Figure 22. — Curves of apparent growth of age sockeye salmon captured 

 in tow nets in sampling units (combined) of West End 1962-63. 



North Arm. — Although North Arm is the largest 

 basin in the Naknek system (about one-quarter of the 

 system's surface area), apparent growth curves and 

 length frequency curves could not be constructed be- 

 cause so few fish were captured in tow nets. The only 

 sample captured near 1 September that had more than 

 20 age fish was obtained in 1963; the average length 

 of these age fish (Table 21) was close to the average 

 for the system. Only one large sample of age 1 sockeye 

 salmon was taken in tow nets in North Arm and this 

 happened during a daytime test tow on 13 August 

 1963. The average length of 225 fish in this tow was 93 

 mm. The average length of 18 fish taken during regular 

 tow netting in North Arm on 1 September 1963 was 94 

 mm. The average lengths of the age I fish in these two 

 samples were several millimeters longer than the aver- 

 age lengths in similar samples elsewhere in the system. 



Northwest Basin. — Northwest Basin is small and 

 relatively insignificant in the production of sockeye 

 salmon in the system and consequently was not sam- 

 pled as intensively as the other basins. Too few data 

 were obtained to permit construction of growth or 

 length frequency curves. Average size data are avail- 

 able for two dates — 20 August 1961 and 1 September 

 1963 (Table 21). The size of age fish was about aver- 

 age for the system. Age I and older sockeye salmon 

 occurred only occasionally in tow net catches in 

 Northwest Basin and the general size of these fish was 

 similar to those from Iliuk Arm and South Bay. 



Brooks Lake. — Juvenile sockeye salmon were gen- 

 erally as scarce in tow net catches in Brooks Lake as 

 in North Arm, but because more tow netting was done 

 in Brooks Lake data were sufficient to permit con- 

 struction of growth curves. The calculated curves de- 

 scribing the apparent growth of age sockeye salmon 

 in Brooks Lake (Fig. 24) were based on samples of one 

 or more fish. Although the minimum sample size was 

 small, all the points fall close to the calculated curves. 

 These curves show the typical (for the Naknek sys- 

 tem) declining rate of growth in late August. The aver- 

 age lengths of age sockeye salmon on 20 August and 



1 September were generally about average for the sys- 

 tem (Table 21). Age I and older fish were seldom cap- 

 tured in Brooks Lake, but in general they were about 

 the same size as comparable fish in Iliuk Arm and 

 South Bay. 



Causes of Differences in Size of Juvenile 

 Sockeye Salmon on 1 September 



Differences in the size of juvenile sockeye salmon 

 within a year between areas and within areas between 

 years have commonly been reported for other sys- 

 tems. These differences may be due to one or more 

 factors, of which I will consider the following for the 

 Naknek system: (1) real differences in rates of growth, 

 (2) differences in time of recruitment of fry and result- 

 ing differences in number of growing days by a given 

 date, (3) differences in rates of dispersion of large and 

 small or fast- and slow-growing fish, and (4) differ- 

 ences in size of fry at time of emergence. 



Real differences in rates of growth. — Differences in 

 the rates of growth of juvenile sockeye salmon within a 

 system are most likely due to differences in the avail- 

 ability of food and in water temperatures. A reduction 

 in the average size of juvenile sockeye salmon has 

 often been directly or indirectly attributed to large 

 numbers of feeders, both sockeye salmon and other 

 species such as sticklebacks. Some examples in sys- 

 tems of western Alaska are the Wood system (Burg- 

 ner. 1964); the Kvichak system; 11 and the Chignik sys- 

 tem (Narver and Dahlberg, 1964). Examples in other 

 areas are: British Columbia, Babine Lake of the 

 Skeena system (Johnson, 1958) and Cultus Lake of the 

 Fraser system (Foerster, 1944), and the east coast of 

 Kamchatka Peninsula, USSR, Lake Dalnee (Krogius, 

 1961). 



For the seven largest lakes of the Naknek system 

 (Northwest Basin is excluded because of too few sam- 

 ples), the mean surface water temperatures in the 

 month of August, the mean number of age and age I 

 sockeye salmon per tow, and the mean fork lengths 

 of the age fish for the years 1961-63 are shown in 

 Table 22; the mean number per tow of the three 

 species offish most commonly taken in tow nets with 

 the juvenile sockeye salmon (pond smelt, threespine 

 sticklebacks, and ninespine sticklebacks) are also 

 shown in the table. Some of the differences in the size 

 of the age sockeye salmon are probably due to real 

 differences in growth rate. The largest age fish gen- 

 erally occurred in Coville Lake and West End (Table 

 22). These two basins also had the greatest average 

 combined catches of sockeye salmon juveniles and as- 

 sociated species in tow nets and the highest surface 



"Orra E. kerns. 1966. Abundance and size of juvenile sockeye 

 salmon and major competitor species in Iliamna Lake and Lake 

 Clark. 1964 and 1965. Univ. Wash., Fish. Res. Inst. Circ. 66-15. 

 35 p. 



41 



