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APPENDIX 



COMPUTATION OF SPAWNING STRESS (OR SENESCENT) MORTALITY, AND SUBSEQUENT 



DERIVATION OF LONG-TERM MEAN AGE COMPOSITION OF FULLY EXPLOITED PORTION 



OF POPULATION 



Some comments by the reviewers of this paper indicated that the concepts of 

 spawning stress mortality and long-term mean age structure of the exploitable 

 portion of the fish population are not fully understood by all. Furthermore, 

 different practices of the application of mortalities are in existence. Most 

 commonly, the "natural mortality" is assumed to be constant whereas fishing 

 mortality is assumed to be age specific. In this paper, the reverse is assumed. 

 These above-mentioned concepts are clarified in this appendix. 



In most modern fisheries (trawling, purse seining, etc.) the fishing on 

 fully recruited year classes can be considered as taking an equal fraction from 

 each year class (i.e., it is not age (size) specific). Obviously, the lower size 

 limits depend on mesh size used, and thus affect year classes which are not 

 fully recruited to the fishery. There are obviously cases where the fishery is 

 somewhat size selective, such as a fishery on spawning concentrations only, and 

 on fish species where there is a pronounced segregation of age (size) groups 

 with depth. In some species (e.g., cod) old, large individuals are assumed 

 to inhabit deeper depths, and thus might be subject to the fishery in a lesser 

 degree than younger fish. This condition has never been quantified. Furthermore, 

 the amount of biomass (and numbers) in these old year classes is low and the 

 error introduced by assuming age-constant fishing mortality will be small. 

 However, in some species which are predominantly caught with size-selective 

 gear (gill nets, long lines), the fishing mortality is age specific. 



