PHOTOPERIOD CONTROL OF SEXUAL-MATURATION OF THE ATLANTIC HALIBUT (HIPPOGLOSSUS-HIPPOGLOSSUS) - PLASMA THYROID-HORMONE AND CALCIUM LEVELS

Three groups of Atlantic halibut (Hippoglossus hippoglossus) were expo sed to simulated natural (n = 225), four-month advanced (n = 225) and four-month delayed (n = 40) annual photoperiod cycles. Of these, 30, 3 0 and 15 tagged individuals, respectively, were studied for sexual mat uration and sampled for blood every four weeks for over three years to obtain repeated annual maturational profiles, plasma thyroid hormone and calcium profiles. Most tagged individuals (28, 24 and 12, respecti vely) became sexually mature during the study and the male:female rati o was found to be 1:3. The spawning of the control group took place du ring March, April and May. The photoperiod manipulation significantly shifted the timing of spawning already in the second spawning season, and after three years, spawning of the advanced and delayed groups com menced 114 days before and 130 days after the control group, respectiv ely, in close concordance with the shift in annual photoperiod cycles by 120 days. A distinct annual profile in total plasma calcium levels, reflecting changes in circulating vitellogenin levels, was seen in sp awning females in all treatments with highest levels of 4.5 mM observe d one to two months prior to spawning and lowest levels of 2.8. mM abo ut a month after the end of the spawning period, Distinctly different patterns of circulating thyroid hormones (TH) were observed in relatio n to spawning, depending on sex. Under the simulated natural annual cy cle photoperiod, male halibut exhibited autumn-winter peaks in plasma triiodothyronine (T-3) and thyroxine (T-4) (23 ng T-3/ml and 28 ng T-4 /ml) which declined to an annual low-point in late spring (7 ng T-3/ml and 12 ng T-4/ml), around the time of spawning. The T-3:T-4 ratio was fairly consistent around one throughout the year. In contrast, female s had generally high circulating T-3 levels (32 ng T-3/ml) and low T4 levels (< 10 ng T-4/ml), hence, T-3:T-4 ratios exceeding 3:1 during th e months prior to spawning. This relationship was also present in fema les on advanced and delayed annual photoperiod cycles, suggesting invo lvement of T-3 in female reproductive processes, possibly by way of in creased peripheral monodeiodination of T-4. (C) 1998 Elsevier Science B.V. All rights reserved.