Spirulina (Arthrospira platensis) and by-products from the fish processing industry as alternative protein sources to marine-origin meals in the weaning diets of mullet (Liza aurata)

The aquaculture industry is a major provider of fish for human consumption and is essential for meeting the increasing food demands of the global population. This study evaluated the use of Arthrospira platensis (Spirulina) and fish processing by-products as alternative protein sources to squid meal in weaning diets for Liza aurata (golden grey mullet) larvae. Larvae (initial weight: 1.92 ± 0.12 mg; length: 10.47 ± 1.01 mm) were reared from 31 to 52 days post-hatching (dph) under controlled laboratory conditions. Five experimental microdiets (MDs) were tested: a control diet containing 100 % squid meal as the protein source; three diets in which squid meal was partially replaced by 10 %, 20 %, or 40 % A. platensis (Arth10, Arth20, Arth40), all supplemented with lipid and vitamin–mineral premixes; and a Circular diet in which 90 % of the protein source was derived from fish by-products and formulated without additional lipid or micronutrient supplementation. Co-feeding with enriched Artemia sp. metanauplii was implemented and gradually reduced until MDs became the sole feed. Parameters assessed included growth, survival, skeletal anomalies, biochemical composition, stress response, and histological analysis of liver, pancreas, and intestine. The results indicated that A. platensis can replace up to 40 % of squid meal without compromising growth or survival, while larvae fed the non-fortified Circular diet showed significantly lower growth despite similar survival rates. Indicators of larval quality, including skeletal condition and histological features, were generally comparable among treatments, although differences in lipid deposition were observed and were primarily associated with dietary formulation. Reduced growth and altered biochemical composition in larvae fed the Circular diet were consistent with an energy and/or micronutrient limitation. Overall, these findings support the use of Spirulina as a partial replacement for marine-origin protein sources in mullet weaning diets when adequately supplemented and indicate that fish processing by-products require appropriate nutritional fortification to be effectively incorporated within circular-economy approaches in larval aquaculture. • Arthrospira platensis can replace up to 40 % of squid meal in Liza aurata weaning diets without affecting growth or survival. • Spirulina-based diets supported larval growth and survival rates comparable to the control diet. • Fish-processing by-products maintained survival but resulted in reduced growth. • High deformity incidence was observed across treatments, with no major differences among diets. • Alternative protein support more sustainable and circular aquafeeds when properly formulated.