214 Ross F. NiGRELLI 



author has observed more than 150 species of enidosporidians 

 (myxo- and microsporidians) in more than 1,000 species of 

 fishes (see Walford, 1958). These parasites are often tissue- 

 and cell-specific and have been found in all the tissues and 

 organs, including the eyes, brain and heart. The parasites 

 produce a variety of lesions, the extent of which varies with 

 the species of parasite and degree of infection. They may 

 cause no more damage than the development of a simple 

 cyst, or they may cause acute and chronic diseases. For 

 example, some may produce cellulitis, cystitis, nephritis, 

 hepatitis, enteritis, pericarditis and endocarditis; others may 

 induce tumours of the infected organs and/or the surrounding 

 tissues, many of them bordering on true neoplasia; still others 

 may cause hyaline degeneration of muscle and other tissues. 



It is safe to say that all fish harbour one or more kinds of 

 parasites. The resistance of fish, or the rate at which they can 

 acclimatize to changes in the environment, appears to be 

 related to their parasitic load. Experience has shown that as 

 a rule a 10 per cent mortality can be expected when fish are 

 first netted or trapped randomly, another 10 per cent as the 

 result of handling and shipment, and 10 per cent more will 

 succumb in the first few weeks of life in the tanks. Such fish 

 tend to show a relatively high degree of parasitaemia, and 

 their ability to withstand shock is related to the intensity and 

 the site of infection or infestation, the rates being highest for 

 those fish in which infections are localized in the kidneys, 

 gills and skin, which are important organs of osmoregulation. 

 Selective methods of trapping, handling and shipping usually 

 result in higher survival values. For example, fish caught in 

 traps rather than in nets are less subject to trauma and can 

 be transferred to holding-pens with very little injury. Survival 

 rates are further increased if the fishes are starved for a period 

 of time and before shipping are transferred to waters with 

 slightly altered densities. Relatively young fish, as related to 

 potential age, are better risks than yearling or older (larger) 

 fish. 



