This study investigated the effects of dietary 6‐phytase, produced by a genetically modified Komagataella phaffii, on growth performance, feed utilisation, flesh quality, villus morphometric properties, and intestinal mRNA expression in rainbow trout. Six iso‐nitrogenous, iso‐lipidic, and iso‐caloric diets were formulated and fed to triplicate groups of juvenile rainbow trout weighing 32.57 ± 0.36 g (mean ± SD) for 90 days. The dietary treatments included two positive controls (PC), one formulated with 400 g/kg of fish meal named T1, and the other formulated with 170 g/kg of fish meal plus 1% avP derived from monocalcium phosphate named T2. The remaining dietary treatments consisted of a negative control (NC) formulated with 170 g/kg of fish meal (T3), NC+ 750, NC+ 1500, and NC+ 3000 OTU/kg levels of phytase designated as T4, T5, and T6 diets respectively. Compared to T1, weight gain (WG) increased by 16.29, 13.71 and 11.66% in T4, T5 and T6, respectively (p < 0.05). Feed conversion ratio (FCR) was lowered by 3.2 and 0.8% in T4 and T5 compared to T1 (p < 0.05). WG, feed intake (FI), FCR, final body length, bone ash, bone ash P, and intestinal morphometry were negatively affected in T3 fed fish (p < 0.05). Whole‐body fish nutrient, bone ash, bone ash phosphorus (P) compositions and mucosal villus morphometric properties improved in rainbow trout fed diets supplemented with phytase dose ranging from 750–3000 OTU. Bone ash increased by 6.12% in T5 compared to T1 (p < 0.05). Phytase inclusion enhanced the profitability of feeding juvenile rainbow trout such diets as it reduced the feed price and economic conversion rate. Dietary inclusion of phytase down‐regulated mRNA expression of genes responsible for fatty acid synthesis and lipogenesis in juvenile rainbow trout. Dietary phytase up‐regulated the mRNA expression of genes (SLC4A11 and ATP1A3A) responsible for nutrient uptake and down‐regulated intestinal expression of MUCIN 5AC—like genes (mucus secreting genes) in juvenile rainbow trout. Along with improving performance parameters, the inclusion of phytase in rainbow trout diet containing plant‐based protein sources, can preserve intestinal morphology by regulating the mRNA expression of genes responsible for fatty acid synthesis, lipogenesis and nutrient uptake and transport.

The aim of the present study was to investigate the effects of seasons (summer and winter) on the proximate, amino acid and fatty acid compositions of gilthead sea bream fish fed with commercial diets in sea cages and earthen ponds in the Aegean Sea in Türkiye. In general, the nutrient composition of the diets met the nutritional needs of the fish. Proximate analysis of fish fillets revealed that fish reared in earthen ponds had the lowest moisture and highest lipid content in the winter season (p < 0.05). Although fillet protein contents were comparatively higher in the winter (p < 0.05), there was no significant difference between systems in the same season (p > 0.05). Growing gilthead sea bream in marine cages produced fillets with high contents of the essential fatty acids (EFA) eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3). On the other hand, the 18:1n-9 and 18:3n-3 contents of fish reared in earthen ponds increased by 25.64 and 18.80% respectively, compared to those in marine cages. Fillet content of total n-3 fatty acid were comparatively higher (p < 0.05) in gilthead sea bream from marine cages, and this increase was even more evident in the winter months. The n-3/n-6 ratios of fillets were significantly high in gilthead sea bream from marine cages in the summer. The levels of the essential amino acids (EAAs) arginine and leucine were higher in fillets from both systems in the winter. On the other hand, fillet contents of lysine were higher in the summer. In general, the total EAA content of fillets was higher in fish from the sea cages in summer. EAA/NEAA ratios in both systems in summer and winter were found to be at good levels in terms of fish fillet quality. Fillet contents of EFA and EAA from both systems and seasons were found to be good for human consumption and will have positive effects on human nutrition. In conclusion, commercial diets used in feeding gilthead sea bream improved the proximate, fatty acid and amino acid compositions of fillets to a good extent. Different seasons and culture systems may affect the nutritional composition of fish fillets, especially on fillet EFA contents. Results of fillet samples in marine cages and earthen ponds in summer and winter showed that the fish were of good quality for human health. Evaluation of the results in the present study shows that that gilthead sea bream can also be successfully cultured in earthen ponds in areas unsuitable for agriculture.

The purpose of this study was to determine the effects of replacing fish meal with red lentil meal (RLM) as an alternative plant protein source in diets for juvenile rainbow trout (10.14±0.04 g mean initial weight) on growth performance and amino acid composition of fish. Four iso-nitrogenous and iso-lipidic experimental diets were prepared to include 15% (RLM15), 20% (RLM20) and 25% (RLM25) of fish meal. At the end of the 60 day feeding trial, the highest mean individual weight gain (30.55±0.08 g) of fish was found in control group but not significantly different from RLM15. Crude protein level of whole body/fillet gradually decreased with increase in RLM percentages in the diets. Generally, essential amino acid (EAA) profiles of whole body/fillets reflected the dietary EAA profile. EAA profile of fish fed RLM15 diet was close to control group (P>0.05). However, lysine levels of fish decreased with increasing dietary RLM levels. RLM20 fed fish had the highest body contents of phenylalanine (P