Abundanise and aj^e composition of stock . - When the weighted, combined, 

 lerigt1i-'f'"requency™iTtrlbution has been converted to age=f requency we have 

 a series of season's curres representing the relative abimdance of each 

 age of fish in the stooko These are useful in deducing the relations be-= 

 tween abundance and spawning and between abundance and competition effects. 

 This is indicated by broken lines in the diagram and will be discussed 

 later. At the moment we shall pass on to detennination of mortalities 

 and recruitments. 



Compute *•' Vital statistics." - By vital statistics we refer to re- 

 crui 'tifien t~"and mortaiityT"^-^ fisheries counterpart of the actuary's 

 births and deaths. One might also use the term '*population dynamics .** 

 The computations involved are adaptated from methods previously developed 

 or employed by BaranoVj, 1918 1 Thompson and Bell, 1934.; and Rieker, 1940. 



Total mortality is readily computed in situations where recruitment 

 and m-ortality can b e reasonably assumed to have been uniform during as 

 many years (prior to the year in question) as there are ages present in 

 the stock. Then, it is necessary to have only one season's age distribu- 

 tion, for each age class will be a constant percentage less numerous than 

 the next younger. The constant percentage then is the annual mortality 

 rate. 



Such simple situations are rare, but, fortunately, they are not the 

 only ones amenable to analysis for mortality. That statistic may be readily 

 deduced from the age frequencies of two successive seasons, providing 

 triat they truly represent the stock in the sea. Here the decline would 

 be measured by the relative numbers of a given year=class in the two 

 frequencies and the percentage reduction of a year=class from the first 

 to the second season would represent the annual mortality rate. If all 

 year classes have been fished with equal intensity, hence have suffered 

 equal satoh mortality, and also have experienced equal natural mortality, 

 the indicated total mortality will be sensibly equal for all but the 

 ''entering'* year classes o Inequalities, therefore, may be ascribed to 

 ago^selective, natural mortality or age -selective fishing intensity. 

 Correlation of the absolute quantities of each year=class in the catch 

 with their respective mortalities should distinguish between these two 

 age-selective processes. 



Recraitmento = As used herein, recruitment means the nvmber of sar- 

 dine 3~TeaGlain g~c'ommei ciai age each yearo Oiven ag-: -distributions that 

 reliably represent the abundance and age composition of the stock, the 

 recruitment is directly proportional to the numbers of individuals in 

 the year-class that has, for the first time, appeared in the commercial 

 catch. The only complicating feature is that there has been nothiiig in 

 the previous adjustments to insure that the entering year-class will be 

 fully represented if, in its first season, it is only partially available 

 to the fishery. Hence, in this respect, we cannot consider the weighted 

 cui-Ties as reliably representing the stocko However, from a series of 

 age-distributions it should be possible, without great effort, to dis- 

 tlnguish the age at which a year^'class becomes fully available, deduce 



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