Fish Hatchery Management - fisheries & aquaculture

306 FISH HATCHERY MANAGEMENTreservoirs of infection. Furunculosis may break out in virtually any freshwaterfish population, including warmwater species, if conditions such ashigh temperature and low dissolved oxygen favor the pathogen.Among the eastern salmonids, brook trout are the most susceptible to infection,brown trout are intermediate, and rainbow trout are least susceptible.Atlantic salmon also are susceptible. Furunculosis has been reported inmost of the western salmonids. In addition to salmonids, the disease hasbeen reported in many other fishes, including sea lamprey, yellow perch,common carp, catfish, northern pike, sculpins, goldfish, whitefish, and variousaquarium fishes.Sanitation provides the most important long-range control of furunculosis.If a population of trout at a hatchery is free of furunculosis and if thewater supply does not contain fish that harbor the pathogen, strict sanitationmeasures should be used to prevent the introduction of the disease viaincoming eggs or fish. Eggs received at a hatchery should be disinfectedupon arrival. Iodophors used as recommended are not toxic to eyed eggsbut are highly toxic to fry.Maintenance of favorable environmental conditions for the fish is ofprime importance in preventing furunculosis outbreaks. Proper water temperatures,adequate dissolved oxygen, efficient waste removal, andavoidance of overcrowding must be observed. In areas where the disease isendemic, strains of trout resistant to furunculosis are recommended. However,regardless of the trout strain involved, acute outbreaks of furunculosishave occurred when conditions favored the disease.Sulfamerazine (10 grams per 100 pounds of fish per day) in the diet hasbeen the standard treatment of furunculosis for years. In recent years, becauseof sulfa-resistant strains of A. salmonicida, Terramycin (3.6 gramsTM-50 or TM-50D per 100 pounds of fish per day for l0 days) has becomethe drug of choice. Furazolidone (not registered by the Food andDrug Administration) has been used successfully under experimental conditionsagainst resistant isolates of the bacterium. Furox 50 (also not registered)at 5 grams active ingredient per 100 pounds fish per day has beenused successfully under production conditions with Pacific salmon. Drugsare effective only in the treatment of outbreaks. Recurrences of furunculosisare likely as long as A. salmonicida is present in the hatchery systemand environmental conditions are suitable.ENTERIC REDMOUTH (ERM)Enteric Redmouth disease refers to an infection of trout caused by an entericbacterium, Tersinia ruckeri. Initially, the disease was called Redmouth; laterthe name Hagerman redmouth disease (HnU) was used to differentiatebetween infections caused bv Tcrsinia and those caused by the bacterium