120 BULLETIN OF THE BUREAU OF FISHERIES. 



killing the fish, and he states that " in smaller lakes in the vicinity which are fed by 

 springs, the fish are comparatively free from such enemies." In such instances, 

 therefore, it is evident that the heat weakens the fish and renders it possible for the 

 parasite to kill it if the latter is present in sufficient numbers. 



After all, however, it is in restricted areas like artilical fish ponds, hatcheries, 

 aquaria, etc., that these parasites become most troublesome. Here every condition 

 deleterious to the fish is advantageous to the parasite. The artificial surroundings 

 make more or less of a drain upon the fish's vitality, and since the number of fish is 

 restricted, there is a resultant concentration of the evil effects produced by the 

 parasites. They are always assured of a suitable host and are enabled to choose the 

 strongest and healthiest fish, thereby lowering the general tone. If the breeding of 

 Arguli is once started in such a place it will rapidly assume dangerous proportions 

 unless checked at the very outset. This truth was clearly stated by the first observer 

 of the Arguli. Leonard Baldneur, in a manuscript dated 1666, deposited in the 

 public library at Strassburg, while speaking of what he calls the '"pou des 

 poissons" (.1. Jbliaceus), says that it is seldom found except upon trout, which it 

 frequently kills, especially if they are kept in ponds. The United States Fish 

 Commission has had several such experiences and is constantly on guard against 

 these Arguli. Sometimes it is necessary even to remove fish from the aquaria and 

 clean them, as was recently the case at Ann Arbor, Mich. The fish were Amia 

 calva and the parasites ArguVus americanus. 



It is chiefly to the natural enemies of the Arguli that we must look for a practical 

 solution of this important problem in fish economy. The author has elsewhere 

 recorded" some of these enemies discovered while investigating the cause of a serious 

 mortality among the fish in a small artificial pond at Warren, Mass. Here the death 

 was caused by a superabundance of ,1. catostomi. After much trouble the cause of 

 the increase in these parasites was found to he the removal of all the small surface 

 species, dace, roach, etc., which had been seined and sold for fish bait. These small 

 fish feed upon the- newly hatched Argulus larvae, and as soon as they were restored 

 to the pond the Arguli were reduced to their normal numbers and no further trouble 

 has been experienced from them. 



These facts have recently been confirmed by observations upon two species of 

 Finiihiliis at Woods Hole. In September, 1902, many specimens of A. fwndulus 

 were obtained from Fundulus majalis and F. Jieteroclitus, both in the salt water of 

 Woods Hole harbor and the almost fresh water near the head of Great Pond in 

 Falmouth. On leaving Woods Hole, the writer attempted to take away some of 

 the Arguli alive, for this purpose placing nine of the parasites upon two rugged 

 specimens of F. heteroclitus, which were taken in a large fruit jar to Westfield. 

 Mass. (12;"> miles), and placed in a salt-water aquarium. Everything progressed well 

 for about three weeks, hut not much food was given to the fish through fear of 

 contaminating the limited supply of salt water. Consequently they became quite 

 hungry, and one night deliberately <i/i all the parasites. 



From these observations we are enabled to draw the following conclusions: 



1. Under ordinary conditions it is not probable that the Arguli Occasion their 



-i Proceedings of the United States National Museum, vol. xxv, p. 651. 



