Fish Hatchery Management - fisheries & aquaculture

FISH HATCHERY MANAC;EMEN'I'COI,UMNARIS DISEASEThe causative agent of columnaris disease historically has been namedChondrococcus columnaris, or Cytophaga columnaris, but now is classified asFlexibacter columnaris in Bergey's Manual of Determinatiue Bacteriology. Theagents are long, thin, gram-negative bacteria that move in a creeping orflexing action, and that have a peculiar habit of stacking up to form distinctivecolumns, hence the name "columnaris."Columnaris most commonly involves external infections but can occur asan internal systemic infection with no visible external signs. Externally, thedisease starts as small, grayish lesions anywhere on the body or fins; mostcommonly the the lesions occur around the dorsal fin or on the belly. The lesionsrapidly increase in size and become irregular in shape. As the lesionsget larger, the underlying musculature can be exposed. The margins of thelesions, and occasionally the centers, may have a yellowish color due tolarge aggregations of the bacteria. Frequently, lesions may be restricted tothe head or mouth. In Pacific salmon and warmwater fish, particularly catfish,lesions may be confined to the gills. Lesions on the gills are characterizedby yellowish-brown necrotic tissue beginning at the tip of the filamentsand progressing toward the base.Columnaris disease usually is associated with some kind of stress conditionsuch as high water temperature, low oxygen concentration, crowding,and handling. Under appropriate conditions, columnaris may take an explosivecourse and cause catastrophic losses in I or 2 days after the first appearanceof the disease. Therefore, it is incumbent upon the fish culturistto maintain the best possible environmental conditions for the fish and tominimize any stress conditions.Although columnaris disease attacks practically all species of freshwaterfish, catfish are particularly susceptible. In warmwater fish, most outbreaksof columnaris occur when the water temperature is above 68'F, but thedisease can occur at any time of the year. Columnaris disease is common insalmonids held at water temperatures above 59"F. Progress of the diseaseusually is faster at the higher temperatures.Flexibacteria are common inhabitants of soil and water. They commonlyare found on the surface of fishes, particularly on the gills. The stress ofcrowding, handling, spawning, or holding fish at above-normal temperatures,as well as the stress of external injury, facilitates the transmissionand eruption of columnaris disease.Presumptive diagnosis of columnaris is accomplished best by microscopicexamination of wet mounts of scrapings from lesions and detection of manylong slender bacteria (0.5 x l0 micrometers) that move by flexrng orcreeping movements and form "haystacks" or "columns."Preventative measures include maintenance of optimum water temperaturesfor salmonids, reduced handling during warm weather, maintenance