The numbers of 2- and 3-ocean female sockeye 

 salmon in the escapements to the Nushagak dis- 

 trict are known for 1946-57, and the potential egg 

 deposition by each age group has been estimated 

 (table 32). With potential egg deposition in bil- 

 lions as the independent variable ( E ) , the formu- 

 la which describes the relation between return 

 and potential egg deposition is 



R = 1.7456Ee-''-'''»'"^ 



The curves describing this relation permit the 

 estimation of the optimum number of eggs on the 

 spawning grounds. To convert number of eggs to 

 number of spawners, an average fecundity of 

 4,000 eggs per female and a sex ratio of 1 :1 are 

 assumed. This calculation also yields an esti- 

 mated optimum escapement of about 900,000 

 fish. 



Since 1956 the high seas fishery has taken a 

 substantial portion of the Bristol Bay sockeye 

 salmon run each year. We approximated the higlj 

 seas catch of fish from the Nushagak fishing dis- 

 trict by prorating a portion of the estimated high 

 seas catch of each ocean age group of Bristol Bay 

 sockeye salmon according to the ratio of Nusha- 

 gak district returns to the total Bristol Bay 

 returns. The optimum escapement was then com- 

 puted with these corrected returns ; it was again 

 found to be between 0.9 and 1.0 million fish. 



Table 32.— Ntimber of 2-ocean and 3-ocean females in sockeye 

 salmon escapements and estimated potential egg deposition, 

 Nushagak district, 19^6-57 



Two-ocean femalps ' Three-ocean females • „ 

 V Potential 



Year 



egg 



Fish Eggs Fish Eggs deposition 



Thousands Afillions Thousands Mitliojis Million.^ 



1946 2,638 9.600 429 1.840 11,440 



1947 850 3,093 704 3.020 6.113 



1948 109 397 958 4,110 4.507 



1949 32 116 50 215 331 



1950 163 593 158 678 1,271 



1951 110 400 187 802 1,202 



1952 123 448 116 498 946 



1953 126 459 338 1,450 1,909 



1954.. 321 1,168 60 257 1,425 



1955 836 3,042 228 978 4,020 



1956 ,.., 358 1,303 248 1,064 2,367 



1967 U5 418 164 704 1.122 



' The average fecuadities of 2- and 3-ocean females are 3.639 and 

 4,290 epgs on the assumption of mean lengths of 490 and 5n0 mm., 

 respectively (Mathisen, 1962). 



The four reproductive curves, all similar, indi- 

 cate that an escapement of about 0.9 million 

 sockeye salmon to the Nushagak district will 

 yield the highest surplus of return over parent 



escapement. Escapements ranging from 0.6 to 

 1.2 million would theoretically produce about the 

 same surplus because the curve is nearly flat in 

 this area (fig. 22). 



A drastic decline in the retui-ns to the Nusha- 

 gak district occurred in 1946-54. In fact, the 

 returns did not equal the parent escapements in 

 1946-48. The returns from the relatively large 

 escapements of these 3 years may have been 

 abnormally low. If data from additional years 

 indicate that it is more common for relatively 

 large escapements to produce large returns, the 

 calculated optimum escapement will be higher 

 than 0.9 million. 



The escapements of sockeye salmon to the 

 Wood system have averaged about 70 percent of 

 the total for the Nushagak district in recent 

 years ; this fact suggests an optimum escape- 

 ment for the Wood system of about 630,000 sock- 

 eye (70 percent of 900,000). The only other 

 period when escapements were counted for the 

 Wood system was in 1908-19, when adults were 

 counted at a weir on Wood River by the U.S. 

 Bureau of Fisheries (no estimates of escape- 

 ments were made for the other systems of the 

 Nushagak district during these years) . Although 

 the average catch in the Nu.shagak district ap- 

 proached 5 million fish in 1908-19, escapements 

 to the Wood system averaged only 750,000 sock- 

 eye salmon (Bower, 1920). The escapements to 

 the Wood system from 1908 to 1919, therefore, 

 averaged only slightly higher than our calculated 

 optimum based on recent data. After 1919, the 

 average catch in the Nushagak district dropped 

 and remained at about 3 million until 1946 — we 

 have no satisfactory explanation for the higher 

 production in 1908-19. 



INTERPRETATION OF FINDINGS AND 

 APPLICATION TO MANAGEMENT 



In this final section we consider those charac- 

 teristics of the sockeye salmon populations, the 

 spawning grounds, and the nursery lakes that 

 bear on carrying capacity and optimum escape- 

 ment and, finally, give our best estimate of the 

 optimum escapement for each system. The Main- 

 land Systems tributary to Nushagak Bay — Igu- 

 .shik. Snake, Wood, and Nuyakuk — are discussed 

 together. The Kvichak system of the Mainland 

 group and the Peninsula Sy.stems are discussed 

 individually. The Alagnak and Egegik systems 

 are not discussed. 



SOCKEYE SALMON IN MAJOR RIVER SYSTEMS IN SOUTHWESTERN ALASKA 



449 



