Abstract

Gulf sturgeon (Acipenser oxyrhynchus desotoi) fry (3-6 weeks old) were presented to the fish disease diagnostic laboratory at the University of Florida in April and May 1998. The fry had originated from wild-caught broodstock that were spawned on the University of Florida campus. Resulting fry were divided into two populations, with some of the animals being maintained on campus, and some moved to a satellite facility located in Blountstown, Florida. Both populations were affected with mycotic enteritis caused by Candida.

Mortalities at the Blountstown farm began when fry were approximately 3 weeks old. Fish were housed in 300 gal circular tanks set up on a flow-through system. The outbreak began suddenly with mortality as high as 100 fish per day. Fry were being fed at 5% body weight per day. They were in the process of being "weaned" from a diet of decapsulated brine shrimp to an experimental, commercially prepared diet¹. At the time of the epizootic, fry were characterized as "pinheads" by biologists, and there was significant concern over the percentage of the population that appeared to be "poor doers". Gross examination of severely affected individuals revealed Saprolegnia-like fungal mats visibly protruding from the anus and extending along the ventral aspect of the peduncle to the caudal fin. Internal examination of fish revealed fungal hyphae within the lumen of the gut. Protozoal agents, identified as Hexamita and Tetrahymena using light microscopy, were also seen. Culture of gut contents and posterior kidney resulted in isolation of yeast initially identified as Candida on Sabouraud's agar. Aeromonas hydrophila and A. sobria were also isolated from the posterior kidney and liver of some moribund fish. The stocking density of the fry was decreased and they were returned to a diet of decapsulated brine shrimp only. Mortalities decreased over several weeks, and finally ceased.

Fry from the same spawn, housed in Gainesville, were affected by a similar condition beginning at about six-weeks of age. These fish were housed in 30 gal tanks set-up on a flow-through system. The mortality rate was low, no more than 15 animals per day, and fungal hyphae were never seen protruding from the anus. Prior to death, affected individuals were observed sitting on the tank bottom and curling their tail along their body wall. Affected fish were extremely thin. At the time of the epizootic, the fry had been completely weaned off of the brine shrimp and were being fed the experimental diet only. Candida was cultured from the gut, posterior kidney and liver of moribund fish using Sabouraud's agar. Aeromonas hydrophila and A. sobria were also isolated from some animals.

Microscopic pathology was consistent with the diagnosis of mycotic enteritis. The primary lesion was chronic enteritis and necrosis of the intestine. Hepatic and renal tissues were also abnormal, with cloudy swelling of hepatocytes and evidence of nephrosis and tubular necrosis in the posterior kidney. There was speculation that some changes seen may have been caused by nutritional imbalance, and were not directly related to the fungal infection.

Mycotic enteritis is extremely rare in fish. This case may have resulted from simple oral inoculation of fry with contaminated feed. Cultures of the experimental diet were positive for Candida. There may have been other mitigating factors including crowding, nutritional imbalance, and water temperature. In both cases, daily mortality was fairly low and not all tanks of fry were affected. There was some speculation that the initial case may have been caused by intestinal impaction secondary to improper decapsulation of brine shrimp eggs, however, there was no evidence to support this in the second outbreak. Continued investigation of the nutritional and husbandry requirements of this species is encouraged.

Acknowledgements

The authors thank Dr. Andy Lazur, Mr. Greg Stauffer, and Ms. Debbie Britt of the Mitchell Aquaculture Demonstration Farm (Blountstown, FL), and Dr. Frank Chapman, Department of Fisheries and Aquatic Sciences (Gainesville, FL), University of Florida for their assistance.

References

1.  Bardi RW, FA Chapman, FT Barrows. 1998. Feeding trials with hatchery-produced Gulf of Mexico sturgeon larvae. Progressive Fish Culturist 60:25-31.