Igushik, Snake, Wood, and Nuyakuk Systems 



The Igushik, Snake, Wood, and Nuyakuk sys- 

 tems are all tributary to Nushagak Bay, the 

 source of most of the fish taken in the Nushagak 

 fishing district. These systems have a combined 

 area of 867 km.- distributed as follows : Igushik 

 system, two lakes and 74 km.= ; Snake system, 

 one lake and 89 km.= ; Wood system, five main 

 lakes and 425 km.- ; and Nuyakuk system, three 

 lakes and 279 km.- The Nushagak fishery oper- 

 ates on a mixture of stocks from the four sys- 

 tems, and the catch for the district is assigned to 

 each system on the basis of age composition and 

 escapement levels. Therefore, the several systems 

 of the district are evaluated in part as a unit. 



In the early 1900's the annual commercial 

 catch of sockeye salmon in the Nushagak district 

 averaged about 5 million fish. It declined to an 

 annual average of about 3 million in 1920-45 

 and suffered another marked decline in the next 

 10 years. Since 1955 the annual catch has been 

 generally increasing but has not yet reached the 

 pre-1945 levels. 



The recent annual escapements to systems of 

 Nushagak Bay have been distributed about as 

 follows: Wood system, 70 percent; Igushik sys- 

 tem, 23 percent; Nuyakuk system, 5 percent; 

 and the Snake and several minor systems, 2 per- 

 cent. The Nuyakuk and Snake systems histori- 

 cally may have produced at much higher levels. 



Because of the selective action of the commer- 

 cial fishery (larger fish are more susceptible to 

 capture, and the time and route of migration 

 and, therefore, exposure to the fishery may vary 

 among the stocks), fishing mortality has not 

 been uniform among the stocks in the Nushagak 

 district. In 1948-60 the percentages of the large 

 3-ocean adults returning to the Wood, Igushik, 

 and Nuyakuk systems were about 30, 60, and 70 

 percent, respectively (from data compiled by 

 Burgner, footnote 7). 



Studies of migration routes and timing of 

 adult sockeye salmon in Nushagak Bay have 

 shown that runs to the Wood and Igushik sys- 

 tems are partially segregated in the fishing areas 

 (unpublished data of the Bureau of Commer- 

 cial Fisheries Biological Laboratory, Auke Bay, 

 Ala.ska) . There is evidence that in the past, runs 

 to the Igushik system may have been subject to 

 lower fishing intensity than those from the Wood 

 system. Were the intensity of fishing the same 

 on the two runs, a higher percentage of the fish 



going to the Igushik system would be captured 

 because of the predominance of the 3-ocean fish. 



Since .sockeye salmon in the escapement to the 

 Nuyakuk system are primarily 3-ocean fish, and 

 this run of larger fish is presumably fished as 

 intensively as the run of smaller fish in the Wood 

 system, the run to the Nuyakuk system probably 

 suffers a consistently heavier fishing mortality 

 than the run to the Wood system. Over the years 

 this heavier mortality could have caused a de- 

 cline in the relative contribution of the Nuyakuk 

 system to the Nushagak fishery. 



Certain spawning areas within the Wood sys- 

 tem consistently contain more 3-ocean than 2- 

 ocean fish. Thus, fishing mortality may vary be- 

 tween groups within a system as well as between 

 systems. 



Estimates of the capacities of the potential 

 spawning areas in each system in the Nushagak 

 district indicate that the potential area greatly 

 exceeds that used at the current levels of escape- 

 ment. 



The factors that limit production of juvenile 

 sockeye salmon in systems tributary to Nusha- 

 gak Bay appear to lie in the rearing areas of the 

 lakes rather than in the spawning areas. Our 

 studies of the limnology of the lakes of these 

 Mainland Systems indicate that they are gen- 

 erally poorer in minerals than lakes of the Penin- 

 sula Systems and have a higher calcium-to- 

 sodium ratio. The watersheds in Nushagak Bay 

 have mostly sedimentary rocks, which contribute 

 smaller amounts of minerals to the lake waters 

 than do the igneous rocks common in the water- 

 sheds on the peninsula — an area of recent vol- 

 canism. Lakes of the Igushik and Wood systems 

 nevertheless ranked fairly high in primary pro- 

 ductivity. Lakes in the Nuyakuk system had the 

 highest mineral content of lakes tested in the 

 Nushagak district, but rates of carbon fixation 

 were low. Lakes of the Nushagak Bay area have 

 less of an oceanic climate than the other lakes 

 studied — ice covers the lakes longer, and thermal 

 stratification during the summer is more pro- 

 nounced. 



Although the four main lakes in the Wood 

 .system have similar limnological characteristics 

 and similar numbers of sockeye salmon per 

 square kilometer of lake in the escapements, they 

 contrast sharply in the proportion of kinds of 

 spawning grounds. The similarities in numbers 

 per unit of lake area in the average escapements 



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U.S. FISH AND WILDLIFE SERVICE 



