REPORT OF COMMISSIONER OF FISH AND FISHERIES. 139 



Mr. Cokor iuvcsti<>"ated a ))ariuu'lc (Dickclasjris) pai'a.sitic on the 

 gills of the common cdiblo oral). From a report submitted by Mr. 

 Coker it appears that this parasite affects over 50 per cent of the male 

 crabs and about 90 per cent of the females; that it is not found in 

 3'ouno- crabs, being thrown off by the frequent molting; that crabs 

 whose gills are heavily burdened with the parasite have less vitality, 

 are sluggish in their movements, and are the first to die in captivity. 

 While the usual number of barnacles found in one crab is from 2 or 3 

 to 8 or 10, in some the gills are tilled to overflowing and may contain 

 500 to 1,000 of the parasites. 



The new laboratory buildings on Fivers Island were nearly com- 

 pleted by the end of the year, and on May 26 it was practicable to 

 throw the laboratory proper open to investigators. Prof. H. V. Wil- 

 son, the director, having gone abroad. Dr. Caswell Grave, of Johns 

 Hopkins University, was appointed to the position. The operations 

 of the laboratory during the last few weeks of the tiscal year 1902 will 

 be referred to in the report for the next year. 



WORK IN FISH PATHOLOGY. 



The occurrence of serious disease among fishes at the hatcheries of 

 the Commission and elsewhere has required the almost constant atten- 

 tion of Mr. M. C. Marsh, the assistant assigned to this subject, and 

 has shown the wisdom of making special provision for the study of 

 this increasingly important branch. 



A part of the summer and fall was spent by Mr. Marsh at the North- 

 ville (Michigan) station of the Commission in considering the disease 

 aflecting the brook trout, A bacterial organism was isolated from 

 the dying fish and the disease was reproduced in healthy trout by 

 inoculation. On the recommendation of this division, two ponds were 

 constructed entirely of concrete and cement for the purpose of exclud- 

 ing disease-producing Ijacteria, and the ponds were stocked with 

 healthy trout from the Au Sable River and from a private trout farm 

 at Osceola, Wis. Mr. Marsh visited this farm to inspect the fry and 

 yearling fish prior to securing a supply for Northville. The large 

 spring pond constituting the main water supply was drawn down, 

 cleaned, and thoroughly disinfected with chlorinated lime. Patholog- 

 ical material and cultures were brought to the Washington laboratory, 

 and a studj^ of the the ofi'ending organism was taken up. This genu 

 can not be identified with any hitherto-known species, and a full 

 description of its form and behavior is substantially completed. 



At the meeting of the American Fisheries Society, held at Milwau- 

 kee, Wis., in July, Mr. Marsh l)rought the brook-trout disease to the 

 attention of the assembled fish-culturists. 



An investigation of the mortality among brook trout at the Paris 

 station of the Micliigan Fish Commission disclosed the same disease 

 as at Northville, l)ut in a milder form. Mr. Marsh visited by request 



