Observations on the Types of Deformed Larvae Encountered during Induced Spawning of a Threatened Catfish, Ompok bimaculatus (Bloch,1794)

Abstract

Larval deformity is a recurring hatchery concern because it reduces seed quality, production efficiency, growth and survival. The present study recorded and categorised the visible morphological anomalies encountered during induced spawning and hatching of the threatened butter catfish, Ompok bimaculatus. Broodfish weighing 90–120 g were collected during July, and females were induced with Ovatide at 1 mL/kg body weight. Eggs obtained after stripping were fertilised with sperm suspension prepared from harvested testes and incubated in a flow-through plastic-tub hatchery. Approximately 300–400 fertilised eggs were placed in each hatching tub, and hatching was allowed for 24–26 h. Deformed larvae were collected from four breeding attempts, segregated initially using a magnifying glass and confirmed under a stereozoom microscope. The observed deformities were classified as teratoma, acephala, dwarf larvae and axial deformity. Teratoma was characterised by an undifferentiated embryonic mass without distinct body parts; acephala showed an indistinct head region; dwarf larvae were smaller than normal hatchlings; and axial deformity involved bending or multiple curvatures of the trunk and/or tail. Normal larvae were 4–5 mm long, whereas some deformed larvae were 2–3 mm or 2.6–3.1 mm long. Deformity was generally 5–7% during usual breeding operations and increased to 10–15% during late breeding. The observations indicate that visible larval deformities are important practical indicators of hatchery seed quality in O. bimaculatus.