30 THE NATURAL HISTORY AND HABITS 



occurred with individuals reared in similar ponds 

 supplied with water from a rivulet, the temperature 

 of which throughout the year ranges pretty nearly 

 with that of the river Nith. 



This plainly tells that there must have been some 

 derangement from the temperature of the water, and 

 also other unseen causes, whence some arrived at the 

 smolt state at the end of one year, and others not till 

 the end of two years. This may be caused from con- 

 finement in the ponds, and the limited quantity of 

 food that can be gotten there in comparison to what 

 can be got in the open water. This, combined with 

 an unsuitable temperature, very likely produced the 

 slow progress of the fry in Mr. Shaw's ponds. 



I erected a chain of four artificial breeding ponds 

 on the River Shin. Each of these were about 

 eighteen feet long, nine feet broad, and five feet deep, 

 and supported on the river side by a strong stone 

 wall, of a height above the highest flood mark ; that 

 wall was solidly packed together, so as to prevent the 

 egress or ingress of the smallest insect from or to the 

 river; the ponds were filled within with suitable 

 gravel, and placed so as to have the depth of water 

 one foot at the top or entrance to the pond, and four 

 feet at the lower or farthest down end ; at the entrance 

 of the water from the river to the pond, a strong high 

 wall is built, at the bottom of which is a drain about 

 twelve inches square, strong and closely grated so as 

 to prevent any debris from a flooded state of the river 

 getting in. This drain also regulated the supply of 

 water in all states of the river, so that the current in 

 the ponds continued the same. The inside of the 

 admitting wall was also grated, first with an iron 

 grate, one-eighth of an inch apart, and then a narrow 

 wrought wire grate, so that the smallest fishes could 

 neither get in or out. The four ponds were erected 



