Transition from endogenous to exogenous feeding in hatchery-cultured European eel larvae

The transition from endogenous to exogenous feeding is critical during fish early life, where appropriate feed availability and timing of initiation of feeding influence survival. For European eel (Anguilla anguilla), establishing first feeding culture is at a pioneering state, where successful production of larvae has recently enabled feeding experiments. In the present study, three diets and potential benefits of early feeding during the transition from yolk-sac stage to feeding larvae were explored, including molecular analyses of genes involved in digestive functions and growth. Three consecutive trials were performed using hatchery produced eel offspring. In Feeding regimes 1 and 3, expression of npy and cck (appetite regulation) was higher, while expression of pomca (food intake) was lower in non-prefed larvae, indicating increased fasting and higher starvation risk. In contrast, Feeding regime 2 led to the highest survival ever registered for European eel larvae i.e. 20% at 20 dph, in spite that prefeeding resulted in reduced survival rate during the endogenous feeding stage. This was associated with initial hsp90 (stress/repair) upregulation in larvae receiving prefeeding, however, with subsequent downregulation during exogenous feeding. Notably, the growth related gh expression was higher in prefed larvae, indicating growth benefits of prefeeding. Likewise, prefeeding resulted in pomca as well as try, tgl, and amyl2a (digestion) upregulation, providing evidence of beneficial maturation of gut functionalities. Essentially, Feeding regime 2 demonstrated a continuous upregulation of growth, appetite and digestion related genes, which in combination with the highest survival suggest that dietary requirements were partially met. Moreover, in Feeding regime 2, gh and tgl were expressed at a higher level in prefeeding larvae than in the control, indicating that prefeeding might be advantageous in spite observed mortality, but further research is needed, including timing of feed application.