Triiodothyronine and thyroxine levels and morphological parameters of fish were studied from January to May to evaluate the smoltification of yearling sea trout (Salmo trutta L.) raised in two different rearing systems (recirculating and flow-through) in three different hatcheries in Latvia. Thyroid hormone levels increased from January to May in both rearing systems in fish of different sizes. There was positive correlation between the degree of silvering and fish fork length and thyroid hormone levels, but there was no correlation between condition facor and thyroid hormone levels. Fish reared in recirculating systems were significantly larger (p < 0.05) and exhibited more visible signs of smoltification earlier than those reared in flow-through systems. The thyroid hormone levels and degree of silvering gradually became nearly the same by May in the sea trout from both rearing systems. Changes in thyroid hormone levels or fish size in spring cannot be evaluated separately from other signs to determine smoltification.
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