Hydroxyproline supplementation on the performances of high plant protein source based diets in turbot (Scophthalmus maximus L.)

•Hydroxyproline increased performances of high plant protein diets.•High plant protein diets reduced tissue hydroxyproline levels in turbot.•Hydroxyproline supplementation in diets improved muscle textures in turbot.

Hydroxyproline (hyp) is one of the bioactive molecules rich in fishmeal but low in plant protein sources. With increased utilization of plant proteins in aquafeeds, a better understanding is warranted for the necessity of hyp supplementation in high plant protein based diets. In the present study, isonitrogenous and isoenergetic turbot diets were formulated with 40%, 50% and 60% fishmeal substituted by plant proteins, with (or without) addition of 0.6% hyp. After an 8-week feeding trial in juvenile turbot, hyp supplementation significantly improved specific growth rate (SGR) and feed efficiency ratio (FER) in fish fed diets with 50% or higher fishmeal replaced, but not in the group with 40% fishmeal replaced by plant proteins. Hyp levels in plasma and tissues were reduced after plant protein substitutions and replenished by dietary supplementation. Dietary hyp significantly increased muscle hardness, springiness and chewiness of fish (P < 0.05). These results suggested that dietary hyp supplementation is particularly necessary for fishmeal-replaced diet exceeding a certain high level.