Supplying rapeseed meal to the diets with or without potassium iodide for yellow catfish (Tachysurus fulvidraco)

Abstract

The aims of the study were (1) to determine the effect of rapeseed meal on growth, body composition, thyroid hormones, and expressions of three genes involved in thyroid function and (2) to determine if potassium iodide in the diet counteracts the adverse effect caused by rapeseed meal on growth. One fish meal-based diet was serviced as the control. The other six diets were designed on the basis of 3 × 2 factorial design. The rapeseed meal inclusion (10, 20, and 30%) and potassium iodide inclusion (0 and 72 mg (kg)−1) were set to be two factors. The fish experiment was designed according to a randomized block model with three size groups of fish (27.7, 19.9, and 8.1 g). The feeding period lasted for 71 days. The results showed that final weight, weight gain, specific growth rate (SGR), and feed conversion ratio (FCR) of fish fed with 10% rapeseed meal-based diets were significantly better than those fed with 20 and 30% rapeseed meal. The highest nitrogen retention and protein efficiency ratio (PER) were in the 10% rapeseed group. Inclusion of 30% rapeseed meal in the diets significantly reduced whole-fish dry matter, Ile, and Phe contents. The gene expressions of the thyrotropin receptor (TSHR), sodium/iodide symporter (NIS), and thyroid peroxidase (TPO), as well as the thyroid hormone (T3 and T4) levels in the plasma, were not affected by the different diets. However, TSHR and TPO gene expressions negatively correlated with T4 and lipid retention. NIS gene expression positively correlated with cumulative feed intake, final weight, weight gain, SGR, nitrogen retention, and PER. It also negatively correlated with FCR. In conclusion, the rapeseed meal inclusion level in the diet was suggested to be 10%. Potassium iodide in the diet (72 mg (kg)−1) did not counteract the adverse effects caused by rapeseed meal on fish growth. Correlations provided evidence to support the link between NIS gene and fish growth.