Skin Pigmentation in Gilthead Seabream (Sparus aurata L.) Fed Conventional and Novel Protein Sources in Diets Deprived of Fish Meal

Simple Summary Intensive farming of carnivorous fish species relies on the use of feeds, where fish meals have represented, for a long while, ideal sources of protein to ensure optimal growth, health and quality to cultured fish. Due to the gap between the demand for these commodities by the growing aquaculture industry and the unsustainability of further exploitation of the alieutic resources, their levels of inclusion in fish feeds have been mostly replaced by terrestrial plant protein-rich derivatives and conventional animal processed proteins. Recently, novel ingredients (i.e., insects and microalgae) have been proposed to this end. While the impact of different alternative proteins on fish growth and health has been studied, limited information exists on the effects of such dietary changes on quality traits of cultured fish such as skin pigmentation. The present study was aimed at assessing the pattern of yellow pigmentation of the skin in gilthead seabream fed various alternative protein sources (vegetable ingredients, insects, poultry by-product meal, red swamp crayfish and marine microalgae) included in different proportions in fishmeal-free diets, in order to evaluate new feed formulations on the basis of their coloring capacity, as intense skin coloration have been associated with high-quality of farmed fish products. The pattern of yellowish pigmentation of the skin was assessed in gilthead seabream (Sparus aurata) fed for 12 weeks iso-proteic (45%) and iso-lipidic (20%) diets deprived of fish meal and containing either a blend of vegetable protein-rich ingredients or where graded levels of the vegetable protein blend were replaced by insect (Hermetia illucens-10%, 20% or 40%) pupae meal, poultry by-product meal (20%, 30% or 40%), red swamp crayfish meal (10%) and marine microalgae (Tisochrysis lutea and Tetraselmis suecica-10%) dried biomass. Digital images of fish fed diets differing in protein sources were analyzed by means of an automatic and non-invasive image analysis tool, in order to determine the number of yellow pixels and their dispersion on the frontal and lateral sides of the fish. The relationship between the total carotenoid concentration in the diet and the number of yellow pixels was investigated. Test diets differently affected gilthead seabream skin pigmentation both in the forefront and the operculum, due to their carotenoid content. The highest yellow pixels' number was observed with the diet containing microalgae. Fish fed poultry by-product meal were characterized by the lowest yellow pixels' number, diets containing insect meal had an intermediate coloring capacity. The vegetable control, the microalgae mix diet and the crayfish diet had significantly higher values of yellow pixels at both inspected skin sites.