The population ecology of stream-living Lahontan cutthroat trout (Oncorhynchus clarki henshawi)

The structure, dynamics and persistence of populations of stream-living Lahontan cutthroat trout (Oncorhynchus clarki henshawi) was studied to examine (1) population characteristics in relation to fragmentation of stream habitats, (2) large-scale patterns of occurrence, and (3) population density in relation to body size.; Habitat fragmentation refers to the breakup of formerly interconnected habitats. Fragmentation increases the degree of isolation among habitats and decreases the average size (area) of habitat. Comparative studies of Lahontan cutthroat trout in fragmented and interconnected stream systems indicated that fragmentation led to a loss of recolonization potential and reduced life history and habitat diversity within isolated streams. Fish in isolated streams also tended to be more sedentary. Habitat fragmentation may thus have an important influence on the life history characteristics of Lahontan cutthroat trout, and on long-term population persistence.; To examine large-scale patterns of persistence of Lahontan cutthroat trout populations in relation to habitat fragmentation, data on cutthroat occurrence in 119 streams throughout its range in the eastern Lahontan basin of Nevada were analyzed. This large-scale analysis involved relating Lahontan cutthroat trout occurrence to a number of stream attributes that may possibly be related to population persistence, including habitat fragmentation, habitat size and shape, presence or absence of exotic salmonid fishes, elevation and precipitation regime. The analysis revealed habitat fragmentation, and possibly size, to be the sole factors associated with cutthroat occurrence.; Finally, relationships between body size and population density of stream-living salmonid fishes in the intermountain west were studied. Recent theoretical work indicated that two mechanisms may produce a negative relationship between body size and population density: intraspecific competition for food and/or competition for space. Overall, results of this study were much more equivocal than previous tests of the theory, and in several cases, no significant relationship between body size and population density could be found. These deviations from theoretical predictions were discussed in light of several practical and theoretical problems in the analysis of body size-abundance relationships.