The Application of Synthetic Flavors in Zebrafish (Danio rerio) Rearing with Emphasis on Attractive Ones: Effects on Fish Development, Welfare, and Appetite

SIMPLE SUMMARY: Low aquafeed palatability can adversely affect fish growth and welfare, often resulting in feed waste. Conventional feed attractants, which are regularly included in aquafeeds to improve palatability, rely on natural marine resources. The present study aimed to identify cost-effective synthetic flavors to be included as innovative feed attractants in the zebrafish (Danio rerio) diet. A comprehensive overview on the effects of the tested flavors—two attractive and one repulsive—on fish growth and development, welfare, and appetite regulation was analyzed, comparing zebrafish larval and juvenile stages. Both diets containing attractive flavors enhanced zebrafish feed intake and growth without affecting fish welfare. Maximizing feed intake is a critical point for the aquafeed production sector to reduce feed wastage and minimize the economic impact. ABSTRACT: The aim of the present study was to test synthetic flavors as potential feed attractants in zebrafish (Danio rerio) during early development. Six experimental groups were set up in triplicate: (i) a CTRL group fed a zebrafish commercial diet; (ii) a PG group fed a control diet added with Propylene Glycol (PG); (iii) A1(+) and A2(+) groups fed a control diet added with 1% of the two attractive flavors (A1(+) cheese odor made by mixing Propylene Glycol (PG) with the aromatic chemicals trimethyamine, 2-acetylpyrazine, 2-acetylpyridine, and dimethyl sulfide; and A2(+) caramel odor, made of PG mixed with the aromatic chemicals vanillin, maltol, cyclotene, acetoin, butyric acid, and capric acid with traces of both gamma-octalactone and gamma-esalactone) or the repulsive flavor (A(−) coconut odor, made by mixing PG with the aromatic chemicals gamma-eptalactone, gamma-nonalactone, delta-esalactone, and vanillin with trace of both delta-octalactone and maltol), respectively; (iv) an ROT group fed the two attractive diets, each administered singularly in a weekly rotation scheme. All the tested synthetic flavors did not affect the overall health of larval and juvenile fish and promoted growth. Due to the longer exposure time, results obtained from the juvenile stage provided a clearer picture of the fish responses: zebrafish fed both attractive diets showed higher appetite stimulus, feed ingestion, and growth, while the brain dopaminergic activity suggested the A2(+) diet as the most valuable solution for its long-lasting effect over the whole experiment (60-day feeding trial, from larvae to adults). The present study provided important results about the possible use of attractive synthetic flavors for aquafeed production, opening new sustainable and more economically valuable opportunities for the aquaculture sector.