INDONESIAN HATCHERY REARED SEABASS LARVAE ( Lates calcarifer ) , ASSOCIATED WITH VIRAL NERVOUS NECROSIS ( VNN )

Mass tuortality atuongs seabass larvae, LQhs t<tkrtrifer, reared in hatcheries in East Java, and Bali dr'te to viral nervous necrosis were investigated. outbreaks of the disease occurred froDr Ar'rgust to Novetnber 1997. Each time the disease occurred, cumulative mortality of the larvae reached 100'%. Infected fish were churacterized by abnormal behaviour such as swiurnring upside down or sinking to the bottonr. Bacteria or parasites associated with the disease were not detected in infected fish. Histopathologically, necrosis and vacuolation in the brain atrd retina were observed. Abundant spherical virai particles, B0 nm in diameter. were found in the cytoplasm of affected nerve cells. These findings revealed that the mortalities among seabass larvae were due to viral nervous necrosis (VNN) caused by nodavirus. KEYwoRDS: seabass, Lates carcarifer. vi,ar ner.vo*s nec'osis, .odavi'us.


INTRODUCTION
Seed production techniques for various rnarine fish and shellfish have been developed in Indonesia.Seabass, Lates calcarifer is one ofthe species rnarhed as new and potential cornrnodity for aquaculture.However, mass rnortality in thl larval stages due to unidentified diseases have led to frequent decreases in production in recent vears.
In 1997, rnass rnortalities of seabass larvae, Lates calcarifer occvtred at hatcheries in Situ_ bondo, East Java, and in Gerokgak, Bali in August and then in Banvuwangi in Novernber.The rnortalitl,' rate reached 100% in all cases.Clinical signs of the irrfected fish were characterized by abnorrnal behavior such as swirruning upside d.own or sinking to the bottorn.
Mass rnortalities in seabass larvae due to a viral infection called viral nervous necrosis (VNN) (Glazebrool< et al., 1990; Renarrlte el at., LggI;  \4rrnday et a1.., 1992) have been reported in Australia and the Mediterranean.VNN has also been reported in a wild variety of cultured marine frsh species, and has caused serious damage to the industry in Japan (Yoshihoshi & Inoue.1990:  Mori el o/., 1991).
In this study, a histopathological study of the rnass tnortalities at three seabass hatcheries in Indonesia was conducted to elucidate the cause of the rnortalities.

Fish Exarnined
One to two-month-old larvae of seabass reared irr East Java and Bali showing abnormal behavior such as swimrning upside down or sinking to the bottom, were exarnined for this study.The origin of the eggs distributpd to the these hatcheries identical.Water temperatures were 27-2g0 Cwhen outbreaks of the disease occurred.

Parasitological and Baeteriological Exanrinations
The gills and body surface were examined for parasites by usirrg light microscope.Bacteria frorn the liver and brain were isolated using rnarine agar and TCBS agar, then incubated at 27o C and 35"C respectively for 48 hours.

H istollathologicnl Examination
All of the exarnined fish showed same histo- pathological appearance.Vacrrolation and degeneration of nerve cells were obserr.'edirr the brain (Figure 1) and retina (Figure 2), but no histopathological ehanges were observed in other organs.Nurnerous round-shaped, rrnenveloped virr.sparticles, about 30 nrn in diarneter, were observed in the cvtoplasrn ofthe degenerated nerve cells (Figure 3).In both the brain and retina, the virions had the satne size and rnorpholopy.Norlnal appearance of the brain and retina of seabass was shown in Figure 4 Recently, a viral infection, viral nervous necrosis (VNN) was reported in larvae and juveniles of Japanese parrotfish (Yoshihoshi & Inoue, 1990t' irt stripped jack (Mori el al., 1992),larvae and jtrveniles of redspotted grorrper (Mori et al', 1991), help grouper and tiger puffer (Nahai et al',19941, adult   sevenband grollper (Fukuda et aI., 1996)' barrarntrndi in Australia (Glazebrook et    IFR Journal Vol.IV No.1, 1998   (b)   of the brairr (a) and retina (b)  observed in the cyptoplasrn of affected nerve cells (Nakai et aI.,1995 jn Fukrrda et al., t9g6).Fukuda et aI. (1996)reported that fish infected with VNN were characterized by upside down swirnming behaviotu, swirnbladder inflation and degeneration of nervous tissue (necrosis and vacuolation).
In the present study, the infected fish were characterized by abnorrnal swirnrning or sinhing to the bottorn, anorexia, slight blackening of the body.and degeneration of brain tissrre and retina (necrosis and vacuolation).No parasites nor bacteria associated with the disease were found, but small round-shaped virus particles (30 nm in Renanlt et a1,., L99l\.This indicates, that present seabass disease was associated with VNN.This is the first report ofVNN ofseabass larvae reared irr a hatcherv in Indonesia. Fnktrda et al. (1996) suspected a highei rearing water ternperature in summer season (25-280 C) is a possible predisposing factor which enabled the viral invasion into the central nervous system.
Mori el c/. (1991)also reported that disease occurred in larval and juveniles of redspotted grouper were reared at 25-27oC in August to September.The effect of rearing water temperature on the defence mechanisms of seabass larvae should be exarnined in further experiments.VNN in Japan was detected frorn gonads of broodstocks (Mon e t aL, 1 997; Watan ab e e t al., IggT't. indicating possible vertical transrnission of the virtr.sfrom spawners to the offsprings.One effective rnethod to prevent VNN is by using broodstocks free of the virus in the hatchery.But, Glazebrcok et al. (1990) reported that the virus is highly infectious, being transrnitted frorn diseased to healthy fish within 4 days of contact.Up to now, no chernical agent has been found effective against this disease.