the relation in numbers between males and females in the various age 

 groups J it is, therefore, possible to determine for each of these 

 groups how many females there wereo With the aid of Redeke's 

 observations (1909) the number of sexually mature females could 

 be determined (column 6)0 Franz (1908) determined the average 

 number of eggs contained in females of different ageso Therefore, 

 we could ascertain the total number of eggs produced by 1,000 two- 

 year-old plaiceo 



Table 13 shows that about l.S million eggs are sufficient to 

 produce 1,000 two-year-old plaiceT Therefore, 1,300 eggs are necessary 

 to produce one plaice j each female, therefore, has to produce git least 

 2,600 eggs if the stock is to remain constanto According to Franz 

 (1908) a sexually mature female has at least 9,000 eggSo Thus, only 

 ^ third of the total nimber of females has to reach the spawning stage 

 to keep the plaice stock at the same strengtho This is considerably 

 less than that necessary for the haddock and probably explains why 

 interference of the breeding production, through intensive fishing, 

 always appeared sooner for the haddock than for the plaiceo 



In table 14 it has been detennined to which age the young plaice 

 has to be protected by various stages of fishing intensity, if the 

 breeding production is not to declineo Due to Franz's observations, 

 we could account here for the changes in the numerical relation 

 between the sexes, which we could not do for haddocko 



The table shows that if the total annual mortality is 40 percent, 

 397 one-year-old plaice are sufficient to give an egg production during 

 the course of the years which will deliver 1,000 plaice « With a total 

 mortality of 50 percent, 1,344 one-year-old or 672 two-year-old plaice 

 are therefore necessaryo Tfe have only 1,000 one-year-old plaice and 

 can only obtain 672 two-year-old plaice by protection of the fish during 

 their second year. If we wish to retain 672 two-year-old plaice from 

 the 1,000 one-year-olds then the mortality may not rise above 30 percent© 

 With a natural mortality of 10 percent, the mortality through fishing 

 may not be larger than 20 percent. That is half of the value assumed 

 in these series for the following years o Roughly, we may say that the 

 plaice must be protected during the first year and during half of the 

 second year. A minimum size of 10 to 15 centimeters would, therefore, 

 be necessary depending on the rate of growth. 



If the total mortality has reached 60 percent, then there will 

 be an egg production for 1,000 one-year-old plaice only if, at the 

 beginning of the fourth year, 935 plaice are present. In other wordS^ 

 the 1,000 one-year-old plaice have to be protected in their second and 

 third years, and--since the natural mortality will have reduced the 

 1,000 fish to 729--also during a part of the fourth year. This would 

 make a minimum size of 25 centimeters necessary. 



66 



