Prevalence of mycobacterium in wild and captive delta smelt

Infections associated with Mycobacterium were examined in wild and captive delta smelt, Hypomesus transpacificus. Mycobacterium was not detected from any of the fish examined immediately after collection from the Sacramento-San Joaquin Estuary or during captivity of broodstock 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 experimental groups maintained at 17°C. Mycobacterium and mycobacteriosis were more prevalent among groups frequently handled for physiological experiments. Broodstock groups that were less stressed exhibited a lower prevalence of the bacterium and the disease. These findings suggest that Mycobacterium may be present in a latent state in the wild population of delta smelt and infections may progress from asymptomatic to clinical under intensive culture conditions. Two species of non-pigmented and rapidly growing Mycobacterium (Runyon Group IV) were isolated from the liver, spleen, and kidney tissues. The most abundant isolates were identified as Mycobacterium chelonae and a less dominant group was designated as Mycobacterium sp., based on biochemical and growth properties and chromatography of lipids from representative strains. A disease control strategy that combined rearing of small isolated groups at lowered water temperatures, minimal handling, and periodic prophylactic treatments after handling minimized the impact of Mycobacterium in delta smelt broodstock reared in captivity.