Do young on-grown eels, Anguilla anguilla (Linnaeus, 1758), outperform glass eels after transition to a natural prey diet?

Abstract

Survival rates among European eels, Anguilla anguilla (Linnaeus, 1758), on-grown using a formulated diet in a commercial aquaculture facility, were compared with glass eels from the same cohort following their transition to a natural prey diet in the laboratory. Treatments included zero, 42-day, and 196-day periods of grow-out prior to 30-day experimental periods when eels were fed Chironomus spp. larvae (10 tanks, each containing 240-L water and 40 glass or 10 on-growing eels; 12:12 hr photoperiod; water temperature 18°C). All glass eels survived, compared to 87% (42-day) and 99% (196-day) for on-grown eels. Although the eels on-grown for 196 days had a high survival rate, they did lose weight. Farm-reared eels may have accumulated sufficient resources over the 196-days to survive the first 30 days after weaning from a formulated diet, but not for an additional 30 days (84% survival). Lack of superior survival rates among on-grown eels challenges the presumed benefits of releasing on-grown eels for population restoration.