110 SALMONID^ OF BRITAIN. 



be a cause of death, among these fishes; thus, on July 3rd, 1866, several salmon in 

 Scotland "were reported to have heen killed by lightning during the intensely hot 

 weather which was then prevalent. Also thunderstorms may cause a rush of 

 waters from roads, lanes and sewers, sweeping with it every kind of filth, without a 

 sufficient flood in the river to carry it ofE. Dynamite employed for blowing up the 

 remains of the Tay bridge destroyed fishes as far as a couple of miles away. 



(3.) Diseases may directly attack these fish whether developing in the eggs, in 

 the stage of infancy, or as they mature, while some may be badly developed 

 occasioning monstrosities, or malformed consequent upon accident (p. 41 ante). 

 Space, however, will not permit an, account of the numerous affections to 

 which fishes are subject, especially the young (see p. 46), from fungus to 

 the many diseases of a contagious or non-contagious character. There are certain 

 animal parasites which aSect fish that may be chiefly divided into such as are 

 internal or entozoa, and others that are external or epizoa, also infusoria and 

 parasitic fungi. Entozoa* are very common, and one form (Bothrioeephalus) is a 

 common resident in the pyloric csBca of salmon : in fact, tapeworms would seem to 

 be numerous, but most of these entozoa undergo transformation after changing 

 their abode the final most frequently occurring in a water bird. Epizoa are 

 likewise seen as small crustaceans, and those which are of most consequence to the 

 salmon are of two classes ; first, such as are more essentially surface forms, as fish 

 lice, that are able to move from place to place by means of their hooked and 

 prehensile antennae, or even leave the fish and swim freely in the water. A 

 parasite of this class, Lepeo^tlieirus stromii, Baird, belonging to the family Cali- 

 gulidse, is found upon salmon when first arriving from the sea,t for they will only 

 live in salt water ; while by means of their foot jaws they adhere to the body, on 

 which they are able to move about from one locality to another. They are most 

 commonly found in the gUl-cavity or inside the mouth, and they generally die 

 soon after being removed from the water. Miiller and Pabricius both supposed 

 that these parasites lived upon the mucus secreted from the fish. The second 

 class of these epizoa are more sedentary forms, belonging to the family Lerneo- 

 podadse, and which frequently have their heads imbedded in their victims' bodies. 

 In fresb waters the gills, more especially of kelts, become attacked by a parasite, 

 Lerneopoda salmonea, Linn., belonging to this family. Some imagine that it 

 chiefly fixes itself to the SahnonidcB prior to spawning, while others believe that it 

 mainly does so to kelts returning seawards : it dies from immersion in salt water. 



Parasitic fungi may also occur, and of late years one of a contagioxts and 

 destructive character has been very virulent in our fresb waters, and epidemic 

 since 1878. This is the Saprolegnia ferax, which has probably always been present J 

 but requires a soil suitable for its germination and growth, and although some of 

 the following are in certain places predisposing causes to this fatal complaint, still 

 it has been observed where none such could have existed. Debility, however occa- 

 sioned, seems to render these fishes susceptible to the disease, especially after 

 injuries causing abrasions, as in kelts after the breeding season or having been 

 netted, also unspent forms, but young fish may be also affected. Frosts, droughts, 

 and polluted waters favour its extension, while, possibly, were fewer kelts pre- 

 served and our rivers purer we should see less of the disease, especially where the 

 currents are rapid. Rock salt has been found to be the best mode of treatment in 

 fish ponds and aquaria ; and, while migrating to the sea would seem to arrest the 

 fungus, it is not certain that it will not reappear on the salmon's return to the river. 

 Once this fungus has attacked a fish, it may become the nucleus from which 

 infection spreads, and within twenty-four hours, thousands of zoospores, which 



* Cooking fish infested with worms destroys the vitality of the latter, while they could not 

 live in the human body. 



■j- It has been assumed, and, I believe, with reason, that when salmon are captured with 

 parasitical Caligi upon them, they cannot have been long from the sea ; still, so far as I know, 

 experiments are still desirable to ascertain whether, it the fish came gradually into brackish and 

 subsequently into fresh waters, these parasites would immediately succumb. 



X It is mentioned in the Fish Commission Eeport, 1860, as existing epidemically in the 

 Severn four or five years previously (pp. 233, 234). 



