Prevalence of Mycobacterium in wild and captive delta smelt

Infections associated with Mycobacterium were examined in wild end captive delta smelt, Hypomesus transpacificus, Mycobacterium was not detected from any of the fish examined immediately after collection from the Sacramento-S an Joaquin Estuary or during captivity of broodstock groups at water temper atures of 9-12 degreesC. However, Mycobacterium was Isolated from, and myco bacteriosis occurred in, broodstock held at 16 degreesC during spawning sea son and in experimental groups maintained at 17 degreesC. Mycobacterium and mycobacteriosis were more prevalent among groups frequently handled for ph ysiological experiments. Broodstock groups that were less stressed exhibite d a lower prevalence of the bacterium and the disease. These findings sugge st that Mycobacterium may be present In a latent state in the wild populati on of delta smelt and infections may progress from asymptomatic to clinical under intensive culture conditions. Two species of non-pigmented and rapid ly growing Mycobacterium (Runyon Group IV) were isolated from the liver, sp leen, and kidney tissues. The most abundant isolates were identified as Myc obacterium chelonae and a less dominant group was designated as Mycobacteri um sp., based on biochemical and growth properties and chromatography of li pids from representative strains. A disease control strategy that combined rearing of small Isolated groups at lowered water temperatures, minimal han dling, and periodic prophylactic treatments after handling minimized the im pact of Mycobacterium in delta smelt broodstock reared in captivity.