Seaweed-based diets lead to normal growth, improved fillet color but a down-regulated expression of somatotropic axis genes in rainbow trout (Oncorhynchus mykiss)

Abstract

Effects of diets incorporating seaweed (macroalgae) on growth performance, fillet composition, digestive capacity, serum biochemical parameters, and expression of somatotropic axis genes (Gh, IGF-1, Ghrelin) of rainbow trout (Oncorhynchus mykiss) are reported. Experimental diets containing biomass of one of the three seaweeds (Gracilariopsis persica, GP; Hypnea flagelliformis, HF; Sargassum boveanum, SB) at 5% wt/wt (diets GP5, HF5, SB5) and 10% levels (diets GP10, HF10, SB10) were compared with a standard control diet free of seaweed biomass. Rainbow trout juveniles were fed the prepared diets for 83 days and the effects of treatments were measured during short term (47-day) and long term (83-day) feeding durations. Fish fed on seaweed-based diets had the same weight gain, feed conversion ratio, the specific growth rate, and protein digestibility as the fish fed on control diet, but protein efficiency was reduced in the groups receiving the SB diets. Fillet analyses revealed that crude protein and ash content were not affected by the diets, but in long term (83-day) feeding trial, the HF10 group showed a significantly reduced fat content. After 47-day feeding, the GP5 and GP10 groups showed significantly reduced fillet dry matter content relative to control, but in 83-day feeding trial no significant differences were observed between the control and the treated groups. HF at both levels for 47 days and SB5 for 83 days, improved the color of the fish fillets. The levels of cholesterol and triglycerides did not differ significantly among treatments, but aspartate aminotransferase level was higher in fish fed on GP10. Moreover, inclusion of HF10 and SB10 resulted in significantly higher level of pituitary Gh expression relative to control fish in 83-day feeding trial. The expression level of IGF-1 was lower in fish fed SB10 for both feeding durations. All seaweed diets reduced expression of Ghrelin in comparison to control fish, but the difference was significant only for the shorter feeding duration with SB10 diet. Overall, the inclusion of HF showed better results.