■17- 



The spawning stress mortality can be determined from the long-term age 

 composition of a stock. To obtain a long-term age composition, the observed 

 year class strengths during at least 10 years (greater number of years for 

 species which have great variations in year class strength) are summarized and 

 normalized. Furthermore, we need data on the age (size) of maturation (i.e., 

 at which age 50 and 80^ of the stock is sexually mature). 



The features which appear in long-term mean age composition of the population 

 are schematically shown in Figure 11. First we should eliminate partially 

 recruited year classes. The first year class selected as fully recruited can 

 be at times ambiguous, and minor adjustments might be needed. In these species 

 where the maturation (to ca_ 80^ of the population as determined from corresponding 

 empirical data) occurs after the full recruitment to fishery, two different 

 slopes (which are not fully straight lines) of the decrease of the numbers can 

 be estimated. Before maturation, total mortality is mainly due to fishing (the 

 natural mortality from disease and other causes being apparently very small). 

 After maturation, total mortality increases each year. As this mortality 

 increases after maturation, it is called spawning stress mortality by the present 

 author, although some other authors (e.g., Beverton 1963) have called it 

 senescent mortality. Thus, if it is assumed that the fraction of each age class 

 removed by fishing is a constant for each year once the stock is fully recruited, 

 then any increase in mortality can be attributed to spawning stress, or senescent 

 mortality. Once high age classes are reached, the mortality (measured as 

 percentage change in numbers) will appear to decrease. This is due to the 

 "tail end" which is a consequence of some strong year classes which have existed 

 during the period used in summation of the data. As in this region the summation 

 is bounded by zero (no negative year classes possible); the normalized mean in 

 this age region produces the "tail end" which is not fully compatible with previous 

 year class data. 



