Morphological abnormalities in hatchery-bred milkfish (Chanos chanos Forsskal) fry and juveniles

Abstract

Morphological abnormality has been observed in hatchery-bred milkfish juveniles. To characterize and quantify the occurrence of these anomalies, hatchery-bred milkfish juveniles from commercial nursery ponds were sampled, and the development of abnormalities in tank-reared fry and juveniles was monitored. Small specimens were cleared and stained using a KOH-alizarin technique for osteological examination. The occurrence of gross abnormalities in hatchery-bred milkfish juveniles reared in commercial ponds was highly variable (3–26%). These abnormalities were predominantly a cleft on the branchiostegal membrane (CBM) and a deformed operculum (DOp), which was mostly folded. CBM was commonly associated with a deformity or the partial to total absence of its supporting branchiostegal rays (DABr). DOp and DABr, but not CBM, were first detected during the early juvenile stage. Reference samples of wild fry did not develop similar proportions of abnormalities when reared in tanks, indicating that these abnormalities are predetermined or induced before the fry stage. Slow growth and development were observed in fish with opercular and branchiostegal abnormalities. A high mortality rate (70%) of abnormal fish was also observed after handling and transfer of stock. These results demonstrate that morphological abnormalities such as CBM and DOp do not only affect the appearance of milkfish but also interfere with its growth and survival.