SALMON— CAUSES OF DESTEUOTION TO. 109 



While mud or dirt in rivers may occasion deleterious consequences to the fish, 

 owing to its containing injurious ingredients. 



The young par takes any hait with avidity, and at almost any time, even 

 when the trout refuses to rise, while their stomachs are often found gorged 

 with the larvae of aquatic insects, even when the milt is exuding from the fish. 

 Mr. Tegetmeier observed (Proo. Zool. Soo. 1868) that the smolts which left the 

 Stormontfields ponds in May that year were much larger than those of the 

 previous season, due to their diet having been changed from boiled ox-liver rubbed 

 down to coarse powder, for this season the aquatic weeds in the ponds had become 

 covered with Limnea ovata Yax.,peregra, on which they fed greedily and to which 

 the great increase in size was undoubtedly to be attributed. 



The causes of destruction among these fishes may be disease, or consequent 

 upon the modes of capture adopted by man or the lower animals, and can 

 be divided into, (1) those consequent on the condition of the waters they inhabit, 

 (2) atmospheric disturbances and accidents, (3) diseases, (4) misplaced energy in 

 fishing or the work of poachers, and (6) injuries occasioned by the lower animals. 



(1.) Waters may be of such a directly poisonous character,* as to at once 

 kill the fish in them, or be rendered mechanically unfit for their residence, as by 

 the presence of mud which checks or even stops respiration at the gills. Or 

 the water may be so polluted that the living food which ought to be present has 

 been destroyed. The Salmonidce will perish in waters wherein some fish will 

 live, while possibly bull-heads Oottus gdbio, gudgeonsf Gohio fluviaUUs, 

 and loach Nemacheilus harhatula, will thrive where salmon would die. Also 

 what would be poisonous to the fry may be faced with greater impunity by 

 the old fish. Among the substances which have proved directly poisonous are 

 the refuse of gas tanks, mine washings, chloride of lime, caustic potash, and the 

 refuse from manufactories, paper mills, bleaching grounds, tanneries, or sewers. 

 Also artificial root manure washed into rivers from cultivated fields, sheep 

 dippings, and other destructive agencies. The more rapid the current, the more 

 quickly ai'e the poisons dispersed and diluted ; consequently the less chance of their 

 being immediately fatal to the fish, but they may be permanently injured thereby 

 and possibly give rise to a debilitated offspring and infirm race (see p. 27 ante). 



(2.) Atmospheric disturbances, accidents, and a great rise or a similar fall in 

 the temperature may be destructive, thus a severe frost may freeze the redds 

 wherein the eggs are deposited and destroy their vitality (p. 35 ante), while too 

 high a temperature is very fatal to the young. Electric distnrbances may likewise 



* "In Ireland, during July and August, brown salmon from the south coast are frequently 

 poisoned and consigned to England for sale : any fish having white gills and white eyes are 

 certainly poisoned fish and naturally unwholesome as ioodi."— Field, July 19th, 1884. 



■j- I observed in British and Irish Fishes, p. civ, that "when a river in India becomes unduly 

 full of mud the crabs retire to the banks, and even the eels leave the stream for the wet grass in 

 the vicinity. This attempt to escape from water loaded with ingredients inimical to life has 

 likewise been observed among the invertebrate forms of Europe, as was some years since pointed 

 out by M. Gerardin, in Prance. A series of experiments and investigations showed that colour, 

 taste, odour, or chemical composition cannot invariably be accepted as criteria of whether water 

 is wholesome or the reverse, but that such must be looked for in its eflect upon the animals and 

 plants which reside in it. When fish died from river pollution it was observed that molluscs 

 sometimes saved themselves by hiding under leaves and waiting there until the danger had passed 

 away : thus, in July, 1869, Limnea remained five days out of the water. 



Among plants, one of the most delicate was found to be the watercress, and it was remarked 

 that when some deleterious substance from a starch factory obtained access to the Croult above 

 the cress-beds of Gonesse, all these plants died within a few hours : the pollution removed, the 

 cress-beds again flourished. Pond weeds and veronicas only live in water of good quahty ; mints, 

 rushes, and water lilies, accommodate themselves to mediocre water ; Garex is stiU less sensitive ; 

 and lastly, the most robust of water plants is a species of reed, Arundo phragmites, which resists 

 the most infected water. Among molluscs, the Physa fantinalis lives only in very pure water, the 

 Valvata piscinalis in that which is healthy, while others can reside in that which is of mediocre 

 quality ; no mollusc will live in what is thoroughly polluted. The phanerogamous or flowering 

 plants thus sketch in distinct traits the character of different streams ; but infusoria and 

 cryptogams, and particularly alga, may also enable one to judge in the matter by the modifications 

 to which they are subject from alterations of the water. Those lower organisms survive after 

 the disappearance of fish, of molluscs, and of green herbs." 



