Il6o BULLETIN OF THE BUREAU OF FISHERIES. 



type in contrast with the fasting Rhine fish." Parasites are also found in the 

 intestine behind the pylorus, where the Rhine salmon remains free from para- 

 sites. Among the intestinal parasites of the Baltic salmon also are included no 

 true limnetic species. Such only lie encapsulated in various organs. This indicates 

 that the infection with the true fresh-water parasites, CucuUanus, Tricenophorus , 

 Ascaris aculeati, actually occurs in the rivers. The Baltic salmon comes into 

 fresh water as richly laden with parasites as the fish caught in the lower stretches 

 of the Rhine in Holland. While the parasitic fauna of the Rhine salmon decreases 

 in proportion as it ascends the stream, that of the salmon in many other rivers 

 is enriched by numerous limnetic elements. The natural explanation lies in 

 the fasting of the Rhine salmon, whereas its relatives in other streams do not 

 cease taking food. The Baltic salmon, having returned to the ocean, loses the 

 limnetic parasites of the open intestine but retains those located in the closed 

 organs of the host. 



The material is too scanty to determine a seasonal distribution, if any 

 exists, and in fact the food of the Baltic salmon undergoes little change through- 

 out the entire year, so that no general modification would be expected in the 

 parasitic fauna, variations being merely of an individual or casual type. 



Upon a careful study of the individual species the parasitic fauna of the 

 Baltic salmon manifests a more varied aspect than that of its relative. There 

 are 2 pure marine forms, in contrast to 8 in the Rhine salmon, 2 pure limnetic 

 species as agairist not a single one in the other host, 6 parasites found in both 

 marine and fresh-water fishes, and 3 parasites found only in the Baltic salmon, 

 with a fourth which can not be assigned with certainty to either type of environ- 

 ment. It is very striking that the purely marine Tetrarhynchi so abundant in 

 the Rhine salmon have not yet been demonstrated in the Baltic fish. These 

 relations are indicated in the appended table of parasites from the European 

 salmon, collated from various authors. 



The Rhine salmon shelters a purely marine parasitic fauna, while the Baltic 

 salmon reckons many limnetic forms among its parasitic guests. This remark- 

 able condition finds its explanation in the continued feeding of the latter type, 



"■ One should not forget in estimating this factor as presented by Zschokke that in one important 

 respect conditions are not identical. The Baltic salmon are still in salt water; not until they enter 

 some estuary and begin the ascent of some river do they meet the fresh water environment to which 

 the Rhine salmon investigated by Zschokke are subject. To secure an exact parallel one should 

 compare the Baltic salmon with such of the Rhine variety as may be captured in the North Sea. 

 Zschokke refers in a later paper to some taken from this body of water and notes in their case also that 

 the average degree of infection with parasites is greater than in the case of those fish taken from the 

 Rhine stream itself. This fact only emphasizes the immediateness and definiteness of the effect on 

 the parasitic fauna of the salmon which is produced by the fresh water environment and abstinence 

 from food. 



