On the basis of tests of the amount of food used for growth and 

 that used for maintenance j one may calculate the length of this period© 

 Russell (1932) found that the period must be at least 60 growth days 

 for plaice J, and we may accept that it will be of the same order for 

 haddocko A year may be calculated at about 200 growth daySo Therefore, 

 thinning out to reduce the amount of food used for maintenance is 

 harmful if it takes place within 3 years of the time of market readiness, 

 Sine© plaicop at the earliest;, reach a reasonable length of 25 

 centimeters at the beginning of the fifth year^ one should not be 

 allowed to destroy it after the second year of lifeo Haddock becomes 

 marketable within 3 years and therefore may not be destroyed under= 

 sized at any ageo For plaice, the results of Russell are completely 

 in agreement with the observations of BQckmana^who arrived at them in 

 an entirely different way© B?lckmann(1932) computed the size and 

 classification of the catches for the hypothetical cases that the 

 whole fish stock is being fished with one of the netsi Ap B or C 

 (table 9) o The chance for escape of undersized fish is smallest with 

 net Ap larger with net B, and largest with net Co The loss, Qriginat= 

 ing from the destroying of undersized fish^ was included in the calcula- 

 tions o Net Ap the steam trawler net used in the North Sea^ retains 

 all plaice over 17 centimeters j net Bj, a net used for the catching of 

 solOp catches all plaice from a length of 20 centimeters i Net C, used 

 for the catching of plaice by draggers, retains all plaice at 29 

 centimeters o 



All plaice below 25 centimeters must be computed as being under<= 

 sizedo Assuming that the strength of this group is a million fish 

 at the end of the first year and further assuming that 50 percent 

 of the stock is caught every year and that the natural mortality is 

 10 percent until the fourth year but only 5 percent afterwards, we 

 find that the largest catches are made with the largest size meshes? 

 that iSj, when the smallest number of undersized fishes are destroyed 

 (l)o This appears also to be the case if the fishing rate is increas= 

 ed to 75 percent (lll)o 



BSckmannias also observed how much the catch will be if one thins 

 out the and I groups3^of the plaice (ll)o It is shown th«n that the 

 catches with the nets A and B become somewhat larger than a t I, the 

 catches with the net C somewhat smaller© The results with nets A 

 and B show that it may have some advantage not to leave both the 

 youngest groups protectedo The results with net C give the impression 

 that no loss will appear through the moderate thinning out of both 

 youngest year groups o 



2/ The first year fish is indicated as O=groupo the second year 

 rish as I^group^ etco 



