Influence of temperature and feeding on early sexual maturation commitment in male Atlantic salmon (Salmo salar, L.) during the freshwater stage
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- Master theses 
The intensification of Atlantic salmon (Salmo salar) post-smolt production to enhance growth performance has promoted a rise in precocious male maturation rates. Unlike traditional parr maturation, which seems to be highly dependent on genetic background, post-smolt maturation seem to be more linked to intensive rearing conditions. This study focused on the relationship between different temperatures and feeding rations on precocious male maturation and early gonad development in Atlantic salmon pre-smolts. Early gonad development was used as an indicator for post-smolt maturation. Juvenile salmon (n = 1800) were reared at three different temperatures (8, 12.5, and 18°C) and two different feeding rations (67%, 100%), producing six experimental groups (8-67%, 8-100%, 12.5-67%. 12.5-100%, 18-67%, 18-100%). An LD24:0 photoperiod was maintained throughout the experiment with a five-week winter signal (LD12:12) induction in February-March to promote developmental events. Growth (body weight, CF), hepatosomatic index (HSI), gonadosomatic index (GSI), gonadotropin receptors transcription (fshr, lhr), gonadotropin-responsive transcription (amh, gdsf1, gsdf2, igf3), plasma 11-Ketotestosterone (11-KT) concentrations, and spermatogenic activity were used as indicators for maturational advancements. According to the present results, the intensive rearing groups (18°C-100%, 18°C-67%, 12.5°C-100%) experienced a high developmental rate, stimulating spermatogenetic advancement. In comparison, less intensive rearing groups (12.5°C-67%, 8°C-100%, 8°C-67%) displayed lesser physiological development with corresponding low or no spermatogenetic advances. Results suggest high temperatures (18°C) to be one of the main contributors to trigger precocious male maturation in Atlantic salmon, controlling the rate and magnitude of gonadal development independently of the feeding ration. Intermediate temperature (12.5°C) seems to be more dependent on intensive feed rations to fully mature, as the full-fed group displayed a moderate percentage of maturational advancements with corresponding physiological development than the restricted feeding group. This proposes that the relevance of feed rations on precocious male maturation may be dependent on temperature. Low rearing temperatures (8°C) seemed to impair the maturational process independently of feeding rations, further supporting the importance of temperature as a precocious maturation trigger. For the salmon industry, this means that intensive rearing may enhance growth, but at the cost of a high proportion of early maturation. By rearing fish at lower intensities (12.5°C-67%, 8°C-100%, 8°C-67%), it is possible to achieve growth and avoid maturation simultaneously.