Trout and Fish Lice 



Nathan Fasten, 

 University of Washington, Seattle 



Trout are attacked by a great many parasites. Those often met with 

 in our streams and hatchery ponds are commonly known as fish-lice, and 

 technically as copepods. These parasites often become so serious a pest 

 in hatchery ponds that they kill thousands of fish. A knowledge of the 

 life history of these animals is therefore not only desirable, but highly 

 important from the standpoint of fish culture. 



The fish-lice which attack the trout belong to the family I.ernaeo- 

 podidae, the most highly specialized group of copepods. They spend 

 nearly their entire life on the host, attacking such delicate structures as 

 the gills (Figs. 1 and 2), fins (Fig. 5), and the membranes of the mouth 

 (Fig. 3). They have a particular fondness for regions which are richly 

 supplied with blood vessels, for the blood is their sole food. The adult 

 females (Fig. 4) are found in greatest abundance on the fish and may be 

 easily recognized. They are quite large, yellowish-white, and a few milli- 

 metres in length ; they possess two cigar-shaped egg-sacs which dangle 

 from the posterior end of the body. Within these egg-sacs the embryos 

 mature and complete their life-cycle. 



For a number of years the writer (Fasten, 1912-1916) has concerned 

 himself with a fish-louse which attacks the brook-trout (Salvelinus fon- 

 tinalis) found so commonly in the middle western states. Technically this 

 parasite goes under the name of Salmincola edtcardsii Olsson, and it has 

 proved a serious stumbling block to trout culturists through many regions 

 of the United States. In our own state of Washington the author has dis- 

 covered a similar copepod (Salmincola falculata Wilson), which infests 

 the steelhead trout (Figs. 3-5). This fish louse has been previously 

 recorded by Wilson (1908. 1915) on the salmon of Washington and the 

 trout of California, but was not known to exist on the trout of this state. 

 The copepod was observed on some Washington steelhead trout (Salmo 

 gairdneri) from Lake Samish, which were on exhibition in one of the 

 aquaria in the state hatchery building at Seattle during the fall of 1914. 

 It has since been seen on steelhead trout exhibited in various restaurants 

 in Seattle. From all appearances the life histories of Salmincola edwardsii 

 and Salmincola falculata are similar and will be briefly sketched. 



The embryos found within the egg-sacs of the adult females hatch 

 into perfect larvae, which are capable of swimming about freely in the 

 water. These larvae are minute, about l/35th of an inch in length, and 



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