HATCHERY OPERATIONS 89 



Most fish diseases are water-borne and are readily transferred from one 

 rearing unit to another by equipment such as brushes, seines, and dip nets. 

 All equipment used in handling and moving fish can be easily sanitized by 

 dipping and rinsing it in a disinfectant such as Roccal, Hyamine, or so- 

 dium hypochlorite. Solutions of these chemicals can be placed in containers 

 at various locations around the hatchery. Separate equipment should be 

 provided for handling small fish in the hatchery building and should not 

 be used with larger fish in the outside rearing units. Detailed procedures 

 for decontaminating hatchery facilities and equipment are presented in 

 Chapter 5. 



Dead and dying fish are a potential source of disease organisms and 

 should be removed daily. Empty rearing units should be cleaned and 

 treated with a strong solution of disinfectant and then flushed before being 

 restocked. Direct sunshine and drying also can help sanitize rearing units. 

 If possible, ponds and raceways should be allowed to air-dry in the sun for 

 several weeks before they are restocked. To prevent long-term buildup of 

 organic matter, ponds typically are dried and left fallow for two to five 

 months after each harvest. Many times, the pond bottoms are disked, 

 allowing the organic matter to be oxidized more quickly. After the pond 

 soil has been sun-baked, remaining organic material will not be released 

 easily when the pond is reflooded. 



Disinfection of warmwater fish ponds is a process by which one or more 

 undesirable forms of plant and animal life are eliminated from the environ- 

 ment. It may be desirable for several reasons: disease control; elimination 

 of animal competitors; destruction of aquatic weeds, among others. Disin- 

 fection may be either partial or complete, according to the degree to which 

 all life is eliminated. It is impractical, if not impossible, to achieve com- 

 plete disinfection of eathern ponds. 



Disinfection of ponds with lime is a common practice, especially in Eu- 

 rope. This is particularly useful for killing fish parasites and their inter- 

 mediate hosts (mainly snails), although it will also destroy insects, other in- 

 vertebrates, and shallow rooted water plants for a few weeks. Calcium ox- 

 ide or calcium hydroxide are recommended; the latter is easier to obtain 

 and less caustic. Lime may be applied either to a full or dewatered pond 

 (so long as the bottom is wet); in either case, the lime penetrates the pond 

 soil less than an inch. It is most important that the lime be applied evenly 

 across the pond, and mechanized application is better for this than manual 

 distribution. Except for the smallest ponds, equipment for applying lime 

 must be floated. This means that at least some water must be in the ponds, 

 even though lime is most effective when spread over dewatered soils. 



Lime makes water alkaline. If the pH is raised above 10, much aquatic 

 life will be killed; above 11, nearly all of it. Application rates of 1,000 to 

 2,500 pounds of lime per acre will achieve such high pH values. Appropri- 

 ate rates within this range depend on the water chemistry of particular 



