HARDWICKE'S SCIENCE-GOSSIP. 



129 



THE SALMON DISEASE AND ITS CAUSE. 



By M. H. Robson, Hon. Sec. of the North of 

 England Microscopical Society. 



SOME analogy appears to exist between the spread 

 of fungoid disease, at present so destructive to 

 salmon life, and the seemingly erratic dissemination 

 of ordinary zymotic disorders, which in their origin 

 may probably be all referable to violations of natural 

 laws. 



Unlike, however, the still more subtile characters 

 •of many epidemics affecting the human family, or 

 even rinderpest amongst cattle, the vegetable parasite 

 Saprolegnia fa-ox, at present developing itself 

 with such unprecedented rapidity and mischievous 

 results upon fish in English rivers, is of sufficient 

 magnitude to be dealt with by the ordinary powers of 

 the microscope — and its progress may be observed 

 from the germination of the infusorial spore (fig. 78), 

 throughout with a Jth or ^th objective. 



The causes ascribed to account for this hitherto un- 

 known outbreak amongst salmon are various, but 

 mostly pointing to river pollution. They, however, so 

 far as I am aware, do not indicate any considerable 

 change of condition or circumstance to which the fish 

 have been subject of late years. 



The proximate cause of this disease, and why its 

 ravages should extend so rapidly at this time, is a 

 question of sufficient interest and importance to arrest 

 the attention of all interested in the preservation of 

 our noblest fish and one of the most important sources 

 of food supply. Not that the fungus restricts its 

 attacks to salmon, for many freshwater fish are 

 destroyed by it, and even newts, tadpoles, and fresh- 

 water Mollusca are sometimes attacked ; this is well 

 known to those who have aquaria, as, despite all 

 ordinary care, it sometimes happens that the finny 

 favourites become fluffy and mouldy ; the fungoid pest 

 irritates and destroys the skin, until, seizing upon the 

 gills, the tortured animal can resist the attack no 

 longer — and is soon found floating dead — the fungus 

 developing rapidly over it in woolly tufts. 



Amongst salmon in rivers this pest first appeared 

 so recently as the spring of 1878, in the Carlisle Eden, 

 the Annan, the Mitt and the Lancaster Lune, 

 where large numbers of spawned fish (kelts) and also 

 some salmon smelts and trout were found in pools, 

 and floating down the stream dead or dying. Nearly 

 twenty years ago those engaged in developing the ova 

 of trout, salmon and char, found these attacked by a 

 vegetable parasite which effectually destroyed their 

 vitality ; this was also to a considerable extent cultiva- 

 ting and disseminating the fungus itself. Mr. F. 

 Buckland thus describes the appearance of salmon 

 killed by this disease: "they are all more or less 

 covered with patches of fungus generally circular in 

 form. The tail is nearly always affected, and often to 

 such an extent that the soft parts are eaten away and 



the bony rays left quite uncovered. A bunch of fungus 

 is found generally sitting on the head and nose, and 

 hence the diseased fish in the Eden are called ' salmon 

 with white nightcaps.' " 



Many years ago the Rev. M. J. Berkeley instanced 

 the genera Achlya, Saprolegnia, Pythia, and Aphano- 

 myces as "notoriously antagonistic to animals, es- 

 pecially those of aquatic habits in a low stage of 



Fig. 78. — Salmon-Disease Fungus (Saprolegnia fcrox). 



References to Sketches : 1, Group of threads with Sporangia in 

 different stages of growth ; 2, Formation of Second Sporan- 

 gium ; 3, Infusorial Spores; 4, Same in germination; 5, 

 Oogonia in various stages ; 6, Portion of wall to show the 

 apertures. From drawings by the Rev. M. J. Berkeley, after 

 Pringsheim. 



vitality ; " of these the genus Saprolegnia appears to 

 be most inimical to salmon life. On account of their 

 aquatic habits, and also from the fact that they emit 

 active flagellated spores from the clavate tips of their 

 threads, which swim freely like Infusoria, they were 



