PEARCY and FISHER: MIGRATIONS OF COHO SALMON 



1984 



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SEPTEMBER 



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FORK LENGTH (cm) 



Figure 2.— Continued. 



40 



The Question of "Jacks" 



Are the juvenile coho salmon off Oregon and 

 Washington in late summer relatively nonmigra- 

 tory because they are sexually precocious? GSI 

 were almost always <0.1% for those fish visually 

 classified as "undeveloped". GSI's from fish with 

 "developed" testes ranged from 0.29?^ to 1.0% in 

 July 1981 and 1984 in fish >250 mm FL; from 

 0.3% to 5.6% in August 1981, mostly in fish >280 

 mm FL; and from 2.4% to 6.6% (except for one 

 value at 0.6) in September 1982, 1983, and 1984 

 in fish >300 mm FL (Fig. 3). In August 1981, and 



clearly in September 1982, 1983, and 1984, two 

 distinct groups of fish were evident: "jacks", with 

 developing testes (GSI >0.3% August or GSI 

 >2.0% September), and "nonjacks", which 

 showed no development (GSI <0.1%). 



The total numbers of jacks and nonjacks in each 

 50 mm length group were estimated for the catch 

 during August 1981 and September 1982, 1983, 

 and 1984 from the ratio of jacks and nonjacks in 

 the sample (Table 4). Only 8.4%, 4.8%, 5.2%, and 

 2.8% of all juvenile fish (male and female) were 

 "jacks" in August 1981 and September 1982, 

 1983, and 1984, respectively. However jacks com- 



181 



