(29°) 



300 



200 



g. 100 



V X 



Age-group 



2000 



1500 



« 1000 



500 



o> 300 



200 



100 



005 100 1-50 



073 r 



500r 



400 



50 100 150 200 250 



Cod-end mesh size (mm) 



Fig. 171A. Mortality rate of plaice. Natural logarithms of average number caught of each 

 age-group of North Sea plaice per 100 hours' fishing, 1929-38. The slope of the line gives 

 the estimate (F+M) = 0-83. The first three points are lowered by absence of fish from 

 the exploited area (some individuals still remaining in the nursery areas) and, possibly, 



by rejection at sea. 



Fig. 171B. Growth in weight of plaice. Average weight of fish of each age-group, 1929-38, 

 and the fitted von Bertalanffy equation for growth in weight. 



Fig. 171c. Annual steady yield of plaice plotted against fishing intensity. Yield of plaice 



per recruit (Y W IR) as a function of fishing mortality coefficient (F) with t p ' = 3-72 years, 



corresponding to a 70 mm gauge mesh in trawls, double twine. The vertical line at F = 0-73 



corresponds to the average pre-war fishing intensity. 



Fig. 17 id. Effect of growth rate changing with density. The yield of plaice per recruit 

 (Y W IR) is shown as a function of mesh size with fishing intensity constant at F = 073, 

 but in calculating curve (b) the growth-rate was assumed to be reduced progressively as 

 the density of stock increases. The differences from the use of constant growth-rate, curve 

 (a), are considerable. Not only is the benefit from increase of mesh estimated to be less, but 

 the maximum yield is reached at a considerably smaller mesh size. 



