Brewer's spent grain and brewer's spent yeast as partial fishmeal replacers in juvenile goldfish (Carassius auratus): tissue-specific redox responses and intestinal histomorphology

Reducing fishmeal inclusion is a major objective for improving aquaculture sustainability. Alternative protein sources derived from industrial by-products may support fish growth while maintaining health and welfare. This study evaluated the effects of partially replacing fishmeal with brewer's spent grain (BSG) or brewer's spent yeast (BSY) in juvenile goldfish (Carassius auratus L.) diets. Three isonitrogenous diets were tested: a control diet and two experimental diets in which 10% of fishmeal protein was replaced by either BSG or BSY. Growth performance, proximal gut histomorphology, and tissue redox status were assessed. Intestinal sections were stained with hematoxylin–eosin and periodic acid–Schiff to evaluate morphology and glycan-containing cells. Redox status was determined by measuring reactive oxygen species (ROS), hydroperoxides (HPs), protein carbonyls (COs), susceptibility to oxidative stress, and total antioxidant capacity (TAC). Neither BSG nor BSY affected body weight or total length, indicating that both ingredients can partially replace fishmeal without impairing growth. Redox responses were tissue-specific. In muscle, BSG increased HPs and susceptibility to oxidative stress, whereas BSY increased ROS but enhanced TAC and reduced oxidative damage, suggesting activation of antioxidant defenses. In the proximal gut, both diets reduced ROS but increased susceptibility to oxidative stress. Histological analysis showed preserved intestinal morphology in the control and BSG groups, while BSY increased lamina propria thickness. Both supplemented diets increased PAS-positive cells, particularly BSG, suggesting a local inflammatory response. In the mid-intestine, both diets reduced ROS production and susceptibility to oxidative stress. However, BSG increased HP and CO levels, indicating oxidative damage, whereas BSY maintained a more balanced redox status. Overall, both BSG and BSY supported fish growth and represent promising ingredients for circular aquafeeds. Nevertheless, BSY exhibited a more favorable redox profile across tissues, highlighting its potential as a valuable alternative protein source for sustainable aquaculture.