Figure U. — Location and U.S. Fish and Wildlife Service reference number of spawning 

 streams in Port Gravina, Prince William Sound. 



Calculation of the intertidal spawning popu- 

 lation required the subtraction of the number 

 of tagged fish passing through the weirs and 

 also the subtraction of the total number of 

 emigrants from the total number of fish 

 tagged. Eighty-seven tagged fish were counted 

 through the West Fork weir and 111 through 

 the East Fork weir. 



Fifteen pink salmon tagged in Olsen Creek 

 were found in the three streams on the west 

 shore of Port Gravina (fig. 11). Nine of these 

 were found in stream number 24, which is in 

 Olsen Bay. No tagged fish were found in any 

 of the streams on the east shore of Port 

 Gravina. 



In the intertidal area 37,524 pink salmon 

 carcasses were examined (339 bore tags), 

 in the West Fork 9,216 were examined (45 

 bore tags), and in the East Fork 12,688 were 

 examined (34 bore tags). 



Calculating escapement at Olsen Creekfrom 

 sample tag ratios resulted in the following 

 estimates, where N represents the population 

 estimate, N the upper confidence interval, and 

 N the lower confidence interval: 



Intertidal 

 N = 100,147 

 N = 111,341 

 N = 90,088 



West Fork : 



N = 17,730 (weir count: 16,888) 

 N = 20,736 

 N= 11,604 



East Fork : 



N = 26,620 (weir couit: 18,870) 

 N = 37,182 

 N= 19,064 



Confidence intervals (N, N) at the 95-per- 

 cent level were calculated using the normal 

 hypergeometric approximation. According to 

 Chapman (1948), this approximation is prefer- 

 able for larger values of n (5,000-10,000 or 

 larger). 



The tag ratio estimate for the West Fork 

 came quite close to the weir count, but the 

 estimate for the East Fork was much greater 



13 



