related only in a general way to the abundance of po- 

 tential spawning grounds per unit lake area (Table 8). 

 The largest catches of age fish came from the lake 

 with the greatest amount of probable spawning 

 grounds per unit of lake area — Coville Lake yielded 

 about 96 fish per tow and has 3.32 ha of spawning area 

 per square kilometer of lake. The lowest densities of 

 age fish were generally in basins that had the lowest 

 ratios of spawning grounds to lake area — Northwest 

 Basin has 0.02 ha of spawning ground per square 

 kilometer of lake and North Arm has 0.04. The excep- 

 tion to this is Brooks Lake which has an intermediate 

 abundance of spawning area (0.24 ha), but a low abun- 

 dance of young sockeye salmon. The other lakes had 

 variable catches of age sockeye salmon, seemingly 

 independent of the abundance of their spawning 

 grounds. 



The lakes fall into three groups in terms of abun- 

 dance of age I sockeye salmon (Fig. 6): (1) lakes that 

 never have many age I fish and usually none after 

 July — Coville and Grosvenor Lakes and Northwest 

 Basin; (2) lakes that usually have a few age I fish all 

 summer — Brooks Lake, North Arm, and West End; 

 and (3) lakes that have many age I fish through the 

 summer — Iliuk Arm and South Bay. The last two ba- 

 sins constitute only about 25% of the system's surface 

 area, but contain about 90% of the age I sockeye salm- 

 on in the July to September period. The decline in 

 abundance of age I fish in Iliuk Arm and South Bay 

 each summer is concurrent with the downsystem mi- 

 gration of age I fish into these lakes from Grosvenor 

 Lake and the continued outmigration of smolts from 

 the system via the Naknek River. 2 



Abundance in Each Lake of the System 



The preceding section described in general terms 

 the abundance of juvenile sockeye salmon in the 

 pelagic areas of the system as a whole. This section 

 will discuss the abundance of juvenile sockeye salmon 

 in the lakes and connecting rivers in detail and some of 

 the factors affecting it. To facilitate comparisons 

 among the lakes. Figure 7 shows the number of age 

 fish for the early, middle, and late time periods by 

 sampling unit in each lake of the Naknek system for 

 1961-63. The 1964 data are not shown in Figure 7 be- 

 cause they are complete only for Coville Lake. 



Coville Lake. — Studies of juvenile sockeye salmon 

 in the Naknek system were gradually concentrated in 

 Coville Lake because it seemed to have special fea- 

 tures which would facilitate understanding the 

 dynamics of the population. These features are: (1) an 



Table 8. --Relative abundance of spawning grounds 

 and average catch per unit of effort of age 

 sockeye salmon in early July 1961-63 in lakes of 

 the Naknek River system. 



2 The main smolt migration is complete and sampling was usually 

 ended by late July, but the migration was sampled intermittently in 

 August 1956 and 1958 and to 7 September 1962. The smolt migration 

 extended through August, but involved relatively few fish. (H. W. 

 Jaenicke. National Marine Fisheries Service. Auke Bay Fisheries 

 Laboratory, Auke Bay, AK 99821, pers. comm.l 



abundant population of fast-growing juvenile sockeye 

 salmon and associated species; (2) a single major 

 spawning stock of sockeye salmon; and (3) a narrow 

 lake basin with the major source of sockeye salmon fry 

 at the end opposite the outlet. This combination of 

 characteristics simplified sampling and offered a better 

 opportunity for detecting gradients in biological condi- 

 tions. 



The mean rate of catch of age sockeye salmon in 

 tow nets in sampling units of Coville Lake is shown in 

 Figure 7 for the standard time periods for 1961-63 and 

 in Figure 8 for several time periods for 1961-64. In 

 1961 the abundance differed markedly from the other 

 years in that an early-season (about mid-July) high was 

 not observed. Although it may be that sampling in 

 1961 began after the early-season maximum of abun- 

 dance, the pattern of recruitment of fry from American 

 Creek, the major source of fry to the lake, and mortal- 

 ity in the lake may have been quite different in 1961 

 than in 1962-64, as indicated by the greater abundance 

 of age fish at the end of the summer in 1961 (Fig. 7). 

 The catches of age sockeye salmon in the lake de- 

 clined markedly through the summer in 1962, 1963, 

 and 1964 and were similar at the end of August each 

 year. The analysis of variance showed significant dif- 

 ferences in abundance due to time only in 1963 (Table 

 6). 



Because the major source of juvenile sockeye salm- 

 on in Coville Lake is at the end opposite the outlet, a 

 gradient in abundance and possibly in size of juveniles 

 might be expected. To increase the chance of detect- 

 ing such a gradient, the downlake sampling area — unit 

 C-2 in 1961 and 1962 (Fig. 1) — was divided into three 

 units in 1963 and into four in 1964 (Fig. 2). In Figure 8, 

 C-l is the sampling unit closest to American Creek in 

 all 4 yr and C-2, C-2L, and C-5 are the units closest to 

 the outlet of the lake in 1961-62, 1963, and 1964 respec- 

 tively. 



Neither abundance nor size of age sockeye salmon 

 in Coville Lake showed a gradient from the source to 



16 



