To find the average length of the various age groups, under the 

 changed circumstances of table lEb, it was assumed that the rate of 

 growth (measured as weight-increase-per-year) decreases in reverse 

 proportion to the density -increase of the sttcko This is a simpli- 

 fied illustration which, however, seems to be reasonable ( B^^ckmann, 

 1932), The densities of both stocks are equal to the sums of columns 

 6 of each table, that is as 2,883 + 3,625, With this number the 

 intervals between the average weights of the various age groups 

 (column 8, table 12a) must be multiplied to arrive at the intervals, 

 and consequently the weights, under the new conditions (coliann 9, 

 table 12b), The average sizes and the starting lengths of each 

 age group can be_found again from these weights » 



Our tabulations show that by fishing with the wide=meshed net, 

 about 115 kilograms of haddock bring in 2olO guilders more than 

 when the small-meshed purse is used (see table 12a). This means 

 a profit of 294,000 guilders on the approximately 14,000,000 

 kilograms which could be landed in Holland (1930). 



This is not the only advantage o The most important result is 

 that because of the protection, the haddock stock becomes lo3 times 

 as dense. That is to say, the same catches can be made in only 78 

 percent of the time formerly necessary. In the above tabulations 

 it was assumed that after the start of the conservation regulations, 

 as many young haddock would get into the nets as in the past. 

 However, this will not be the case, because, as we have said already, 

 the fishermen will avoid the schools of worthless young haddock. The 

 loss resulting from the imperfect selectivity of the nets, therefore, 

 will be less than was assimied in the tabulations! consequently, the 

 profit will be greater. This review leads us to the conclusion 

 that the facts presently available show that a minimum mesh size of 

 8 centimeters and a minimimi fish size of 27 centimeters are necessary 

 to protect the haddock sufficiently. This will only be valid as long 

 as the total mortality of 50 percent is not exceeded, 



bb» Plaice 



As for the haddock, we first have to determine the lower limit 

 of the minimian size. This is determined by the condition that the 

 egg production must remain on one level, 



BRckmann (1939) gives a summary of the strength of the year 

 classes during the period 1925-36, From this picture shown in 

 figure 9, it does not appear that the average annual strength declined 

 after 1930, Therefore, we may assume that the egg production of the 

 plaice was sufficiently large to maintain the supply of young. 

 Statistics can be taken from BQckmann's work (1932) about the average 

 decline of a year class. With the aid of these numbers, table 13 

 shows how a group of plaice, with a strength of 1,000 at the end of 

 the second year, declines from year to year, Franz (1908) has given 



S3 



