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TABLE 2. Mortality of brown trout fry incubated with rugose plastic sub- strate and two treatments of smooth-screened substrate. Mortality is cumu- lative, covering the time from first feeding to seven weeks. Cumulative Mortality Plastic substrates Smooth screen, trays Smooth screen, baskets 3.9% 5.6% 7.7% DISCUSSION Gravel in the redd plays a particularly important part in creating the appropriate environment for salmonid eggs and developing alevins. The chinks between stones provide resting places where alevins can remain still. Given such inactivity more of the yolk material should be converted to body tissue and less should be expended on maintaining vital functions. Similarly, hatchery incubation systems providing a rugose substrate that simulates gravel appear to produce fry which have a larger size and which may have a greater capacity for growth and survival than do fry raised in smooth-substrate incubators. Bonney and Leon (1979) found that Atlantic sal- mon alevins incubated in matrix substrates were 45% heavier at swim up than alevins incubated in smooth bottomed units, and that mortality rate was only 4% after sixty days of feeding. The control group, incubated without matrix substrates, had a mortality rate of 22% during the same period. Emadi (1971) found that frequent rubbing of the yolk sacs of pink, chum and chinook salmon on smooth screen incubators caused an abrasion that allowed water to enter the yolk sac causing coagulation, elongation and malformation. This condi- tion interferred with the normal circulation of blood and was a contributing cause of mortality. Both Snyder (1977) and Fuss and Johnson (1982) found that by adding Bio- rings, Bio-saddles or Vexar netting to Heath Tecna incubators the quality of chum salmon fry could be significantly improved. Fry were 22% larger at swim up and did not exhibit the constricted yolk sacs found in fry from the smooth- substrate control groups. Unpublished data at the Durango State Hatchery in Colorado (Colorado Division of Wildlife) and the Whitman Lake Hatchery in Ketchikan, Alaska show similar results with Kokanee and Chum Salmon. Our observations with brown trout alevins are very similar to those of others. It appears that rugose substrates create a more suitable hatchery environment for incubation, and consequently the incidence of yolk sac mal- formation and coagulation may be reduced. Subsequent growth should be in- creased and mortality should be reduced by incubation systems that produce healthy alevins free of yolk sac afflictions. ACKNOWLEDGMENTS Thanks to the Wyoming Game and Fish Commission for supplying brown trout eggs. Thanks to all th~ personnel at the Rifle Falls Hatchery for their cooperation, and to Larry Harris and Kathy Hector for their technical assistance. 89