Abstract
The development of rainbow trout ovaries and testes, and changes in plasma calcium and testosterone concentrations were investigated under a 3-month advanced annual photoperiod cycle with extreme fluctuations in yearly water temperatures. According to the morphometrical measurements of the gonads (oocyte size and thickness of its zona layer, the diameter of testis lobules, and the size of germ cells) the fish of the advanced photoperiod group matured earlier. Their gonads, in particular their ovaries, began to grow earlier and faster than those of the control fish in the normal photoperiod cycle. Also, in spring plasma total calcium and testosterone concentrations reached their maximal values earlier in the advanced group than in the control group fish. Because of the low water temperature (0.4°C) at that time the fish in the advanced photoperiod cycle, in spite of their early maturation, did not spend their eggs until the normal time, when the water temperature was >4°C. The fish which were removed into a constant water temperature of 10°C half way through the experiment were all running about 4 months earlier than normal in spite of their photoperiod cycle.
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