Lonzarich, David George. 1994. Stream fish communities in Washington: Patterns and processes. Ph.D.
Field surveys and experiments were conducted in streams of western Washington to describe (1) fish assemblage structure and dynamics (2) habitat use, and (3) factors that influence species and age-class distributions. Based on the expectation that high flows act as a major influence over stream assemblage structure, I set out to study fish assemblages and patterns of microhabitat use by fishes in streams with dramatically different flow regimes. Assemblages in these streams were dominated by cottids, salmonids and cyprinids, and showed little variation in species composition, densities and relative abundance across three summers. These results indicate predictably structured assemblages, especially in two of the three streams, and that flood regimes did not profoundly influence these patterns.
Consistent with the observed predictability in assemblage structure, habitat use and segregation by fishes showed little variability within streams. By contrast, spatial variability in habitat use and segregation was often pronounced. These differences can be attributed in part to differences in habitat availability among streams, but the results of this study suggest that biotic interactions can be very important.
Among the more surprising results in the study of habitat use were those examining patterns of segregation among benthic and water-column species. Interspecific segregation among benthic species was always detectable at the habitat scale (i.e., pools, riffles); however, similar sized water-column fishes typically occupied the same habitat s but often segregated at the microhabitat scale. There was little evidence of intra-specific habitat segregation by adult and YOY benthic fishes but segregation between age-groups in all water-column species was pronounced. Related to this, water-column species often showed very strong size-depth correlations. With only one exception, no such relationship existed for benthic species. A hypothesis is developed to explain the different scales at which benthic and water-column species partition resources which incorporates differences in morphology, behavior and size-specific interactions among species in the two guilds.
Logging and other land-use activities have been associated with the degradation of many streams in the Pacific Northwest. Evidence for the ecological effects of such degradation ahs come primarily from studies on salmonid fishes. Although not directly examined, the ecological consequences of habitat degradation as measured at the community level are inferred from several findings of this study. One consequence is that species richness and assemblage stability appear to be positively correlated with habitat complexity. The loss of habitat complexity may reduce opportunities for spatial partitioning and influence coexistence. Likewise, the loss of in-channel structure and deep water habitat may expose fishes to more intense competitive (Chapter 4) and predatory interactions (Chapter 3) which can lead to shifts in habitat use and assemblage structure.
Streams in Washington sustain populations of several economically important salmonid species, which have been the principal focus of research and conservation efforts. This study, the first of its kind in this region, provides insights into the temporal and spatial variability in assemblage structure and resource use by many species common to small streams of Washington, and offers several reasons for these patterns. The results of this study may have some utility as guides to future research and conservation efforts aimed at stream fish communities in this region