Flow dynamics and stocking density influence survival, morphology, and deformities of European eel larvae reared in a Kreisel tank

In the quest to close the life cycle of European eel ( Anguilla anguilla ) in captivity, reproduction techniques have advanced, but mortality and deformities during larval development remain significant challenges. We hypothesized that flow dynamics and stocking densities affect larval survival and ontogeny. Flow fields within the 8 L Kreisel tank were first characterized using fluorescent dye visualization. Then, the effects of flow rates ( Q = ~ 200, 400, 600, 800 mL/min) and stocking densities ( D = ~100, 150, 300, 600 larvae/L) on survival and ontogeny of eel yolk-sac larvae (1–11 days post hatch (dph), up to ~6 mm in body length) were examined from hatch until first-feeding. Most mortality occurred within 5 dph, accounting for 60% and 70% mortality in the flow-rate and stocking-density trials, respectively. Larval performance was significantly influenced by flow rate. At 11 dph, mean survival declined from 17% at 200–400 mL/min to 4% at 600–800 mL/min, while the proportion of normal larvae decreased from 40% to 14%. Stocking density also affected larval performance, with mean survival decreasing from 10% at 100–300 larvae/L to 4% at 600 larvae/L. Flow rate and stocking density significantly affected larval notochord length, whereas oil droplet area mostly remained unaffected. Spinal curvature and tail abnormality were the most prevalent deformity types, while coiling and dwarfism were least observed. Notably, the flow rate of 600 mL/min caused significantly increased deformity severity, which on the contrary was not affected by stocking densities. In conclusion, flow rates of 200–400 mL/min and densities up to 300 larvae/L are recommended for early-stage eel larval rearing, balancing survival, deformities, and efficient tank utilization.