Carob seed germ meal in diets for meagre (Argyrosomus regius) juveniles: Growth, digestive enzymes, intermediary metabolism, liver and gut histology

Carob seed germ meal (CSGM) was evaluated as ingredient for meagre juveniles' diets. For that purpose meagre was fed diets with increasing levels of CSGM (75gkg^-1, 150gkg^-1 and 225gkg^-1) replacing fish meal. Effects on growth performance, feed efficiency, activities of digestive enzymes and key liver amino acid catabolism enzymes were assessed. Digestive tract and liver histology were also evaluated for morphological alterations. Dietary CSGM inclusion up to 225gkg^-1 did not affect growth performance or whole-body composition. Nevertheless, N retention was lower in fish fed the diets with more than 75gkg^-1 CSGM. Digestive enzymes activity decreased with the increase of dietary CSGM. Liver histomorphology and lipid deposition were not affected, while distal intestine showed hyaline droplets of varying sizes with the increase of CSGM in the diets. No signs of inflammation were recorded. Overall, the present study indicates that inclusion of CSGM in meagre juveniles' diets, replacing dietary fish meal as protein source, was suitable up to 225gkg^-1. However, the decrease of digestive enzymes activity, together with decreased protein retention in fish fed the higher levels of CSGM suggest that problems may arise with long-term feeding high levels of CSGM, and this should be confirmed in further studies. Statement of relevance: The successful inclusion of carob in aquafeeds may reduce the amount of fish meal used, having the advantage of producing more cost-effective and environmental friendly aquafeeds. Additionally, meagre is as important candidate for Mediterranean aquaculture diversification and if aquafeeds for this species would benefit from using locally produced feedstuffs such as carob, both aspects will contribute to the industry's sustainability.