xi MORTALITY OF FISH 291 



recruitment is not simultaneous or sudden. For the plaice in the North 

 Sea, with 70 mm mesh, R has been computed at 280 million fish for the 

 area considered, with mean age of 3 -7 years. A check from consideration 

 of numbers and mortality of eggs gave 320 million. Theoretically, we 

 also require to know the length of the life of the fish. Few plaice are 

 caught older than 15 years and almost none over 20 years. Since 

 there are very few in these older age-classes the upper limit is un- 

 important. 



The coefficients of mortality M and F are estimated from the 

 numbers {N t ) of the original recruits R that survive at time t, N ( = 

 R e —M (t — t p ), where e is the natural base of logarithms and t p the 

 age at entry of the recruits. The total mortality (F-\-M) (due to natural 

 causes (M) and to fishing (F)) is estimated from the catches (Fig. 

 1 71 a). The part due to fishing alone is found by consideration of how 

 the catch varies when there are known differences in the fishing effort, 

 for example in the number of hours fished. In the extreme case, we 

 have the war-time periods, when there was no fishing. Confirmatory 

 evidence of the likelihood that a fish will be caught can be obtained from 

 marking experiments, giving the time between release and recapture. 

 For the plaice (F-\-M) has been estimated over the years 1929-38 as 

 0-83. This is the mean for all the age-classes together, though for 

 some purposes it can be calculated for each separately. That the rate 

 is indeed constant is shown by the close fit of Fig. 171 a. The first 

 three points do not fit because recruitment of these classes was still 

 incomplete. This method of treatment, in which the contributions of 

 year classes of the same age recruited in different years are considered 

 together, has the effect of averaging out this variation in recruitment, 

 which would be difficult to estimate. 



The value of the natural mortality of plaice cannot be obtained for 

 these years from variations in fishing effort, since these were only 

 slight. However, since there was almost no fishing in the southern part 

 of the North Sea from 1940 to 1945, we can compare the age groups 

 V and VI of samples taken before the war with their surviving fellows 

 in the years after 1945. This gives a value for A/of o-i, and subtracting 

 this from 0-83 as the total mortality we obtain 073 as the mortality 

 due to fishing. Marking experiments give a value of 0-69, which is a 

 satisfactory agreement considering that the conditions were not en- 

 tirely similar. Moreover, the conclusions from marking involve other 

 problems, such as damage to the fish and rate of movement away 

 from the point of release. The conclusion that fishing is the most 

 important source of mortality in such areas is not new, but is obviously 



