FECUNDITY OF NORTH AMERICAN SALMONIDAE 



By George A. Rounsefell, Fishery Research Biologist 



This paper is the first in a projected series in 

 whicli the autlior proposes to compile and evahiate 

 the pubhshcd information on various phases of 

 the hfc history and conservation of North Ameri- 

 can salmonids. The available information is so 

 widely scattered that merely bringing it together 

 will facilitate the expanding research. Further- 

 more, even a hasty perusal of the literature reveals 

 large gaps in our knowledge. Once these gaps are 

 clearly seen, there is a much better chance of 

 their being filled. 



The primary purpose, however, is to discover 

 througii comparison of the same life phases of the 

 different species and genera, the relation between 

 the fish and the ecological factors in their environ- 

 ment. Since emphasis has been placed on material 

 that would aid in developing principles, and as 

 I am making the study as complete as possible 

 without assistance, I am not including minor 

 items of information. Original data are presented 

 for Karluk River sockeye. 



Although not indigenous, the brown trout, 

 Saimo Irutta, is included in this study as a thor- 

 ougldy naturalized species. European and Asiatic 

 literature is used sparingly, either to aid where 

 knowledge of the North American stock is de- 

 ficient, or to corroborate tlie North American 

 findings. 



Fecundity is an especially interesting topic in 

 the Salmonidae because the comparatively small 

 number of large eggs suggests (as other re- 

 searchers have proved, e. g., Rounsefell and Kelez, 

 • 1938, Rounsefell, 1949 and in ms.') a demonstrable 

 relation between the reproductive potential of the 

 spawning stocks and the numbers of young sur- 

 viving. Neave (1948) has also pointed out that 

 the variation in egg number between species of 

 Oncorhynchus is related to the varying vicissitudes 

 of their life historv. 



 Factors causing decline in sockeye salmon of Karluk River, Alaska. 

 V. S. Department of the Interior, Fish and Wildlife Service, Washinpton, 

 D. C. 



Approved for publication. February 8, 1957. 



The relation between size of spawning stock 

 and number of young produced is fundamental 

 to studies of changes in population size. The 

 survival from spawnings cannot always be de- 

 termined at an early stage, but is more usually 

 measured at some later stage of the life history. 

 In this paper we are concerned with quantitative 

 measurement of the reproductive potential of the 

 spawning stock. Such measurements are usually 

 gross estimates derived from one of the following 

 bases : 



1. Relative abundance of the adult population. 

 This will usually be in pounds of fish caught by 

 some standard amount of fishing effort (a stand- 

 ardized unit of gear fishing a certain period of 

 time). 



2. Relative abundance of the eggs or larvae. 

 This usually is a summation of the density of 

 eggs (in the case of pelagic eggs) per cubic meter 

 over the water area inhabited by the particular 

 population under consideration. Estimates of 

 abundance of species spawning in the littoral 

 zone, e. g.. Pacific herring {Clupea pallasi), 

 may be based on miles of shoreline utilized for 

 spawning. 



3. Actual numbers of mature adults. These 

 numbers may be an actual count of the individuals 

 or maj- be statistical estimates of population 

 size. 



These measures of reproductive potential are 

 each based on one or more of the following 

 assumptions: 



1. That the number of eggs spawned is in direct 

 proportion to the number of mature adults and 

 their mean weight (or length). For this to be 

 true, the relation between size of fish and fecundity 

 must be linear. Moreover, if the size composition 

 of the adult population varies from year to year, 

 then the theorem is true only if the regression 

 of eggs on size passes through the origin, i. e., 

 the regression formula must be of the form 

 y = bx. 



451 



