Physiology of Maturation 



The age at which salmon mature and the physiological 

 processes that govern their maturation have been studied 

 for several years. The ability to determine maturation at 

 sea and to predict the year of spawning is necessary for 

 accurate forecasts of runs and for efficient use of stocks. 

 In salmon, the weight of the reproductive organs (gonads) 

 is generally an index of maturity, but it is reliable only 2 

 or 3 months before spawning (fig. 16). Earlier detection 



FIGURE 16.— Biologist removing gonads of freshly caught 

 salmon for weighing aboard ship. 



of maturity — as much as 6 to 12 months before spawning — 

 is now believed possible through biochemical techniques that 

 examine the SM antigen in blood serum and the quantities 

 of hormones in the pituitary gland. 



SM antigen in blood serum. — During sexual matura- 

 tion, new proteins appear in the blood of female salmon. 

 These proteins are produced in the liver, carried by the blood 

 to the eggs, and concentrated in the yolk. One protein, the 

 SM antigen, is present in maturing females but absent in 

 males, immature females, and spawned-out females. 



Our physiologists studied more than 6,500 sockeye salmon 

 between 1961 and 1968. They concluded that the presence 

 or absence of SM antigen in the blood of oceanic female 

 sockeye salmon (caught from late February or early March 

 to time of spawning) separated most of the females that 

 are maturing from those that are not maturing that year. 



Pituitary hormones. — To determine if maturation can 

 be detected earlier than by the SM antigen, we are investi- 

 gating hormones from the pituitary glands (found at base 

 of brain) of salmon. A marked difference in hormonal ac- 

 tivity was found between immature and mature fish. The 

 method is promising because of its sensitivity and potential 

 reliability. 



Identification of Ages and Stocks 



Quick determination of the age and geographic origin 

 of salmon caught at sea is required for effective manage- 

 ment of the fisheries. A Pacific salmon captured in the 

 ocean could have been born in either Asia or North America. 



Because some species of salmon born in a given year do 

 not all mature and return to fresh water after a specified 

 time, fish of different age and geographic origin may be 

 mixed in some catches. Thus, research on ocean distribution, 

 abundance, and utilization of stocks must rely on good classi- 

 fication systems. 



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