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Yasmina Nirmala Asih*, Daniar Kusumawati and Suko Ismi
Institute for Mariculture Research and Development (IMRAD)
Dsn. Gondol, Ds. Penyabangan, Kec. Gerokgak, PO BOX 140, Singaraja Bali

Decrease in water quality such as increasing levels of ammonia, nitrite and nitrate during grouper larval rearing could lead to toxic environment. Artificial feed (pellet) is the main source of nitrogen converted to ammonia, nitrite and nitrate. In standard management of grouper larval rearing, feeding pellet started at day 8, while siphoning started at day 12. The objectives of this experiment were to evaluate whether delayed in feeding pellet would lead to better water quality and higher survival rate.

Cantik hybrid grouper (Epinephelus fuscoguttatus x E. polyphekadion) larvae were reared in concrete tanks filled with 5000 l of sand filtered seawater. Phytoplankton as green water was added from day 2 to day 25. Siphoning was started at day 12 onwards. Water circulation was performed bit by bit, started 10% at day 8 and reached 100% at day 31.  There were 2 treatments of feeding pellet: A. started at day 8 and B. started at day 10. Parameters observed were levels of ammonium, nitrite and nitrate at 5-day intervals.

The results showed that ammonium level of treatment B was slightly better than treatment A. Even though at the end of experiment nitrite and nitrate levels of treatment B was at the same as treatment A, during 25 days of observation treatment B had lower levels of nitrite and nitrate than treatment A (Figure 1). However survival rate of treatment A was higher (42%) than survival rate of treatment B (35%). To sum up, delayed feeding pellet resulted in better water quality but not better survival rate. Also grouper larvae, particularly cantik hybrid grouper could survive after being exposed to relatively high levels of ammonium, nitrite and nitrate.

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