Graded levels of fish protein hydrolysate in high plant diets for turbot (Scophthalmus maximus): effects on growth performance and lipid accumulation

• In high plant protein diets, fish protein hydrolysate (FPH) replacing fish meal by 10% of total dietary protein did not compromise the growth performance of juvenile turbot.
• The dietary FPH treatments, especially at high levels, modulated the lipid accumulation and fatty acid compositions in various turbot tissues, in a dose- and tissue-dependent manner.
• This study provided evidence for the fish meal protein sparing effects of FPH in fish diets with very high levels of plant protein.
• This is the first study evaluating the modulating effects of fish protein hydrolysate on lipid accumulations and fatty acid compositions in various fish tissues.

A 12-week feeding experiment in indoor flowing seawater system was conducted to investigate the effects of graded levels of dietary fish protein hydrolysate (FPH) on growth performance and lipid accumulation of juvenile turbot (Scophthalmus maximus) (initial body weight 4.16 ± 0.01 g). Four isonitrogenous and isoenergetic experimental diets with high plant protein were formulated to contain graded levels of FPH, meanwhile the fish meal was replaced correspondingly by 0% (Diet FPH-0, control), 5% (Diet FPH-5), 10% (Diet FPH-10) and 20% (Diet FPH-20) of total dietary protein, respectively. Quadruplicate groups of 25 fish were fed to apparent satiation twice daily during the feeding trial. The results showed that the specific growth rate (SGR), protein efficiency ratio, and protein retention was not significantly different among group FPH-0, FPH-5, and FPH-10. The highest level of dietary FPH (FPH-20) significantly reduced the SGR but increased the feed intake compared to the control group. The viscerosomatic index in group FPH-10 and FPH-20 were significantly lower than that in the control group. Fish fed FPH-20 also showed significantly lower crude lipid concentration in whole body than fish fed the control diet. The concentrations of total and neutral lipid in gut were significantly lower in fish fed FPH-10 compared to the control group. For muscle lipid, the polar lipid concentration significantly decreased while the neutral lipid concentration significantly increased with increasing levels of dietary FPH, but no significant difference in total lipid concentration was observed among experimental groups. The lipid concentrations in liver were not significantly different among dietary treatments. With increasing levels of dietary FPH, serum triacylglycerol and cholesterol concentrations significantly decreased. The influence of dietary FPH on tissue fatty acid compositions generally corresponded with that on tissue lipid concentrations. In conclusion, these results suggested that in high plant protein diets FPH replacing fish meal by 10% of total dietary protein did not compromise the growth of juvenile turbot. However, a higher FPH level (replacing fish meal by 20% of total dietary protein) reduced the growth and feed utilization but it increased the feed intake. In the present experimental conditions, the FPH treatments, especially at high levels, significantly modulated the lipid accumulation and fatty acid compositions in turbot tissues, in a dose- and tissue-dependent manner.Statement of RelevanceThis study provided useful data for the circulatory use of fish processing by-products in fish diets. Also, based on the results of lipid analysis in fish tissues, potential lipid manipulating feed additives could be explored from fish protein hydrolysates.