FISH-CULTURE. 213 



sooner or later ; and the main inference to be drawn is that ponds 

 must be constructed in such a manner that this temperature never 

 can be reached. This necessitates a combination of supply with 

 depth. Shade is out of the question, at least from trees, as the 

 decaying vegetable matter imprisons if I may use the word 

 the free oxygen. Depth is governed directly by the size of the 

 fish the pond is to be used for ; indirectly, by the facility of 

 drainage. The depth must never be such as to enable the fish to 

 get out of control ; up to this any depth may be used in earth 

 ponds. In plank ponds, however, where the sides are perpendi- 

 cular, other considerations intervene ; but in earth ponds no depth 

 can be too great if the fish are always under perfect control. If 

 the supply is clear, deeper ponds can be used than if the water is 

 muddy. In practice, 5 feet will be found the average limit ; and 

 44 feet for muddy, and 6 feet for clear, spring water, the 

 absolute limits for earth ponds into which feeding-fry are to be 

 placed to grow into yearlings. With water less than 4 feet deep 

 yearlings do not thrive, unless the current is very strong ; and 

 a strong current is too expensive to pay, arid, besides, very 

 dangerous, as the screens are much more apt to choke than with 

 a moderate supply. A strong current is expensive, because it 

 necessitates larger and more costly intakes and outlets ; because 

 the wear and tear is much increased, both of banks and of perforated 

 zinc ; and lastly, because it uses up water that can be much more 

 profitably employed elsewhere. If these limits are exceeded, the 

 fry are not under control at first. They cannot be coEected for 

 feeding, nor their numbers checked daily without drawing down 

 the water, and the labour which this would entail renders it out 

 of the question. 



The collection of the fry for feeding, especially for the first 

 month after they have been turned into the pond, is of great im- 

 portance. It is at this period nearly the whole of the loss occurs. 

 If the fry do not feed when first turned out, they get weak, and 

 never afterwards pick up. This is the prime cause of fungus on 

 three-month-old fry ; some get weak, others take on cannibal 



