SALMON— CAUSES OF DESTRUCTION TO. 109 



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

 owing to its containing- injurious ingredients. 



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

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

 with the larvro of aquatic insects, even when the milt is exuding from tlie Osh. 

 Mr. Teo-etmeier observed (I'roc. Zool. 8oc. 18G8) that the smelts which left the 

 Stormontficlds 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 vfiih. Limnea ovata YViV.,j)cregra, 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 inhaljit, 

 (2) atmosi^heric disturbances and accidents, (3) diseases, (4) misplaced energy in 

 fishing or the work of poachers, and (5) 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 then- residence, as by 

 the presence of mud which checks or even stops respii^ation at the gills. Or 

 the Avater 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 Gottus gohio, gudgeonsf Gohio fluviatilis, 

 and loach Nemacheilus bariatula, 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 livers from cultivated fields, sheep 

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

 quickly are 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 destriTctive, 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 disturbances may likewise 



* " In Ireland, during July and August, brown sahnon from the soutli coast are frequently 

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

 certainly poisoned lish and naturally unwholesome as food." — Field, July 19th, 1884. 



•j- I obsei-ved in British and Irisli 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 France. 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 effect 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 quality ; mints, 

 rushes, and water lilies, accommodate themselves to mediocre water ; Carex is still less sensitive; 

 and lastly, the most robust of water i^lants is a species of reed, Arundo plireujmites, which resists 

 the most infected water. Among molluscs, the Plnjsa fontinalis lives only in very pure water, the 

 Valvata 2^iscinalis 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." 



