INFLUENCE OF EARLY MATURING FEMALES ON REPRODUCTIVE PO- 

 TENTIAL OF COLUMBIA RIVER BLUEBACK SALMON (Oncorhynchus nerka) 



By Richard L. Major and Donovan R. Craddock 

 Fishery Research Biologists, Bureau of Commercial Fisheries 



Isolating the causes of variability in escapement- 

 ret urn relationships is among the most urgent 

 problems in the biology and management of the 

 Pacific salmons (Oncorhynchus spp.)- Of primary 

 concern are the causes of low numbers of fish in 

 the returns. The fresh-water environment is 

 widely thought to be more important than the 

 nuirine environment in contributing to these 

 subnormal returns. 



Age and sex composition of the parent escape- 

 ment are among the factors of recognized im- 

 portance. For example, an escapement of normal 

 size, but one containing females that are relatively 

 few in number or small in size (young-ocean-age) 

 will bring to the spawning ground a subnormal 

 number of eggs. Hence, with all other factors 

 affecting the return being similar, this escapement 

 would produce a smaller return than would an 

 escapement of fish normal in size, age, and sex 

 composition. 



Sahnon runs to manj- Pacific coast streams 

 contain small fish commonly called "jacks," that 

 have matured after an ocean life shorter than 

 normal for the species and the area. The occur- 

 rence of these precocious fish in the runs of Colum- 

 bia River blueback salmon {Oncorhynchus nerka), 

 known also as sockeye or red salmon, is unusual 

 in three respects. First, they are relatively more 

 abundant here than in most other systems, a 

 feature compounded in the escapement because 

 tile gill net fishery below Bonneville Dam (fig. 1) 

 is selective for larger fish. Second, they spawn 

 mostly in a single tributary, the Okanogan River, 

 although we occasionally observe individuals in 

 the Wenatchce River system, the other major 

 blueback spawning area in the Columbia River 

 watershed. Finally, precocious blueback salmon 

 (age 32's)'in the Okanogan River escapomenis con- 

 tain a relatively high proportion of females.' 



Note.— Approved for publication May 12, 1961. Fishery Bulletin 194. 



The high incidence of 1-year-ocean fish in the 

 escapement of 1953 attracted wide attention. In 

 that year, 52,182 small blueback were counted 

 passing over Rock Island Dam on the main Co- 

 lumbia; this was 34 percent of tlie total count of 

 151,747 (table 1). Escapement to the Wenatchee 

 River system reportedly contained few if any 32's 

 in 1953.' 



The Columbia River blueback runs include fish 

 that mature at ages 3, 4, and 5 years. Altliough 

 exact age-composition data are lacking, age 4 fisii 

 are known to dominate the runs. As a result, the 

 relatively low return in 1957 revived interest in 

 the 1953 escapement. Furthermore, examination 

 of the escapement-return data (table 2) reveals 

 that this cycle is one of only 2 out of 18 cycles 

 observed, in which the return failed to exceed its 

 parent escapement (1940 and 1953 escapements). 



Efforts to explain low returns are ordinarily di- 

 rected toward the isolation of one or more abnor- 

 mal conditions which adversely affected production 

 or survival of the progeny in^ fresh water. Among 

 the factors of potential importance are: (1) un- 

 balanced age and sex composition of the parent 

 stock, (2) unfavorable environmental conditions 

 during the early life history* of the young fish and 

 (3) mortalities connected with seaward migration. 



' This method of designating the age of salmon, developed by Gilbert and 

 Rich (1927), is described by Nelson (1959): 



A fish resulting from an egg laid in the spawning gravels in 1950 and that 

 migrated to the ocean in 1952 and returned in 1955 is called a five-two and 

 designated thus. 5?. Such a fish would have emerged from the gravels of the 

 spawning beds in the spring of 1951 and would have spent 1 grow ing season or 

 summer in fresh water. In referring to its fresh-water history it is called a two- 

 fresh-water fish, because it migrated seaward in its second year. It would 

 have spent 3 full growing seasons, i.e., 1962, 1953, and 1954, and part of a 

 fourth year in the ocean; but in referring to its ocean history it is calletl a 

 three-ocean fish, because it returned as an adult in the third year following its 

 seaward migration. .\ flsli that migrated to the ocean in its third year and 

 returned in its sixth is called a six-three and is designated 63. 



' Personnel communication with Leonard A. Fulton, then project leader 

 of the Columbia River hhiebaciv salmon studies, now with the L'.S. Fish 

 and Wildlife Service. Washington. D.C, and .\Ured C. Oastineau, manager, 

 Leavenworth Xational Fish Hatchery, Leavenworth, Wash. 



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