286 FISH HAICHERV MANAGEMENT 



cleanliness. All shipped-in equipment should be decontaminated thor- 

 oughly before it is placed in contact with clean hatchery equipment and 

 water. The liberal use of warm water and soap is recommended. All trucks 

 and equipment should be decontaminated before they enter the hatchery. 

 The drivers and helpers should not be allowed to assist in loading fish. A 

 "KEEP IT CLEAN" motto should be adopted and hatchery staff 

 impressed with the idea that one slip-up in cleanliness may nullify all pre- 

 vious efforts. 



Defense Mechanisms of Fishes 



As with all living organisms, fish stay healthy only if they prevent exces- 

 sive growth of micro-organisms on their external surfaces and invasion of 

 their tissues by pathogenic agents. Invasion is inhibited by tissues that pro- 

 vide a physical barrier and by natural or acquired internal defense mechan- 

 isms. 



Physical barriers are important, but give variable degrees of protection. 

 Fish eggs are protected by the structurally tough and chemically resistant 

 chorion. However, during oogenesis the egg may become infected or con- 

 taminated with viruses and bacteria living in the female. Once hatched, the 

 delicate fry again are vulnerable to invasion. 



Fishes are protected from injury and invasion of disease agents by the 

 external barriers of mucus, scales, and skin. For example, the skin of sal- 

 mon protects against fungi by continuously producing and sloughing off 

 mucus, which allows fungi only temporary residence on the host. Mucus 

 also may contain nonspecific antimicrobial substances, such as lysozyme, 

 specific antibacterial antibodies, and complement-like factors. 



Gill tissue contains mucus cells that can serve the same purpose as those 

 in the skin. However, irritants may cause accumulation of mucus on the 

 gill tissue and lead to asphyxiation. This is an example of a defense 

 mechanism that can work against the host. 



Internal defenses of the fish can be divided into natural nonspecific 

 defenses and induced defenses. Induced defenses can be either specific or 

 nonspecific. One of the primary natural defense mechanisms is the inflam- 

 matory response of the vascular (blood) system. Defense agents in capillary 

 blood respond to invasion of pathogenic agents and other irritants. Dila- 

 tion of capillaries increases the supply of humoral and cellular agents at 

 the focus of infection. The inflammatory response proceeds to dilute, local- 

 ize, destroy, remove, or replace the agent that stimulated the response. 

 Fish, like most animals, have an important defense mechanism in the form 

 of fixed and wandering phagocytes in the lymphatic and circulatory sys- 

 tems. Phagocytes are cells capable of ingesting bacteria, foreign particles, 

 and other cells. Fish also have natural, noninduced humoral defenses 



