70 Thirtieth Annual Meeting 



wood for ])n'8('nt purposes. The idea is merely that a pervious 

 pond retains the infeetion indefinitely and passes it to the fish 

 lield in it. I helieve no brook trout have been raised successfully 

 in such a pond after this disease has once occurred in it. Xow, if 

 these ponds are built of an impervious material, cement, ma- 

 sonry or iron, this source of infection is cut ofl^. Bacterial or 

 other micro-organisms will not penetrate such material and can 

 at most lodge along its surface. Here they may be readily killed 

 by methods of chemical disinfection, which is practically im- 

 ])ossib]e with the other ponds. It is to be understood, of course, 

 that there is no magic in the cement itself to prevent disease any 

 more than in the wooden construction to cause disease. The 

 cement will act merely as a barrier to the invading germs, while 

 the wood or earth on the other hand are their verv vehicle. Our 

 ])ond of masonry or cement then can reasonably be expected to 

 prevent the spread and continuance of a given epidemic. It cor- 

 rals it, so to speak, within a definite space, where it is vulnerable 

 and may be killed by the ordinary methods. I am referring now 

 of course to the germs in the pond but outside the fish. We can 

 be sure of starting in any given case, with a clean, uninfected, or 

 disinfected pond, and if infection enters thereafter it must gra- 

 vitate from the air, or come down with the water su])ply. 



We must consider briefly that })art of the fish cultural water 

 system above the ponds, — the part visually not inhabited by the 

 fish. You will remember I made a distinction between the dan- 

 ger of the primary or original menace of infection and of the 

 secondary or localized one. The former was looked upon as less 

 dangerous l)ut was responsible for the first epidemic, the latter 

 as far more dangerous and almost certain to infect, and implied 

 the continuance or recrudescence of the disease. This distinc- 

 tion corresponds on the one hand to the water supply above the 

 ponds and free of fish, and on the other hand to the pond system 

 itself in which the fish are held. The demarcation is at the point 

 of entrance of water into the pond. Below this line, or in the 

 ponds, the danger of infection is the same as that above ])lus the 

 localization from previous epidemics. Bacteria probably do not 

 travel Tip the conduits, so that above this line of demarcation the 

 danger is ibat wliicb always existed there, namely tbat some ob- 

 scure cliance or accident may start tlic ball of disease rolling by 



