Weaning wild flathead grey mullet (Mugil cephalus) fry with diets with different levels of fish meal substitution

• Wild grey mullet fry were weaned with diets with different levels of fish meal.
• Fry were successfully weaned onto diets with 75% of fish meal substitution.
• The inclusion of alternative plant protein sources did not affect fish performance.
• Plant protein sources did not affect the activity of pancreatic digestive enzymes.

The culture of flathead grey mullet (Mugil cephalus) is based on wild fry captured during their migration into estuarine environments and consequently, optimizing weaning diets is of special importance for this species at this particular stage of development. Thus, authors have tested a weaning protocol for wild flathead grey mullet fry (202 mg initial body weight) during 60 days (18.1 ± 0.3 °C, salinity, 1.2 ± 0.2‰) using compound diets (36% crude protein, 16% crude fat) with different levels of fish meal (FM) substitution by plant protein (PP) sources (50% and 75% of FM replacement by a blend of corn gluten, wheat gluten, soy bean meal and soy protein concentrate). Fry were progressively weaned onto experimental diets during the first 20 days of the trial (25% Artemia metanauplii replacement each 5 days), whereas compound diets were offered at a feed ratio of 5% of stocked biomass until the end of the trial (day 60). A blend of PP sources (corn gluten, wheat gluten and soy protein concentrate) as the main dietary protein sources in combination with crystalline l-lysine and dl-methionine dietary supplementation were as good as a FM-based diet in terms of growth performance, digestive physiology and fish condition (i.e. proximate composition, oxidative stress status). As the cost-benefit proxy analysis of the tested weaning diets indicated, the inclusion of alternative PP was a satisfactory strategy in terms of feed price reduction, since PP50 and PP75 diets were 15.5 and 23.6% cheaper than the FM diet. Although wild flathead grey mullet fry were satisfactorily weaned onto diets containing 75% FM substitution by PP sources, present results indicated that complete FM replacement in weaning diets for this species might also be feasible.Statement of relevanceIn this study, authors have tested a weaning protocol for wild flathead grey mullet (Mugil cephalus) fry based on diets with different levels of fish meal substitution by alternative plant protein sources. This is of special importance due to the importance of the aquaculture of this species in several regions of the word, as well as for the use of wild animals for on-growing purposes. Results showed that diets with 75% of fish meal substitution can be successfully used for weaning and on-growing wild fry without any detrimental effect of fry performance and condition.