Kyphosis in reared sea bass (Dicentrarchus labrax L.):: ontogeny and effects on mortality

The ontogeny and the effects of vertebral kyphosis on the mortality rate of reared sea bass (Dicentrarchus labrax) are described. The first evidence of deformities was observed in individuals of 10 mm total length (TL) in the form of light malformations or bilateral asymmetries of individual vertebrae. At 14 mm mean TL, the 18.2% of the reared fish presented severe vertebral kyphosis at the pre-haemal region. Kyphosis continued to develop and presented its maximum incidence at 17 mm TL (24.8%). The centre of the deformity was mainly located on vertebrae 5-6. Kyphotic specimens presented a lethargic behaviour. After 17 mm mean TL, the incidence of the deformity decreased exponentially over time due to the heavy mortality of the affected fish (24.8% kyphosis incidence in the surviving fish vs. 52.6% in the dead fish collected from the tank, p < 0.01). For the first time, it was shown that although a skeletal deformity could have little effect on the final quality of the reared juveniles, it can significantly affect the mortality rate and thus the productivity of the hatcheries. The results are discussed with respect to the aetiology, applications and research targets for the erasure of skeletal malformations from the reared fish. (C) 2002 Elsevier Science B.V All rights reserved.