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-26- The previous report (Nehring 1991) alluded to the dynamic interaction between habitat availability and size specific competition, Living organisms (trout in this case) rarely respond instantaneously to environmental stress. Rather, the impacts of most habitat stress factors have a slow, cumulative effect over an undefined period of time. These effects may be manifested as increased mortality rates among the most susceptible components of the trout population. I hypothesized that agonistic encounters and aggressive interactions among conspecifics (different sizes of trout of the same species) and between different species would increase in frequency during periods of severe habitat restriction or as density per unit habitat increased. These sorts of interactions have been documented for brown trout (Bachman 1984) as well as between different sa1monid species (Fausch and White 1981, 1986; Fausch 1988). It is reasonable to assume that the competition for physical habitat, i.e., "living space" would be most severe among those size groups of trout with similar habitat requirements. Data presented in the previous report (Nehring 1991) clearly demonstrated the largest declines in density and biomass occurred among the 20-40 cm (8-16 inch) size classes in 1990. Figure 2 shows the habitat suitability index (HSI) curves for brown trout fry (s 50 mm), fingerlings (50-100 mm), juveniles (100-200 mm) and adults (~201 mm) for water depth and velocity. These graphs indicate there should be the least amount of direct competition (or overlap in habitat requirements) between fry and adult brown trout, then fingerlings and adults, and somewhat greater competition between juveniles and adults. However, competition should be the greatest between the small adults (ages 2 and 3) and the largest adults (ages 4-6), since habitat preferences for these two groups would be virtually identical. These brown trout HSI curves are provided for explanatory purposes only. The relationships among the various life stages for rainbow and SRC trout would be similar. Table 13. Dolores River (below McPhee Dam) rainbow and brown trout age specific mortality rates versus mean age specific mortality rates for Colorado's Gold Medal trout streams. Cohort (Age in Years) River 2 3 4 5 Brown Trout Dolores 0.57 0.65 0.76 0.85 Gold Medal" 0.32 0.42 0.63 0.79 Rainbow Trout Dolores 0.54 0.72 0.45 0.56 Gold Medal 0.38 0.33 0.55 0.59 " Gold Medal mortality rates taken from Table Al in Espegren, Miller, and Nehring 1990.