328 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 18.- 



-Potential grayling egg production on various 

 Grebe Lake waterways in 1954 



Note.— Total expected number 10,900,773; maximum, 12,533,944; mini- 

 mum, 9,267,549. 



Fewer hybrid trout were collected for egg counts 

 than grayling. The average number of eggs for 

 each of 10 female trout (average size 12.2 inches 

 total length; range 9.0 inches to 14.8 inches) was 

 approximately 780 eggs (actual count). Potential 

 egg production was derived for the entire system 

 each year (table 19). This expected number of 

 hybrid eggs was about 257,000 in 1953, and 

 143,000 in 1954. 



Egg-Development and Hatching 



Eggs of the grayling become fully water hard- 

 ened at 24 hours and measure 3.74 to 3.85 milli- 

 meters in diameter. Hatching starts on day 16 

 and is completed by day 21 at an average water 

 temperature of 51° F. (range 46°-61° F.) (Wat- 

 ling and Brown, 1955). At Grebe Lake, the 

 earliest hatching is about June 25 in the outlet and 

 the latest, about August 9 on the rest of the tribu- 

 taries except South Creek (latest hatching date 

 observed on South Creek was July 23). Although 

 females spawn approximately in a 1^-montb 

 period, most of the eggs hatch and the fry drift 

 into Grebe Lake within a 10-day period. Thus 

 fry from early spawning adidts apparently have 

 little growth advantage over the fry of late- 

 spawning adults. 



Table 19. — Potential trout egg production in Grebe Lake 

 1953-54 



1 Based on 10 females (average total length 12.2 inches; range 9.0 to 14.8 

 inches,). The range of the number of eggs per female was 335-1,294. 



Determination of Natural Hatching Mortality 



In the Grebe Lake system two methods were 

 used to appraise the efficiency of natural spawning 

 of grayling in producing fry. One was by the use 

 of traps to collect all possible fry of a known num- 

 ber of grayling and the other was by setting drift 

 nets in tributaries at intervals during the time of 

 downstream migration of newly hatched fish. 



Above the intake dam for the fish cultural sta- 

 tion, Hatchery Creek is repeatedly and naturally 

 divided. The many individual tributaries of its 

 system have permanent sources in springs or are 

 of temporary derivation from melting snow. The 

 dam at the station effectively blocks fish from 

 further upstream migration into the tributaries 

 above it. Although ascent may be possible during 

 high water of some spring seasons, no adult fish 

 were observed above the dam. 



The mainstream of Hatchery Creek, 300 yards 

 above the dam, is approximately 2}i feet wide and 

 )'i foot deep during July and August. Since this 

 stream is spring-fed, the temperature remains low 

 and fairly constant in these months with mean 

 daily values approximating 47 degrees F. One 

 section of this creek, about 70 feet long, was 

 separated from the remainder by K-inch hardware 

 cloth screens at its upper and lower ends. The 

 bottom within this enclosure was composed largely 

 of gravel and rubble up to 6 inches in greatest 

 dimension. The current varied with a maximum 

 of about 2 feet per second. Where trash accumu- 

 lated along the screening at the lower barrier, a 

 shallow pool 1 foot long and 1 foot deep was 

 formed. 



Fifty yards downstream from the lower barrier 

 a fry trap of the inclined-plane type (Wolf 1951) 

 was installed (fig. 14). The trap consisted of a 

 board dam from the top of which a screen trough 

 slanted down to the holding pen or pot below the 

 dam. The slant of the trough was adjusted so 

 that most of the water filtered through I be screen. 

 The overflow carrying the fry dropped into the 

 holding pen. Screen for the entire unit (plane, 

 trough, and pen) was 12 meshes per inch. 



On June 22, 1953, five ripe female grayling and 

 five ripe males were placed in the fenced section 

 of the stream. The average size of the females 

 was 11.3 inches (range, 10.4 to 12.0 ins.) and that 

 of the males 12.0 inches (range, 11.2 to 12.3 ins.). 

 When these fish were removed two weeks later 

 (July 2), apparently all had spawned. The 



