J. PISCICULTURE AND THE FISHERIES. 467 



the waters where they breed, or the downward passage of 

 those attempting to descend; (19) that irrigation canals are 

 exceedingly injurious, if they have vertical falls in them, up 

 which fish are unable to ascend ; for, as the old ones descend 

 down stream to feeding-grounds, they find a stone-wall in 

 their way; but a fine stream of water not so obstructed, which 

 leads them into one of these canals, and once over a fall, they 

 can not reascend, but are destroyed there every time the 

 water is cut oif ; (20) that the same destructive plan exists 

 in nearly every irrigated field in India; (21) that there are 

 certain vermin very inimical to fish, as crocodiles and otters, 

 which should always be destroyed ; (22) that in Great Britain, 

 and other civilized countries, the i3oaching of fish is forbid- 

 den; (23) that where local restrictions on poaching fish have 

 been tried in India, the result has been most beneficial." 

 Heport on Fresh-icater Fisheries and Fishes in India and 

 JBurmah^ by Surgeon-Major Francis Day 



INFLUENCE OF EXTERNAL PRESSUEE IN THE LIFE OF FISHES. 



Mr. Carbonnier has lately published an article upon the in- 

 fluence of external pressure upon the life of fishes, in which 

 he takes the ground that when certain fishes are gravid with 

 eggs they come into shallow water, on account of the less 

 distress produced by the external pressure, and that after de- 

 positing their eggs, unless they are so situated as to be able 

 to return to the deeper waters, they sufi*er very seriously, and 

 sometimes perish ; the natural means of reducing expansion 

 of the abdomen being exposure to colder water and increase 

 of pressure externally. Pie makes a special application of 

 this generalization to the salmon and shad, remarking that 

 when their ovaries are developed they ascend the rivers from 

 deep water, and finally reach the localities where the watei* 

 is very shallow. As the fish run up into shallow water, 

 and the expansion of the ovary is a gradual operation, there 

 is little inconvenience experienced ; but as the emptying of 

 the ovary takes place within a period of a few days at most, 

 unless the fish are where they can return readily, and in a 

 sufiiciently short space of time, to the deeper water of the 

 river or to the sea, they are liable to succumb to the shock. 



It is on this tlieory that Carbonnier explains the fact that 

 so many anadromous fish die in the course of the operation 



