Physiological response of rainbow trout (Oncorhynchus mykiss) to graded levels of novel Chlorella sorokiniana meal as a single fishmeal alternative or combined with black soldier fly larval meal

Abstract

Chlorella meal is a potential single-cell protein source for fishmeal replacement in aquafeed. However, it is still challenging to incorporate Chlorella meal in diets of carnivorous fish at high inclusion level due to its rigid cell wall and lower nutritional quality compared with fishmeal. This study was conducted to investigate novel cell wall-ruptured Chlorella sorokiniana GT-1 meal alone or combined with black soldier fly larval meal (BSFM) as substitute ingredient to dietary fishmeal for rainbow trout, Oncorhynchus mykiss. Nine isonitrogenous (43% crude protein) and isolipidic (17% crude lipid) diets were formulated: a control diet (fishmeal-based) and eight experimental diets replacing fishmeal by 25%, 50%, 75% and 100% with C. sorokiniana meal; 25%, 50%, 75% and 100% with C. sorokiniana meal + BSFM blended with 50:50 ratio (C. sorokiniana meal + BSFM). A total of 540 fish (initial body weight of 67.65 g) were randomly distributed to 27 tanks (20 individuals/tank) with the nine diets in triplicate. The fish were hand-fed experimental diets to apparent satiation twice daily for 8 weeks. The results showed improved growth performance and feed intake in rainbow trout fed a C. sorokiniana meal-based diet (C5). The C. sorokiniana meal-based diet also resulted in higher villi height and greater muscular layer thickness of the proximal intestine, improved intestinal barrier integrity, and healthier gut microbiota with increased Lactobacillus and decreased Acinetobacter compared with the controls. There was no improvement in growth performance of rainbow trout fed a diet based on a mixture of C. sorokiniana meal and BSFM (C9), and no significant difference was observed between C9 and the control group. However, decrease in intestinal inflammation and improvement of bacterial diversity were observed by blending C. sorokiniana meal and BSFM. In addition, shifts in amino acid metabolism and elevation of fatty acid metabolism were found in C5 and C9. Upregulation of mechanistic target of rapamycin (mTOR) pathway-related genes in liver and intestine was also observed in C5 and C9. The findings suggest that the novel C. sorokiniana meal as a single substitute ingredient or combined with BSFM has the potential to replace dietary fishmeal at high inclusion levels in rainbow trout.