that leave Grosvenor Lake and from the trends in 

 abundance of the populations in Grosvenor Lake and 

 lliuk Arm. Order of magnitude estimates of the 

 number of age fish at these points — Coville River. 

 Grosvenor Lake, Grosvenor River, and lliuk Arm 

 — in July and August 1961-63 are summarized in Table 

 18. The estimate for Grosvenor River in 1963 is based 

 on the relation of the catches in August and September 

 of 1962 (Table 16) and 1963 and the estimated total 

 migration of 1962. There is no evidence that the 

 number of age fish in Grosvenor Lake increased in 

 August, even in 1961 when the migration from Coville 

 Lake was largest. The number of age fish in lliuk 

 Arm increased during the summer each year, how- 

 ever, and the increase was greatest in the year of mi- 

 gration of greatest numbers offish to Grosvenor Lake 

 from Coville Lake — 1961. Observations of even the 

 general magnitude of the migration out of Grosvenor 

 Lake into Grosvenor River are available only for 1962 

 and 1963 (fyke nets were fished in Grosvenor River 30 

 days in 1962 and 14 days in 1963). These observations 

 indicate that more age fish left Grosvenor Lake in 

 1962 — the year when more entered from Coville Lake. 

 All indications are that most of the age fish entering 

 Grosvenor Lake from Coville Lake in July and August 

 continue downsystem into lliuk Arm the same sum- 

 mer. 



The sizes (length frequencies) of the age sockeye 

 salmon that left Coville Lake and entered Grosvenor 

 Lake are similar to those for age fish taken by tow 

 nets in Coville Lake and those leaving Grosvenor 

 Lake. All three differ, however, from the samples col- 

 lected with tow nets in Grosvenor Lake (see length 

 frequency graphs in later section of this paper). The 

 fish migrating from Coville Lake were either not pres- 

 ent in the parts of the lake sampled by tow nets in 

 Grosvenor Lake, or the number present in these areas 

 at the time of sampling (the "instantaneous" number) 

 was too small to be significant in the catches. The 

 latter is likely because visual observations, beach sein- 



ing, and trap netting along the shores of Grosvenor 

 Lake all indicated very few age sockeye salmon in 

 the littoral areas — the area not sampled by tow nets. 



Diel Timing of Migrations 



Although juvenile sockeye salmon usually migrate 

 downriver only during dusk or darkness (Hartman, 

 Heard, and Drucker, 1967), the interlake migrants did 

 not always follow this pattern. A restriction of down- 

 stream migration to the dark period of each day was 

 clearly the case for presmolts in Grosvenor River and 

 just as clearly not the case for similar fish in Coville 

 River. Results of sampling in Coville River in 1961 and 

 1962 and in Grosvenor River in 1962 show the diel 

 timing of this migration (Table 19). In Coville River no 

 consistent differences in intensity of movement 

 occurred — age sockeye salmon migrated in great 

 numbers in both daylight and darkness. In Grosvenor 

 River, however, relatively few migrants were captured 

 during daylight, but large catches were made during 

 darkness. 



Differences in the abundance and size of juveniles in 

 tow net catches near the outlets of Coville and Gros- 

 venor Lakes probably resulted from the differences in 

 their diel migratory behavior. Unusually large catches 

 of age sockeye salmon were made with tow nets near 

 the outlet of Grosvenor Lake on 3 nights during the 

 period when large catches of migrants were made with 

 fyke nets in Grosvenor River. The rate of catch in tow 

 nets decreased progressively as fishing was done 

 farther from the outlet of the lake. The length fre- 

 quency distributions of fish from these large catches 

 were similar to those in samples of fish from Gros- 

 venor River and unlike those from Grosvenor Lake. 

 Although juvenile sockeye salmon were abundant in 

 the sampling area near the outlet end of Coville Lake, 

 tow netting in the immediate vicinity of the outlet did 

 not produce unusually large catches. It appears that 

 migrants accumulated at the outlet end of Coville 



Table 18. --General magnitude of age sockeye salmon in interlake migrations and of lake 

 populations in July and August 1961-63, Covile River-Iliuk Arm area. 



Product of average catch per standard tow and number of standard tow volumes to a depth of 

 10 m. There are about 270,000 such standard tow volumes in lliuk Arm and 200,000 in Grosvenor 

 Lake. 



29 



