178 



Part HI. — Fifteenth A nn tad Report 



During my experiment last summer, 1 obtained most satisfactory 

 movements in the water through the effect of variations in the tempera- 

 ture of the surrounding air. [See description of the arrangements.] 



Another point in connection with the water supply is the filtration, 

 which, if thoroughly applied, will remove, perhaps, all organic and inor- 

 ganic matters, leaving the water as a pure and simple solution of salts. 

 There may be no harm in this when food is put in artificially, and it cer- 

 tainly offers conveniences in several ways. But for rearing purposes I 

 find that unfiltered water that is relatively free from decomposing organic 

 substances is all that is wanted. 



The answer to the question whether the presence of abundant light or 

 occasional exposure to the direct rays of the sun is of importance to the de- 

 velopment of larval and post-larval fish produced from pelagic eggs is 

 given by nature herself. In the sea the eggs of a plaice, for instance, are 

 developed near the surface in full exposure to the daylight. The larval 

 stage (during the absorption of the yolk) is also passed much in the same 

 level, and though the post-larval fish may occasionally be found at a 

 greater depth, I think I am safe in saying that this stage should also be 

 called a pelagic one. And since this early development is accomplished by 

 nature in the full light of day, it would be a simple waste of energy even 

 to attempt a rearing experiment in a dull or much shadowed place in a 

 laboratory. 



In accordance with this, it is a long recognized fact in hatcheries that 

 such apparatus give by far the best result that are most abundantly sup- 

 plied with light. The presence of abundant light is of importance for the 

 well-doing of the larvae directly, and it also has a marked effect upon the 

 minute organisms that compose their food; but experiments are still 

 required to show to what extent a dull light would be effective. 



It is well known that a high temperature increases the rate of growth 

 of the embryonic and larval fish, and it is therefore only reasonable to 

 suppose that the chances of success in a rearing experiment with the post- 

 larval stage would be increased relatively as the time was shortened. And 

 we know that eggs and young larvai are also able to resist considerable 

 changes in the temperature. But this, of course, is limited ; and it must 

 be remembered that in a rearing experiment the circumstances have also 

 to be made suitable for the living food. The various species of food fishes 

 will here appear more or less different, so it will be necessary to experi- 

 ment with each of them separately in order to be able to give even 

 approximate figures for the most suitable temperatures. With the plaice 

 I found it risky to allow the temperature to rise above 16°C when dealing 

 with the post-larval stage, while it is known that the same fish, after the 

 transformation has taken place, will thrive well in much hotter water. 



What has now been said respecting the water supply, the influence of 

 light and a high temperature on the success of rearing experiments on a 

 small scale, is also proved by the success of my experiments in 1896, of which 

 I shall now give an account. The question of food can then be more con- 

 veniently dealt with. 



The apparatus and arrangements employed for the rearing experiment 

 were very simple. As a rearing-jar, a large carboy-shaped glass vessel was 

 used, with a diamster of 20 inches ; it was a little flattened at the bottom, 

 and 18 inches deep; the opening of the neck was 9 inches. This vessel 

 or jar was placed in a shallow tub, filled with sand, so that its bottom was 

 evenly supported all round to a height of about three or four inches, and 

 it was placed at a height of about two feet above the floor, near a corner, 

 having a window on each side four feet distant from the jar; one window 

 faced to north and the other to east. Light was supplied from the back 



