Bacterial gill disease plagues salmon and trout in many hatcheries: some infections are sporadic, but others are continual. An inexpensive, easily applied, stable, safe chemical would be highly advantageous for treatment.
The use of Roccal as a 1-hour treatment for bacterial gill disease (Fish 1947) was developed at the Leavenworth (Washington) Station of the Fish and Wildlife Service in 1942 and was quite successful. Since then, Roccal has been used extensively; but because of variability in composition, its efficacy is not consistent (Rucker et al. 1949). The objection to the variability of Roccal was overcome by using another compound, pyridylmercuric acetate, which was suggested by Van Horn and Katz (1946) as having some therapeutic therapy. Pyridylmercuric acetate was tested experimentally at the Leavenworth Station and was found to be very effective for bacterial gill disease. This compound had highly differential toxicities for bacteria and fish but was quite expensive (Rucker 1948, Burrows and Palmer 1949, Snieszko 1949). Another objection to pyridylmercuric acedate was its toxicity to rainbow trout—not to other species of trout or to salmon—at the concentration necessary to control the bacteria (Seaman 1950, Rodgers et al. 1951, Bryant 1951, Foster and Olson 1951).
