ABSTRACT 



This is a study of certain features of the grayling population in Grebe Lake 

 (near the center of Yellowstone National Park) fundamental to its management 

 as a recreational resource. Field work was done in the summers of 1952, 1953, 

 and 1954. 



Grebe Lake is an eutrophic body of water at an elevation of 8,000 feet. It has 

 an area of 145 acres and a maximum depth of 32 feet. The lake was covered with 

 ice fyi months annually ; the highest surface temperature recorded was 67° F. Pre- 

 dominant insects by volume and number were the Tendipedidae ( = Chironomidae), 

 larvae and pupae. By July 14, 1953, an oxygen deficiency in the waters below a 

 depth of 20 feet made them unsuitable for fish life. The grayling was introduced 

 in the lake in 1921. Rainbow trout were planted in 1907, and cutthroat trout in 

 1912 (only hybrid trout are present now). 



In the population estimates of 1953-54 trap nets were utilized along with a mark- 

 and-recapture method. Approximately 28,000 grayling were in the lake in 1953 

 and, 27,000 in 1954. The population of hybrid trout was estimated at 2,000 in 

 1953 and 2,500 in 1954. The standing crop of fish in Grebe Lake each year approx- 

 imated 81 pounds per acre of which 73 pounds per acre were grayling and 8 pounds 

 per acre were trout. 



Both the grayling and the hybrid trout spawned in all four tributaries and in 

 the outlet of Grebe Lake between mid-May and late June. The number of grayling 

 spawning in 1953 was 7,081 and in 1954, 7,878. Enumeration of the trout at the 

 same times showed 674 and 405, respectively. 



During 1954 the natural fry production of grayling in the tributaries of the 

 lake was estimated as 236,500 or 2.5 percent of the estimated number of eggs 

 produced by the spawning adults. The greatest cause of mortality during the 

 early developmental period was attributed to dislodgment of the eggs either by 

 subsequent spawners or by changes in water level. 



The time of annulus formation for grayling younger than 3 years was prior to 

 June 25. Accurate interpretations of age from scales of grayling after their third 

 year was impossible because of the small annual growth both in body and scale 

 lengths. An additional complication often resulted from an absence of a scale 

 annulus to designate the first year of life. This occurred on fish that were too 

 small to have had scales at the start of their second growing season. The grayling 

 reached the legal size of 6 inches during its second summer of life and matured 

 during its third year of life. 



The hybrid trout grew slower than the grayling during its first three years, but 

 faster after age 3. The legal size (6 inches) was attained either in the second or 

 third year of life. Some males of age-group II had matured, but no females 

 matured until they were 3-year-olds. 



Neither the grayling nor the trout was predatory. The food of the grayling 

 during its first year consisted of small nymphs of aquatic insects, amphipods, and 

 Daphnia. The major item in the diets of adults of both kinds of fishes was found 

 to be Diptera larvae and pupae. 



Fishing pressure on Grebe Lake was light with a harvest of only 7.1 percent of 

 the estimated population or 6 pounds per acre in 1953, and 9.9 percent or 8 pounds 

 per acre in 1954. 



The combined mortality from anglers and predators in 1954 approximated 4,200 

 fish or 14.2 percent of the estimated adult population of 29,500 fish. 



