318 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



used. Ricker (1948) considered errors introduced 

 through unequal vulnerability of fish of different 

 sizes not to be serious. 



Table 9. — Analysis of variance of mean sizes of fish 

 captured by various methods in 1954 



FACTORS CONTROLLING COMPOSITION 

 OF THE GREBE LAKE POPULATION 



SPAWNING OF GREBE LAKE FISHES 



An initial determinant of fish population size is 

 the reproductive success of its components. In 

 the Grebe Lake system, ample spawning facilities 

 appear to exist for both the grayling and the hybrid 

 trout. Yet, in a search for factors limiting the 

 abundance of the grayling (which fluctuates dras- 

 tically) thought must be given to undesirable com- 

 petition, and possibly to predation by trout on the 

 spawning grounds that are shared in a very con- 

 centrated way with the grayling. 



Most of the information on the spawning popu- 

 lations of fishes in Grebe Lake was obtained from 

 weirs. One of these fish traps was on Hatchery 

 Creek where grayling spawn has been collected 

 annually since 1931 . Additional weirs or blockades 

 were installed in each of the tributaries of Grebe 

 Lake and in the outlet (the Gibbon River) about 

 150 yards downstream from the lake (fig. 5). 



In 1953, when the snow melted, the large volume 

 of water from the surrounding mountains poured 

 into the inlets, clogged the traps, and allowed 

 many fish to bypass them. Only the installations 

 on Hatchery Creek (fig. 8), the outlet (fig. 9), and 

 South Creek withstood the high waters and en- 

 abled analysis of the spawning population. The 

 weirs and blockades (fig. 10) installed in the 

 Grebe Lake system in 1954 were more effective 

 than those of 1953, since improvements had been 

 made in construction and the spring runoff was 

 spread over a longer period of time. 



Spawning Areas 



Grayling and trout spawn in all inlets and in the 

 Gibbon River below Grebe Lake. The area of 

 each waterway available for spawning was esti- 



mated by the following method. During the time 

 of hatching and while fry were still present in the 

 streams, the farthest upstream locations of fry 

 were determined for each tributary. Measure- 

 ments were then made from the first natural bar- 

 rier above these fry to the lake. All distances were 

 recorded by wading the creeks and measuring with 

 a steel tape the distance traveled. On 3 of the 4 

 tributaries, greatest widths were taken at each 

 100-foot interval even to include undercut regions 

 beneath banks. On the fourth, Northwest Creek, 

 and on the Gibbon River, widths were measured 

 every 357 feet. 



The variability of bottom types and of currents 

 made some sections unacceptable to spawning 

 fish. In computing the amount of suitable spawn- 

 ing grounds, the • sections between each width 

 measurement station were evaluated separately 

 and the results were combined to obtain totals 

 (table 10). Visual estimates of suitability were 

 based on: (1) type of bottom (best-to-worst: 

 gravel, sand, and rubble), (2) speed of current 

 (slow or medium current preferred to a fast 

 velocity), and (3) general characteristic of the 

 stream for that section (riffles attracted more 

 spawning fish than did pools). In general, the 

 suitability of a section corresponded to the com- 

 parative numbers of grayling seen spawning in 

 the same or similar sections the preceding spring. 



Table 10. — Lengths, areas, and estimated amount of stream 

 bottoms suitable for spawning by grayling and hybrid trout 

 in Grebe Lake waterways (July 23-25, 1958) 



Time and Water Conditions 



The spawning season for both grayling and 

 hybrid trout in Grebe Lake in 1953 and 1954 was 

 between mid-May and late June (fig. 11). Daily 

 average water temperatures during this interval 

 ranged from 40° to 57° F. (tables 1 1 and 12) . The 

 temperatures mid-lake and just beneath the 



