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ings, this mortality is avoided. It is most probable that 

 during their pelagic life the eggs and larvse 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 larvae 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 larvse hatched through their metamorphosis all that 

 is gained in the hatchery is the immunity of the eggs and 

 larvae 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 b}' artificial opera- 

 tions according to our present ideas and methods. 



