of the Fishery Board for Scotland. 



121 



During the experiments the temperatures were never steady. The 

 hatchery water was obtained from an outside reservoir, the temperature of 

 which varied with the temperature of the air. The water pumped into the 

 reservoir from the sea was usually at a higher temperature than the water 

 that was standing in it. The cooled water varied also, both in consequence 

 of the variation in temperature in the water and air of the hatchery, and 

 also in consequence of the method of cooling. The cooling apparatus was 

 described on p. 103. Its action depended upon the ice supplied to the tins. 

 Between one changing of the tins and the next, the temperature quickly 

 fell to a minimum, and then, as the ice got melted, gradually rose till it 

 reached the maximum temperature, that was just before recharging. The 

 temperature of the water was taken just before recharging, and again some 

 time, about an hour usually, after that operation. The temperature was 

 also observed at other times. Recharging was necessary twice and three 

 times in the twenty-four hours. 



The question might be asked how far a rise and fall in the temperature of 

 the cooled water is to the advantage of the ova. Would a regular low 

 temperature of, say, 2° C. be fatal to the eggs, while a range of which the 

 average was 2° C. proved safe ? I think not. 



There was a large mortality among the eggs. It was not, however, con- 

 fined to the cooled spawn, although in some batches of these all the eggs 

 died before hatching. Death could not be ascribed in large measure to the 

 cooling. The main cause of the mortality was deficient aeration. This 

 occurred through the adoption of an egg-box (fig. 102), which required a 

 strong current of water flowing through it before a complete circulation of 

 the water was attained. In the cases where the boxes received uncooled 

 water, when the current was reduced the eggs began to die off. The boxes 

 were useful for reducing the temperature of the water, since they were 

 packed outside with ice, but the water in the lower portion of the box was 

 noticed on several occasions to be at a less temperature than the surface 

 portion. In this way a body of dead water existed in the box, and it offered 

 resistance to the inflowing current of water upon which the aerating cir- 

 culation depended. In one box the water at the bottom was at — 1°, while 

 at the top it was — *2° C. ; in another case the temperatures were T5 0 

 below and 2° C. on the top. 



Only a small current of cooled water was available. It varied from about 

 1 gallon (4| litres) in three minutes to 1 gallon in five minutes. Where 

 the egg-boxes were used with the glass plates set in the inclined grooves, 

 nearly all the eggs died. One of the boxes was used successfully with a 

 plate lying horizontally near the top of the box, so arranged that the inflowing 

 water fell on to the plate (IVcf 1 ). In another case the box was half filled 

 with pebbles, and some lumps of spawn were put on top of these (Vc 1 ). 

 The intention in using the boxes was to get advantage of the ice jacket. 

 A white compound formed on the surface of the metal. This may not have 

 been of advantage. 



On the whole, the most efficient aeration was obtained with the Dannevig 

 hatching-boxes,* used as shown on p. 102. They were employed with 

 success for the spawn on the gravel, It is not, however, so easy to keep 

 down the temperature of the water with these, since ice cannot be applied 

 directly. When the fry begin to hatch, moreover, the current of water 

 eventually drives them up against the sieve bottom, i.e., the outlet. It is 

 preferable that the arrangement should allow of the fry being carried away 

 at once by the outflow into a large rearing tank. The fry should not, in 

 my opinion, be handled if it is possible to avoid it. 



*The Dannevig hatching-box is described by Ewart in " On the Artificial Hatching 

 and Rearing of Sea-fish." Fifth Annual Report of the Fishery Board for Scotland for 

 1886. Edinburgh, 1887, p. 230. 



