Aquatic biodiversity continues to decline as humans modify the landscape. A population-level approach is necessary to address the mechanisms of impairment in urban stream habitats. When estimating population-level parameters, incomplete detection of individuals must be accounted for to ensure unbiased estimates. In this thesis, I examined differences in the detection probabilities of stream fishes, and used estimates of size/age-specific detection probabilities to reduce bias in estimates of the reproductive success of various fish species in urban and forested stream habitats. In Chapter 1, I examined differences in detection probabilities of stream fishes among electrofishing passes and size/age groups in the middle New River basin, Virginia. I also examined differences in detection probabilities between two physiographic regions: the middle New River basin, and the upper Wabash River basin, Indiana; and evaluated differences between single- and multiple-season estimation methods. I found that for most species, detection probabilities do not differ among electrofishing passes, size/age classes, between the two regions, or between single- and multiple-season estimation methods. I used size/age-specific estimates of detection probabilities to remove bias from relative abundance estimates of steam fish populations in Chapter 2. In Chapter 2, I examined the reproductive success of six lithophilic and speleophilic fishes in urban and forested reaches of 2nd-4th Strahler-order streams in the middle New River basin. I found that binary classification is a poor method of quantifying reproductive success, that the age distributions of many lithophilic and speleophilic species are dominated by adult individuals in urban habitats, and that the population growth rates of speleophils is reduced in urban habitats. These results suggest that although detection probabilities may be equal among various sources of variation, managers should verify this assumption before assuming equal detectability. The results also suggest that reduced reproductive success of speleophilic and lithophilic species in urban stream habitats may be a mechanism of their impairment. / Master of Science

Stream fish assemblages exist within stream networks defined by the size and proximity of connected streams (i.e., stream network topology). The spatial position of sites within stream networks may therefore regulate opportunities for fish dispersal to access distant resources or colonize "new" habitats. Such inter-stream dispersal dynamics will influence local fish assemblage structure and the vulnerability of local assemblages to anthropogenic stressors. In this dissertation, I explored the effects of stream network topology on fish assemblage structure in the mid-Atlantic highlands, USA and tested the hypothesis that dispersal would affect the sensitivity of fish-based environmental quality assessments (i.e., bioassessments). In chapter 1, I evaluated the effects of stream networks by comparing fish assemblages between sites with and without large downstream confluences (>3rd order) in western Virginia, USA (i.e., mainstem tributaries and headwater tributaries, respectively). I found that local species richness was higher in mainstem tributaries than headwater tributaries and that these effects could not be explained by variation in local environmental habitat conditions. In chapter 2, I developed and applied a continuous model of stream network topology to explore the effects of downstream size and proximity on local fish assemblage structure within the mid-Atlantic highlands. I found that fish assemblage structure (i.e., Bray-Curtis distances in species abundance) was significantly related to variation in stream network topology up to approximately 9 fluvial km from sites. Chapters 3 and 4 explored the implications of inter-stream dispersal for fish bioassessments. In Chapter 3, I identified 10 fish metrics that corresponded predictably to environmental stressors in the mid-Atlantic highlands. However, headwater tributary assemblages showed stronger relations to local environmental quality than mainstem tributaries, consistent with the hypothesis of riverine dispersal. In Chapter 4, I compared the effects of stream network topology on fish and benthic macroinvertebrate assemblages. Fish metrics were influenced by the size and proximity of connected streams but benthic macroinvertebrate metrics were not. This finding suggests that stream fishes may complement benthic macroinvertebrate bioassessments by indicating environmental conditions at larger spatial grains. / Ph. D.

[Truncated abstract] This study investigated the organisational role of environmental stability on the fish communities that inhabit the Fortescue River, an intermittent and variable system in north-Western Australia. It did so by examining the relationships between pool stability (measured by persistence of water through time, and variation in maximum pool depth through time) and the number and type of species within pools, temporal fluctuations in total fish abundance and intra-specific abundance, population size frequency distributions, and growth rate. It also examined the association between life history traits and the stability of the environments occupied within the river, and the stability of the river at large. The results indicated that environmental stability was the major factor structuring the fish communities. Among-pool comparisons revealed that unstable pools contained fewer species, a greater fraction of juvenile size classes, and underwent greater fluctuations in total and intra-specific numerical abundance through time, than stable pools . . . Stability affected community structure by determining (or describing) the probability that a pool would undergo periods of extreme shrinkage, that is, the likelihood that fish will be exposed to extreme physico-chemical fluctuations and complete eradication. While the physical environment (acting through stability) primarily structured the fish communities of the Fortescue River there was indirect evidence that even within this variable river system, biological interactions played a role, albeit minor. Future investigations into the role of environmental stability will benefit from the use of accepted and quantitative methods by all stream ecologists.