48 NORTHERN FISHES 



much damage if sufficiently abundant, leaving the fish emaciated and 

 liable to fungus infection. 



The larvae or glochidia of fresh water mussels are common skin 

 parasites of fishes. Before these larval forms can develop into adult 

 mussels they must pass through a stage as a fish parasite. The glochidia 

 of some species of mussels have toothed valves and bury themselves 

 in the skin, particularly that of the fins. Glochidia of other species have 

 only small toothless valves and these forms bury themselves in the 

 soft tissue of the gills. Glochidia remain on the host several weeks or 

 longer, depending on the temperature. There is some question as to 

 whether they do much actual damage to the fish. 



There are several species of microscopic protozoans that sometimes 

 infect the skin and gills of fishes. Some of these are large enough to 

 see with a magnifj'ing glass. They cause small spots and pustules to 

 appear on the skin and eventually cause death. They may spread from 

 fish to fish and cause an epidemic. Minute flukes belonging to the flat- 

 worms may also infect the gills and skin of fishes. Some of these cause 

 serious losses in hatcheries. 



A common white fungus (Saprolegriia) may attack the skin of fishes 

 wherever the protective mucous coat is broken by careless handling or 

 by wounds. This fungus is present in most warm waters and lives 

 readily on dead animal matter. It grows rapidly in warm water, but 

 does not thrive in cold water; hence it is most prevalent in summer. 

 For some reason fishes in natural open waters are usually not readily 

 infected with this fungus unless in a weakened condition. However, 

 fishes in confinement are highly susceptible to it. The fungus first ap- 

 pears as a white fuzzy patch which spreads rapidly and unless checked 

 usually proves fatal. The most common remedy is a bath for about 

 two minutes or even longer in a 3 per cent salt solution. Fishes which 

 are already in bad shape may die under the treatment, but they would 

 probably die anyway. 



Bacteria may cause fin rot, gill disease, furunculosis or boils, and 

 various other diseases. They are usuafly more prevalent on fishes con- 

 fined in hatcheries or aquaria than on those in natural waters. Davis 

 (1937) describes many of these parasites in detail and cites the known 

 treatments. 



Threadworms or roundworms {Neniatoda) are very common in- 

 ternal parasites of fishes. They are small slender worms, ranging from 

 microscopic size to several inches or more. Frequently, they infect 

 fishes to such an extent as to kill them or to render them thin and 

 emaciated and susceptible to other infections. These worms can be 

 distinguished from other worms of fish by their shape and their lack 

 of attachment spines. They usually occur in the digestive tract, but 

 may be found in the body cavity and in cysts imbedded in the flesh 



