Hydrolysates of whole forage-fish and Pacific krill are useful to reduce fish meal in practical diets for largemouth bass (Micropterus salmoides), and dietary fish hydrolysate suppresses expressions of intestinal oligopeptide transporter and taurine transporter genes

Abstract

Somatic and expressional effects of replacing dietary fishmeal (FM) with a 1:1:2 combination of soy protein concentrate, corn gluten meal, and hydrolysate of forage-fish (HWF) or Pacific krill (HPK) were investigated in juvenile largemouth bass for 66 days. The control diet (FMC) contained 320 g kg−1of FM. Six extruded diets were produced with 3 rates of replacement at 25 %, 50 % and 75 % protein for each of the two hydrolysate combinations. Feed intake (FI) and weight gain (WGR) did not differ between fish fed HWF diets and FMC. Fish fed HPK50 and HPK75 had lower FI and WGR. FI linearly decreased with increasing HPK, while feed conversion ratio (FCR) increased. No significance was found in the FCR among groups fed FMC, HWF25, HWF50, and HPK25, whereas HWF75, HPK50, and HPK75 groups had significantly higher FCR. The apparent digestibility (AD) of crude protein (CP) increased with increasing HWF while HPK didn’t cause significant change. AD of lipid and energy were not affected by diet, and AD of most amino acids increased proportionally with HWF. No proximate composition or retention of crude or digestible protein or energy other than whole-body ash revealed HWF-related differences. Whole-body CP, lipid, ash contents and CP retentions showed significant dose response in HPK fed fish. The condition factor decreased linearly with increasing HPK. Fish fed HPK50 diet upregulated peptide transporter 1 (pept1) expression in the foregut. Expressions of both taurine transporter (taut) and pept1 were upregulated by increasing replacement from 0 % to 25 % HWF. Further increase in HWF replacement caused linear down regulation in expression of both transporters. In conclusion, HWF and HPK facilitated reduced use of fishmeal in practical diet for largemouth bass. Expressions of pept1 and taut were dose-dependent in fish fed HWF. HPK just caused single diet upregulation.