The effect of different early weaning regimes on skeletal ontogeny in ballan wrasse (Labrus bergylta) larvae

Abstract The use of ballan wrasse ( Labrus bergylta ) as cleaner fish in salmon aquaculture remains constrained by hatchery challenges, particularly high larval mortality and skeletal anomalies. This study investigated the impact of early weaning on larval growth, skeletal development, and physiological responses over the course of a 56-day long start-feeding experiment. Four feeding protocols were tested, differing in the timing of formulated feed introduction. All groups received copepod nauplii ( Acartia tonsa ), enriched rotifers, barnacle nauplii ( Semibalanus balanoides ), and a low-shear extruded diet. Differences in weaning onset did not affect larval growth, and ossification followed expected developmental patterns. However, transcriptomic analyses indicated activation of immune and oxidative stress pathways following the introduction of formulated feed. Despite adequate phosphorus and mineral content in the diets, skeletal anomalies persisted, suggesting a multifactorial origin. These results highlight the importance of optimizing early-life nutrition and rearing conditions to improve larval quality and hatchery outcomes in ballan wrasse production.