1 158 BULLETIN OF THE BUREAU OF FISHERIES. 



In not a single case was a parasite found in the alimentary canal below 

 the pyloric coeca. Indeed, parasites which in other hosts inhabit only the 

 intestine, were found in the Rhine salmon to infest stomach and oesophagus, 

 as if better protected there than in the vicinity of the anus. The Rhine salmon 

 loses its intestinal guests like any fasting fish, and through the exclusion of 

 food any new importation of worms is prevented. From the absence of para- 

 sites behind the pyloric coeca one may conclude indirectly that Salmo salar 

 really fasts in the Rhine. When this species enters the river it is richly laden 

 with parasites. It loses its intestinal guests and these are not replaced by 

 any new supply. There remain only the natural inhabitants of the anterior 

 regions of the canal and those which can withdraw thither. Even these pro- 

 tected species diminish in number of species and individuals as the salmon 

 remains longer in fresh water and climbs higher in the stream, until finally 

 there are left only encapsulated forms. The journey up the Rhine has proved 

 at the same time a means of eliminating the intestinal parasites. Some investi- 

 gators, although without knowledge of these facts, have yet endeavored to 

 explain the migration of many fish as due to the necessity of freeing them- 

 selves from parasites acquired in the ocean. 



Salmon caught in Holland, in the lower reaches of the Rhine, are richly 

 infested with parasites. Several species were regularly found in large numbers 

 and the parasitic fauna recalls strikingly that of the ocean salmon. Distomum 

 varicum was very abundant in the oesophagus and Bothriocephalus infundi- 

 buliformis in the pyloric coeca. But fish from the upper reaches of the Rhine 

 presented a radically different picture. The parasites in oesophagus and 

 stomach were very rare. Distomum varicum had disappeared and Bothrio- 

 cephalus injundibuliformis appeared only as single, weak, emaciated specimens. 

 Often the entire alimentary tract yielded no trace of a parasite. 



The parasitic fauna of the Rhine salmon decreases in proportion as the fish 

 ascends the stream. 



A study of the seasonal distribution of parasites in the Rhine salmon 

 evidences that the number of species present becomes reduced in the winter 

 months, and the number of individuals also falls off markedly. The minimum 

 is reached in November and December, the months of spawning, when the fish 

 has penetrated farthest upstream. It has lost its unbidden guests on the long 

 journey. The maximum of parasitic infection is found in the summer months, 

 May to July, when the schools of salmon enter the river. Naturally the journey 

 exerts no influence upon those parasites which inhabit closed organs. 



The question next considered is the origin of the salmon's parasites, whether 

 marine, limnetic, or indifferent in character. The analysis of the forms recorded 

 indicates that the Rhine salmon does not exhibit a single true limnetic parasite. 



