Abstract

The largemouth bass, Micropterus salmoides, is a fish native to the Southeastern and Midwestern United States and also Northwestern Mexico. It is the most popular sport fish in the country, and because of extensive popularity and preference, there is a great demand that allows a selling price well above many other cultured fish. The efficient commercial culture of this species is of great importance to the state of Florida; therefore, there is a need for a good restocking method. It is advisable to use largemouth bass fingerlings greater than 4 inches in length for release when stocking Florida lakes in order to have a good survival rate. Rearing largemouth bass to this size requires the use of pelleted feeds to produce adequate numbers and at the same time be cost effective. The use of commercial pellets in artificial feeding has been successful in increasing production volume, but a significant percentage of the pellet-trained bass develop abnormal livers. The most distinguishing signs of abnormal livers are pale color and the presence of nodules; thus, the condition was termed Pale Liver Syndrome. This syndrome may be at least partially responsible for poor hatching survival rate in stock enhancement projects. We believe that diet is a major cause of Pale Liver Syndrome. In order to test this hypothesis, feed pellets that had various concentrations and ratios of protein, fat and carbohydrate were designed and produced. Pellets produced in 2001 (SemPro) contained quantities of energy that were well below those usually used in carnivorous fish diets. These pellets were designed to be safe, inexpensive, float, and slowly sink. A commercial pellet (BioDiet, Bioproducts Inc. Warrenton OR) was used as the control for purposes of comparison. In 2002, another pellet was formulated which was similar to the 2001 pellet, but the energy content of the 2002 pellet was elevated to approximate levels found in hybrid striped bass diets. Along with the BioDiet control group, a third group was fed the BioDiet pellet plus 10% fish oil. A fourth group was fed the 2002 pellet plus 5% vegetable oil and 5% fish oil. In 2003, a pellet (Diet-4) was formulated that contained energy levels similar to 2002, but with most of the protein, fat and carbohydrates that came from plant sources removed. Fingerling culture and feed trials were performed at the Richloam Hatchery, Webster, Florida, while feed analysis was performed at ABC Research, Gainesville, Florida and histology performed at the College of Veterinary Medicine, University of Florida. General health indicators such as growth rate, body condition, relative liver size, appearance and histology were determined. All groups tested had some fingerlings with pale, fatty livers and nodules. The energy component of the diet appeared to correlate with increases in abnormalities seen in the liver tissue. The oil component of the diet was associated with the incidence of pale livers. Placing test fish on a natural diet of minnows following the feeding trials appeared to reduce fatty livers but did not eliminate liver nodules.

Acknowledgments

Mr. Eric Polk, Mr. Greg Vermeer and Mr. Joseph P. Cardeilhac provided technical assistance.