of the Fishery Board for Scotland. 



177 



Below I shall mention what should principally be considered in rear- 

 ing experiments with larvae from pelagic fish eggs. It seems strange 

 that a fish in the post-larval stage should be so difficult to keep in con- 

 finement, while the preceding and following stages offer no difficulty. It 

 has been said, but without good foundation, that the difficulty lies in 

 the supply of food alone and sufficient currents of water ; but it will be 

 shown later that the question of water is rather secondary, and that the 

 food can be procured without the least trouble. In fact, when the water 

 for the rearing-jar or tank is taken direct from the sea, it is difficult to 

 avoid having forms of suitable food present. But the quantity of food 

 would have to be increased artificially according to the number of larvaa 

 present. The principal points are — (1) The water arrangements ; (2) The 

 question of light ; (3) The temperature ; and (4) The food. 



An important point is the question whether or not a constant current of 

 water should be employed. A stop was put to my most successful experi- 

 ment in 1894 by an overflow carrying away the fry, which were all lost. 

 This time, therefore, I thought that the constant current of water through 

 the jar might be avoided. It is doubtful whether the passing of perhaps 

 ten or twenty times more water through the jar than is necessary for the 

 fry is of any use — a small number of larval fishes, such as are yet nourished 

 by the yolk, will thrive in a small quantity of water for days, providing the 

 air is not artificially forced out by an increased temperature. And the 

 same is the case with young flat-fishes after they have passed the transfor- 

 mation. So it is difficult to see why similar circumstances should not 

 suit this intermediate post-larval stage equally well. The constant current 

 is clearly not necessary for the supply of air. In a hot season a con- 

 stant run of cold, perhaps artificially cooled water, would be a means of 

 keeping the temperature down as desired ; but this may be done equally 

 well by less injurious precautions. 



There is now left the third point, that may be put forward in support 

 of the necessity of the constant water supply, namely, that the regular 

 current has always been found to'Jncrease the activity of the fry. This is 

 undoubtedly so j the fry are always found more 4 vivid ' in appearance when 

 the water is in movement, than when quiet. In the hatching-boxes, a 

 certain amount of current is always found satisfactory and necessary for 

 the welldoing of the fry. But it is to be remembered that a great 

 number (perhaps a quarter of a million or more) of newly-hatched larvse 

 in this case are crushed into the narrow capacity of about one cubic foot 

 of water. And as these fry, at this early stage, are nursed from the yolk 

 and therefore take no active part in feeding, no attention can be derived 

 from the latter in the perpetual activity on the part of the fry when 

 striving against the constant currents. These, again, are of great advan- 

 tage in preventing the fry from accumulating in great crowds, which 

 naturally prove fatal to a great number of them ; they are simply suffo- 

 cated in the crush. And the perfect condition of the fry when turned 

 out from the hatchery shows, that this exposure to the currents in the 

 apparatus has done no harm. In a rearing experiment such crowding 

 can be avoided, and matters are different as the young larva has then 

 got to look for and catch its own food. When a constant circulation is 

 maintained in a small jar, the small organisms which the larvas require 

 for nourishment are sure to be pressed against the outlet side, and thus 

 be drawn away from the attention of the larvse. 



But I have found that a slight movement in the water, whether con- 

 stant or not, is desirable, as it will always increase the activity amongst 

 food organisms that otherwise remain quiet. Such a movement may 

 easily be arranged artificially. 

 M 



