GRAYLING OF GREBE LAKE 



333 



Table 20. — Spawning populations and estimated fry return 

 to Grebe Lake of grayling in 1954 



i Estimated fry production for Hatchery Creek based on 2.362 percent 

 return in South Creek and expected number of eggs released by spawning 

 females minus the 1,400.000 removed by fish cultural activities. 



Low efficiency in fertilization. — Several authors 

 have commented on the small amount of milt 

 produced by males of the grayling (Brown 1938b; 

 Rawson 1950). However, in nature the efficiency 

 of fertilization seems high. In 59 naturally 

 spawned eggs from Alberta, Canada, Ward (1951) 

 found only two that were unfertilized. In the 

 Grebe Lake hatchery, fertilization, as measured 

 by fry hatch, is usually more than 90 percent. 



Egg predation. — Predation on eggs often reduces 

 the numbers available for hatching in fishes but 

 does not seem to be important in the Grebe Lake 

 system as shown by food studies by me, or Brown 

 (1938a). I recovered only 37 eggs from stomachs 

 of 13 female grayling, and none from 4 trout 

 captured on the spawning grounds. Brown 

 (1938a) found 137 eggs in two Grebe Lake gray- 

 ling taken from spawning weirs and 35 eggs in 

 two fish of six that he sampled in Agnes Lake, 

 Mont. Eggs eaten at spawning apparently rep- 

 resent a "cleaning-up" of those that drift down- 

 stream and would therefore be lost to production. 

 I have never observed a grayling engaged in 

 rooting eggs on the spawning grounds. 



Dislodgment of eggs during incubation. — A factor 

 in the mortality of eggs is movement after they 

 have been laid on the spawning grounds. Botli 

 reproductive activities of the grayling itself and 

 environmental forces are accountable. In areas, 

 such as the tributaries of Grebe Lake, where many 

 adults are crowded for reproduction, it is inevitable 

 that spawnings occur repeatedly over the same 

 sections of stream bottom. After water harden- 

 ing, eggs are not adhesive and, when dislodged by 

 subsequent spawners, are swept downstream. 

 Some of the embryos may be killed by water 

 turbulence and others by sharp contact with the 

 bottom. Still others may be deposited by the 

 current in habitats unsuitable for development. 

 I witnessed dislodgment many times, and Nelson 

 (1954) found evidence of it in the eggs that he 

 collected from pools where the grayling had not 



spawned. In my experiment on Hatchery Creek, 

 829 eggs (of a possible 12,13-9) drifted into the fry 

 collecting basket. That some of these eggs were 

 dislodged by factors other than spawning activity 

 was shown by the fact that 125 of them entered 

 the basket after the adult fish had been removed 

 from the enclosure. Obviously these eggs had 

 become separated from the substratum through 

 agencies other than the action of adults. How- 

 ever, there was no observable movement after the 

 second week of the 5-week collecting period. 

 Slight changes in water level and current velocity 

 seem accountable for part of the displacement 

 which occurred. 



AGE AND GROWTH OF GREBE LAKE FISHES 



A knowledge of the age composition of the fish 

 population as it relates to sexual maturity, legal 

 size limits, and growth rates in other areas was 

 obtained by taking scales from Grebe Lake fishes. 



Scales from hybrid trout and grayling were 

 collected in 1952, 1953, and 1954. A subsample 

 of 15 fish in each 1^-inch size group was utilized 

 in 1952 and 1953. In 1954, five male and five 

 female grayling in each one-half-inch size interval 

 were collected and used. All trout scales obtained 

 were utilized in age and growth studies. 



For use on a micro-projector, imprints of gray- 

 ling scales were made in plastic (0.02 inches thick) 

 by use of a roller press. These plastic impressions 

 were supplemented by water mounts and glycerin- 

 gelatin slides of some scales of grayling older than 

 age-group III when it became necessary to observe 

 whether or not growth had been added in the 

 posterior margin of the scale. Trout scales were 

 cleaned and mounted on glass slides in a glycerin- 

 gelatin medium. 



For calculation of growth histories, trout scales 

 (magnification 82.7) were measured from the focus 

 to the anterior margin along an imaginary hue 

 bisecting the scale (fig. 20). Average scale lengths 

 for each age group were calculated for each previ- 

 ous year of life. These average scale readings 

 were then converted into average fish lengths by 

 the formula : 



Fish length at annulus X= 

 (Total fish length-k) (Scale radius to annulus) k 

 Total scale radius 



The constant (k) is a correction factor derived 

 from the scale-body relation. In 1953 the value of 



