Multidisciplinary Scientific Approaches to Improve Survival of Stocked Hatchery Largemouth Bass

Survival was estimated to be 3% or less in most Florida lakes where advanced sizes (67-335 mm TL) of pellet-reared largemouth bass were released during the first few years of our stock enhancement project.  Multidisciplinary approaches are being used in an attempt to increase post-release survival of these hatchery fish.  Handling and release practices were modified after variable stocking mortality (0-85%) was observed in test cages in stocked lakes.  Hatchery fish are routinely screened for pathogens (e.g., Flavobacterium columnare) prior to stocking to insure that healthy fish are being released.  Pathology indicated fatty liver syndrome and formation of lymphocytes and granulomas in the liver, which raised concerns about the use of artificial diets.  These commercial diets had originally been formulated for cold water species.  The biochemical composition of largemouth bass eggs and warm water prey species was analyzed to help develop the proper nutrition in hatchery feeds that included the appropriate amounts and composition of digestible fatty acids and proteins.  Pellet-fed hatchery largemouth bass had difficulty transitioning from artificial feed to live prey in the wild.  Feeding experiments indicated that behavior can be modified and hatchery fish are now transitioned to live prey at the hatchery.  Out-of-season spawning procedures are being developed to stock a larger age-0 hatchery bass in the spring that is capable of feeding immediately on prey fish.  Additional studies are being conducted to reduce domestication affects on feeding and antipredatory skills of hatchery largemouth bass.  Mitochondrial DNA, allozyme, and microsatellite genotypes were used to resolve genetic structure among 48 populations of largemouth bass throughout the state.  Research results allowed managers to incorporate four genetic management units into stocking protocol to prohibit translocations between regions.  Microsatellite genetic markers were later developed to differentiate hatchery and wild fish in stocked lakes for post-release evaluations.  Telemetry studies indicated that 47% of 30 radio tagged hatchery largemouth bass (mean = 106 mm TL) were due to predation; no predation was observed for 20 tagged wild bass (mean = 111 mm TL).