Fisheries Habitat and Fish Populations 



isted between fiy emergence success and the 

 percentage of substrate materials less than 6.35 

 mm in diameter. (See Figure D-2.) Mean fry 

 emergence success was 76, 55, 39, 34, 26, and 

 4 percent, respectively, in cells containing 0, 

 10, 20, 30, 40, and 50 percent materials less than 

 6.35 mm. We measured no distinct trend in 

 emergence timing, and no significant differ- 

 ences in length or weight of fry emerging from 

 the six gravel mixtures. 



Biologists also examined emergence suc- 

 cess of bull trout fry in relation to varying levels 

 of fine substrate materials in a natural stream 

 environment. We simulated natural incubation 

 conditions in a stream (Chapman 1988) by 

 constructing cells with particle sizes, egg pock- 

 ets, and egg planting depths characteristic of 

 natural bull trout redds. We found a significant 

 negative relationship (p < 0.(X)5) between fry 

 emergence success and the percentage of sub- 

 strate materials less than 6.35 mm in diameter. 

 (See Figure D-3.) Mean adjusted fry emer- 

 gence success was 79, 64, 44, 39, 26, and 4 

 percent respectively, in cells containing 0, 10, 

 20, 30, 40, and 50 percent materials less than 

 6.35 mm. A major portion of the observed 

 mortality resulted from fry entombment in the 

 heavier sediment levels. 



Results from these studies showed an em- 

 bryo mortality of about two-thirds when 35 

 percent of the gravel comprising the incubation 

 environment is smaller than 6.35 mm. At 40 

 percent smaller than 6.35 mm, approximately 

 three-quarters of the embryos deposited did not 

 emerge successfully. 



Freeze Coring in Spawning Sites 



Project personnel collected frozen core 

 samples from migratory westslope cutthroat 

 trout redds in Hungry Horse Creek. This work 

 was recommended to verify the locations and 



structure of egg pockets in natural spawning 

 sites (Chapman 1988). This sampling confirmed 

 the setup used for the incubation studies. We 

 found an egg deposition depth of greater than 

 10.0 cm but less than 20.0 cm. We observed an 

 undisturbed layer of large angular particles 

 (mostly greater than 50.8 mm and some greater 

 than 76.1 mm) in the frozen samples beginning 

 around 17.8 cm below the surface and extend- 

 ing downward. It is likely that these particles 

 prevented deeper excavation by the fish and 

 formed the "floor" of the redds. 



In all samples, less fine material (percent 

 less than 6.35 mm) was present in the 10.0 cm 

 depth band containing the eggs than in the one 

 immediately above it. Likewise the geometric 

 mean (Platts and others 1979) and Fredle index 

 (Lotspeich and Everest 1981), two other meas- 

 ures of particle size, were greater in the egg 

 pocket strata. A comparison of the mean values 

 for egg pocket depth bands and the 10.0 cm 

 strata above it showed significant differences 

 for both the geometric mean particle size and 

 the Fredle index (p < 0.05). 



It is possible that the female fish cleaned the 

 substrate in the egg bearing strata in the process 

 of spawning (Chapman 1988). Although em- 

 bryos may incubate in egg pockets having greater 

 geometric mean particle size and less fine sedi- 

 ment, emerging fry must still migrate up through 

 the material covering the egg pocket to success- 

 fully reach the stream. Entombment by this 

 material resulted in the majority of the mortality 

 observed during the incubation studies. 



Field crews collected frozen core samples 

 from bull trout redds in Lion Creek. We found 

 no eggs in these samples, but other interesting 

 observations were made. Researchers have re- 

 ported that some fish spawning in higher levels 

 of fine sediment may build larger redds, but 

 deposit eggs at a shallower depth (Everest and 

 others 1987). Our work supported these find- 

 ings although our sample size was small. 



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Flathead Basin Cooperative Program Final Report 



