Prevalence of mycobacterium in wild and captive delta smelt

Abstract

Department of Animal ScienceUniversity of CaliforniaDavis, California 95616e-mail: [e-mail] associated with Mycobacterium were examined in wildand captive delta smelt, Hypomesus transpacificus. Mycobacterium wasnot detected from any of the fish examined immediately after collectionfrom the Sacramento-San Joaquin Estuary or during captivity ofbroodstock groups at water temperatures of 9-12°C. However,Mycobacterium was isolated from, and mycobacteriosis occurred in,broodstock held at 16°C during spawning season and in experimentalgroups maintained at 17°C. Mycobacterium and mycobacteriosis weremore prevalent among groups frequently handled for physiologicalexperiments. Broodstock groups that were less stressed exhibited alower prevalence of the bacterium and the disease. These findingssuggest that Mycobacterium may be present in a latent state in the wildpopulation of delta smelt and infections may progress fromasymptomatic to clinical under intensive culture conditions. Two speciesof non-pigmented and rapidly growing Mycobacterium (Runyon GroupIV) were isolated from the liver, spleen, and kidney tissues. The mostabundant isolates were identified as Mycobacterium chelonae and a lessdominant group was designated as Mycobacterium sp., based onbiochemical and growth properties and chromatography of lipids fromrepresentative strains. A disease control strategy that combined rearingof small isolated groups at lowered water temperatures, minimalhandling, and periodic prophylactic treatments after handling minimizedthe impact of Mycobacterium in delta smelt broodstock reared incaptivity.INTRODUCTIONThe abundance of delta smelt, Hypomesus transpacificus, an osmerid fish endemicto the Sacramento-San Joaquin Estuary, California, has declined (Sweetnam 1999).Habitat destruction or degradation associated with inadequate freshwater inflow,availability of food supply, invasion of exotic species, and other environmental factorshave contributed to this decline (Moyle et al. 1992). These factors are similar tothose that have affected striped bass in the estuary - water export, decreased freshwater