2o Tffith the deoreasa of density, an increase in growth rat© 

 occurs e This is caused by the fact that, as the number of fish declines j 

 more food is available for each remaining fish. A fish uses its food 

 for two purposes: for maintenance (tissue reparation, digestion, 

 muscle labor, etco) and for weight increase or growth. Therefore, 

 the food which has become available through the catching of a quantity 

 of fish now becomes an extra ration for the survivors o Furthermore, 

 we saw that by increasing the fishing intensity, the average age of 

 the fish lowers and generally the older fish use a larger percentage 

 of their food to remain alive than the middle-aged fisho A sole of 

 40 centimeters, for instance, increases in one year as much as one 

 of 30 centimeters does, but uses more food during that year. Conse- 

 quently, by an increase of fishing intensity, more food becomes 

 available per fish and a larger percentage of food will be used for 

 weight increase. 



3o The egg production will decrease, as there will be fewer 

 adult fish and the younger ones produce fewer eggs. 



4o Because the average age lowers, death from old age decreases 

 and, as the older fish often are formidable cannibals, the death of 

 young fish decreases also. 



Summarizing, w© may say that when an untouched fish stock is 

 being fished, the result is a decrease in the number of individuals, 

 a decrease in the average age and in egg production, an increase in 

 the rate of growth, and a decrease in natural mortality. 



If the stock after fishing is again unfished, it will gradually 

 regain its maximum strength. The quantity with which the stock may 

 increase during a period — for instance one yeai — that is the rate 

 of increase, is controlled by A + G-M (see above). Also, we have 

 seen that the values of egg production (a), natural mortality (M), 

 and numbers of individuals and rate of growth (product of latter = G) 

 are all dependent on the density of the stock. Therefore, the rate 

 of increase also must vary with the strength of the fish stock. 



Regardless of the large number of growing individuals and the 

 high egg production, the yearly increase made by a very large stock 

 is small since the growth rate is small and the mortality is high. 

 In a medium-sized stock, the increased growth rate and decreased 

 mortality outweigh the lower egg production and the smaller number 

 of individuals. In a very small stock, there are too few fish to 

 give G a greater value, even though growth rate is maximum. The egg 

 production is very small and the mortality higher than it is in a 

 moderately fished stock, since smaller fish fall prey easier than 

 larger ones. Consequently, annual increase is low in a fish stock 

 of small strength. The greatest rate of increase is reached by an 

 intermediate stock which is neither too great nor too small. 



