Abstract

The Florida Largemouth Bass (FLMB) is an important game fish in Florida and throughout much of the United States. Heavy fishing pressure, reduction in environmental quality and other factors have caused a reduction in the FLMB-portion of game fish populations in Florida lakes. The Florida Fish and Wildlife Conservation Commission (FWC) currently stocks FLMB fingerlings in lakes with low FLMB populations or lakes that are missing year classes to enhance population size and reproductive success. Early results of these stocking studies suggested that fingerlings greater than about 100 (+/- 15) mm in length were needed for good fingerling survival. However, producing 100 mm fingerlings requires low pond density culture in outdoor ponds, large quantities of natural forage and consequently large areas of forage-production ponds.

The FWC has developed the capability to intensively rear up to 1-million advanced sized (100 mm) FLMB to support a strong state-wide stock enhancement program. This would require the replacement of natural forage with synthetic diets and especially pellet-diets compatible with the use of automatic feeders.¹ Initial studies found that survival of fingerlings produced using synthetic pellet-diets was often low (about 3%). Diagnostic examination of fingerlings reared on synthetic diets showed that severe lesions occurred only in the livers.² Examples of lesions found (and listed by increasing severity) were: steatosis, lobular inflammation, swelling of hepatocytes, fibrosis and tumors. The livers were grossly pale and the condition was termed "pale liver syndrome."¹

Experimental diets were prepared that had significant changes in the levels of six major nutrient classes: protein, fat, fiber, carbohydrate, ash and energy. A contaminant profile, including the 50 contaminants most frequently found in Florida, did not detect significant levels of any known hepatotoxin. Fat content of the diet was the only major nutrient class that was associated with all liver lesions listed above. Liver tumors were tentatively identified as multiple focal nodular hyperplasia (MFNH) and possibly hepatocellular carcinoma (HCC). It was hypothesized that severe fibrosis and MFNH may progress to HCC. Correlation of fatty acid profiles from experimental diets containing different levels of fat and different blends of fatty acids indicated that linoleic acid concentrations were most closely associated with the incidence of severe fibrosis and tumors. All three livers from a sample of three fish fed a 16% fat diet with a blend containing 12% linoleic acid had the lesions listed above. Additionally the bass had more than 25 MFNH per liver. Three fish fed a 26% fat diet with a blend containing 7.5% linoleic acid had all of the liver lesion listed above but no severe fibrosis or tumors. The results suggested that steatosis, lobular inflammation and some swelling of hepatocytes resulted from an overload of fatty acids (>15% fat in the diet) while severe fibrosis and tumors resulted from an overload of fatty acids for lipid blends containing greater than 10% linoleic acid.^3,4,5

Seven high protein (>48% protein) pellet-diets with less than 15% fat containing lipid blends with less than 7.5% linoleic acid were prepared.¹ These pellet-diets produced fingerling growth rates of approximately 1 mm per day under intensive culture conditions at the FWC's Richloam State Fish Hatchery. Some hatchery-fingerlings ranging from 30 to 250 mm in length and fed these diets had mild steatosis, mild lobular inflammation and mild hepatocyte swelling. The incidence of severe fibrosis and tumors was too low to be detected in 50 liver samples. Twenty liver samples taken from 2007 year-class fingerlings that had been stocked from 30 to 90 days in Florida lakes showed fewer lesions than the hatchery-fingerlings and severe fibrosis or tumors were not detected. It was concluded that fingerlings up to 250 mm in length with only mild liver lesions and a low (<2%) incidence of severe fibrosis and tumors can be reared in roughly eight months using synthetic pellet diets. The study has shown that the hatchery-fingerlings can be stocked in Florida lakes, survive, and any mild liver lesions appear to regress. It was also concluded that mortality in stocked-fingerlings was not primarily caused by synthetic-feed induced liver damage.

Acknowledgements

Mr. Joseph P. Cardeilhac provided technical assistance.

References

1.  Cardeilhac P, Dickson H, Stout R, Hardin S. 2004. A pelleted feed for the high-volume production of largemouth fingerlings free of pale liver syndrome. International Association for Aquatic Animal Medicine. 35 12-13.

2.  Porak W, Johnson W, Crawford S, Renfro D, Schoeb T, Stout R, Krause R, DeMauro B. 2002. Factors affecting survival of largemouth bass raised on Artificial diets and stocked into Florida lakes. American Fisheries Society Symposium. 31 649-665.

3.  Rao MS, Reddy JK. 2001. Peroxisomal beta-oxidation and steatohepatitis. Semin Liver Dis 21: 43-55.

4.  Nehra V, Angulo P, Buchman AL, Lindor KD. 2001. Nutritional and metabolic considerations in the etiology of non-alcoholic steatohepatitis. Dig Dis Sci 46: 5347-2352.

5.  Mitchell LA, Moran JH, Grant DF. 2002. Linoleic acid cis-epoxyoctadecenoeic acids and dihydroxy octadecenoic acids are toxic to Sf-21 cells in the absence of albumin. Toxicol Lett 126: 187-196.