480 FISH HATCHERY MANAGEMENT 



Esophagus The gullet; a muscular, membranous tube between the phar- 

 ynx and the stomach. 



Essential Amino Acids Those amino acids that must be supplied by 

 the diet and cannot be synthesized within the body. 



Essential Fatty Acid A fatty acid that must be supplied by the diet. 



Estuary Water mass where fresh water and sea water mix. 



Etiologic Agent See Disease Agent. 



Etiology The study of the causes of a disease, both direct and predispos- 

 ing, and the mode of their operation; not synonymous with cause or 

 pathogenesis of disease, but often used to mean pathogenesis. 



European Gill Rot See Branchiomycosis. 



Excretion The process of getting rid or throwing off metabolic waste 

 products by an organism. 



Exophthalmos Abnormal protrusion of the eyeball from the orbit. 



Exoskeleton The hard parts on the exterior surfaces, such as scales, 

 scutes, and bony plates. 



Extended Aeration System A modification of the activated-sludge pro- 

 cess in which the retention time is longer than in the conventional 

 process. 



Extensive Culture Rearing of fish in ponds with low water exchange 

 and at low densities; the fish utilize primarily natural foods. 



Eyed Egg The embryo stage at which pigmentation of the eyes becomes 

 visible through the egg shell. 



F| The first generation of a cross. 



F7 The second filial generation obtained by random crossing of Fj indi- 

 viduals. 



Fat An ester composed of fatty acid(s) and glycerol. 



Fatty Acid Organic acid present in lipids, varying in carbon content 

 from 2 to 34 atoms (C2-C34). 



Fauna The animals inhabiting any region, taken collectively. 



Fecundity Number of eggs in a female spawner. 



Feeding Level The amount of feed offered to fish over a unit time, usu- 

 ally given as percent of fish body weight per day. 



Fertility Ability to produce viable offspring. 



Fertilization (l) The union of sperm and egg; (2) addition of nutrients 

 to a pond to stimulate natural food production. 



Fin Ray One of the cartilaginous rods that support the membranes of 

 the fin. 



Fin Rot Disease A chronic, necrotic disease of the fins caused by inva- 

 sion of a myxobacterium into the fin tissue of an unhealthy fish. 



Fingerling The stage in a fish's life between 1 inch and the length at 1 

 year of age. 



