312 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



of cutthroat or rainbow trout. The hybrids have 

 the general appearance of rainbow trout, but 

 with a red slash on each side of the hyoid mem- 

 brane. Hyoid teeth, a cutthroat characteristic, 

 were present in all specimens examined. Body 

 forms range from the typical elliptical shape of the 

 cutthroat to the blunt, stocky, high-shoulder 

 outline of the rainbow. 



Table 3. — Annual removal of grayling eggs and stocking of 

 Grebe Lake, 1931-54 



[Moflett, 1950; stocking records are In Chief Ranger's Office] 



1 Remainder lost. 



' Data on spawntaklng activity is unavailable for these years. 



Grayling were first planted in Grebe Lake in 

 1921, when a million eggs were brought from 

 Anaconda, Mont, (record cards in Chief Ranger's 

 Office, Yellowstone Park). The species was 

 gradually established in the lake and by 1930 

 large spawning runs were observed in the various 

 inlets. In 1932 a few eggs were taken artificially 

 on an experimental basis for hatching by the U. S. 

 Bureau of Fisheries. Results were so encouraging 

 that a permanent hatchery was established at the 

 lake. In 1933, more than 2 million eggs of this 

 species were taken from Grebe Lake (Leach, 

 1934-35). Region II of the U. S. Fish and Wild- 

 life Service and particularly the Park itself re- 

 ceived most of the fish produced. The yearly 

 take of eggs and return of eggs and fry to Grebe 

 Lake (table 3) show how well the species estab- 

 lished itself. An extremely low population oc- 

 curred in 1949 when so few spawning fish entered 

 Hatchery Creek weir that only a portion of the 



grayling egg commitments by the fish culture 

 station could be filled. The lake was then closed 

 to fishing; and restocking, which had been rather 

 neglected for the previous 5 years, was undertaken 

 vigorously. 



RECENT POPULATIONS IN GREBE LAKE 



Knowledge of population size and composition 

 (year-class and species) are fundamental to the 

 prediction of angling success. Since the Grebe 

 Lake problem involved species interaction, knowl- 

 edge of population size was particularly important. 



POPULATION ESTIMATES 



Populations of grayling and trout were approxi- 

 mated by the Schnabel (1938) formula as modified 

 by Chapman (1952). With a population esti- 

 mate of this type, fish are captured in successive 

 time units, unmarked fish are marked, and all 

 individuals are released. The ratio of tagged 

 fish (s) to total captures (n) in any sample pro- 

 vides an estimate of the ratio between the total 

 number of tagged fish in the population at the time 

 of the sample (t) and the total population (N). 

 As the numbers of marked fish in the population 

 increase during the experiment, the ratio of s/n 

 also increases subject to sampling errors. Schna- 

 bel used the method of maximum likelihood to 

 combine these sequential estimates into single 



weighted mean such that N = 



nt 



Chapman (1952) stated that for an unbiased 

 estimate of the true population the formula should 



be N = 



nt 



3+1 



With the use of either the Schnabel 



formula or this modification the author indicated 

 that the confidence interval on N could be deter- 

 mined from the Poisson approximation given by 

 Chapman (1948). 



In 1953, the equipment used for the population 

 study consisted of 5 impounding nets. Two were 

 fyke nets on hoops 3% feet in diameter; each 

 unit was 12 feet long, was made of 1-inch mesh 

 (mesh sizes are stretched mesh), and had two 

 throats. Each net was also equipped with 12- 

 foot wings but no leader. The third hoop fyke 

 net was 4 feet in diameter, 15 feet long, and of 

 2-inch mesh. The wings were 50 feet in length 



