— 3 — KNUT DAHL 



If one or more of these conriitioris are lackinj^ the difficulties of carrying out a 

 profitable fish culture in fresii waters by means of artificial liatcliing increase proportionally. 



The aim of modern fish culture is not to aid the reproduction of fishes. Its aim 

 is to find out, how much fish it is possible to make a water produce, and how this 

 can be done most profitably. 



As regards "wild" waters, e. g. waters which cannot be so fully controlled as the 

 artificial ponds, their size and natural conditions are matters of great importance, and 

 it is not possible to lay down a general rule as to the utility of adding artificially 

 hatched fry to such waters. We may however safely state, that as a rule the difficulties 

 of fish culture or a rational cultivation by means of artificial hatching increase in 

 proportion to the size of the waters and the number of species they contain'. 



Some special circumstances have favoured the employment of artificial hatching in 

 fresh waters. The majority of fresh water fish possess relatively large and hardy eggs, 

 which are hatched resting on the bottom or glued to objects on the bottom. The 

 period of incubation ranges up to many months, a circumstance which tends to increase 

 the importance of protecting them against enemies. 



The greatest importance is also attached to the fact, that the recently hatched 

 young are very active, far developed and hardy organisms, which can easily be reared. 

 They can be kept in thousands in a limited space, are easily handled, and can be fed 

 and reared at no great cost and with a small percentage of mortality, for years if 

 desired. It is just this fact, which is of the greatest importance. The fish culturist is 

 in this way enabled to work with the very stage of young fish that suits him, and it is 

 the reared and far-advanced young fish ("der Setzling" of the German culturist) and not 

 the recently hatched egg or the infant larvae or fry ("die Brut") that is of importance 

 in practical fish culture. 



Even in large waters e. g. rivers and lakes which are not subjected to private 

 enterprise alone' but where the state or public institutions attempt an extensive culture 

 or a maintainance or regulation of the stock by means of artificial hatching, the need 

 of liberating far-advanced young fish instead of eggs or larvae is generally acknowledged. 



The institution of fresh water culture has thus been favoured by certain conditions 

 due to the special physiology of the fre.shwater fish, and also by technical facilities. 



If we compare these conditions with the conditions to which the hatching of sea-fish 

 is subject, we will soon observe the great difference. On the one hand the areas of 

 the sea and its biological conditions are of such a size and nature, that consequently a 

 direct comparison with the intensive freshwater culture is precluded. It would be 

 among the "wild" freshwaters, at most, that points of resemblance to the conditions of 

 the sea might be looked for. And yet at the same time it is obvious that even small 

 areas of sea or ocean must be nearly always considered as large compared to most 

 areas of fresh water. 



On the other hand the very hatching of the sea fish presents technical difficulties 

 which in regard to freshwater fish we do not know. 



Freshwater fish are easily and cheaply hatched, because the demersal eggs of these 

 fish are as a rule large and easily handled, the young are very advanced at birth, are 



^ See; E. Walter, Die Fischerei als Nebenbetrieb des Landwirthes und Forstmannes. 



1* 



