimes from age I year until second sexual maturation at 3 years. Annual photoperiod cycles were compressed into 6 or 9 months, held at 12 months, or extended to 18 months, in each case followed by one 12-month cycle (termed 6 + 12, 9 + 12, 12 + 12, and 18 + 12, respectively). Photoperiod alterations caused shifts in the cyclic patterns of plasma calcium, sex steroid, and thyroid hormones, and also produced correlative changes in the timing of spawning. Initial spawning was advanced in the compressed (6 + 12) photoperiod group, followed by further advancement in the timing of the second spawning. Conversely, spawning was delayed in the 18 + 12 group. Patterns in circulating hormones observed under these experimental conditions were consistent with the coupling of photoperiod to spawning by way of established endocrine transducers. These results demonstrate that manipulation of the annual photoperiod cycle is a powerful hatchery technique in the maintenance of reproductive stocks of Atlantic cod that spawn at various times of the year. (C) 2004 Elsevier Science B.V. All rights reserved.