232 
ings, this mortality is avoided. It is most probable that 
during their pelagic life the eggs and larve are destroyed 
by being eaten by other animals or by physical changes, 
but unfortunately we have as yet no idea of the proportion 
of eggs or larvee so destroyed. Experience in the hatchery 
shews that it is during the period between the absorption 
of the yolk sac and the beginning of the metamorphosis 
that the mortality is greatest. This critical period is 
characterised by the change in the means of nutrition of 
the larva, which having used up the food yolk, begins to 
feed on planktonic organisms. If this mortality exists in 
nature, and if it should become possible to avoid it in the 
hatchery, then the gain would be very considerable, but 
until it shall become possible to rear the greater portion 
of the larvee hatched through their metamorphosis all that 
is gained in the hatchery is the immunity of the eggs and 
larvee and of the fishes in the spawning pond. From this 
latter point of view—the immunity of the fish yielding the 
eggs—the hatchery is to be regarded as a reserve of 
spawners, and its function becomes the more valuable the 
greater the reduction of the fish population in the area 
dealt with. It is to be regarded as effecting the same end 
as would be brought about by protection of mature fish on 
the spawning grounds—protection which at present seems 
impracticable, or protection of the fry caught in the course 
of other fishing—protection also apparently impracticable. 
We refer here to the treatment of restricted areas. Prob- 
ably the effective treatment of such an area as the whole 
North Sea by artificial hatching is at present impracti- 
cable. The deficit which the hatchery would have to 
make good is the assumed reduction of the mature Plaice 
population, and this most probably takes place on a scale 
which it would be difficult to approach by artificial opera- 
{ions according to our present ideas and methods. 
