THE PARASITES OF LAKE FISHES 
A. S. Pearse 
Introduction 
Since the times of Van Beneden and Leuckilart animal parasites 
have been intensively studied, but little effort has been made to 
determine the amount or frequency of parasitic infection under 
natural conditions; except for those species directly related to 
man and his domestic animals. Many species of parasites that in¬ 
fest fishes have been, and are being, described, but few accurate 
observations that relate to their abundance and the factors which 
make them numerous or few have been made. 
Van Cleave (1919) found that half the species of fishes that he 
examined from Douglas Lake, Michigan, were infected with acan- 
thocephalans, and he determined the percentage of infection for 
sixteen species. La Rue (1914), Marshall and Gilbert (1905), 
Smallwood (1914), and Ward (1910) made incidental observations 
concerning the number of parasites present in certain fishes. 
Surber (1913) remarks on the small percentage of natural infec¬ 
tions with glochidia. The white crappie, which carries more species 
of glochidia than any other fish, he found to show an infection of 
only 0.7 per cent, and the sheepshead, known to carry two species 
of glochidia, had 3.7 per cent. 
Zschokke (1902) found that salmon lost a large number of their 
parasites while migrating up the Rhine. But Ward (1909) points 
out that such migrations are not always conducive to parasitic 
losses, for the Alaskan salmon during its journey inland acquires 
a copepod which is never found in salt water. 
Little is known of the effects of seasonal succession on the life 
cycles of the parasites of fishes. Van Cleve (1916) states that 
acanthocephalans vary greatly in this respect and cites two species 
in one genus which, though occurring in the same host, mature at 
different seasons. Hausmann (1897) found that perch had very 
few trematodes in the spring. In studying frogs. Ward (1909) 
found the lowest percentage of infection in late spring or early 
