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Most reviews of stream fish connectivity have highlighted the urgent need for standardized methods to quantify the effects of barriers such as road crossings on fish movement and incorporate the complexity of fish behaviours. A question that has not been fully addressed yet in field assessments of fish stream connectivity is which conditions influence the detectability of road crossing effects. Failure to detect existing road crossing effects can result from shortcomings in sampling design that lead to low statistical power., Here, we propose general barrier dispersal models to allow for asymmetry in barrier permeability and changes in movement behaviours of fish confronting a barrier. Despite the increased realism of these ecological assumptions, it remains to be determined whether asymmetric barriers and altered movement behaviours can be unambiguously detected using mark-recapture trials. We used simulations within a modelling framework that explicitly incorporates barrier and behavioural effects to assess the statistical power of various mark-recapture sampling designs under different combinations of design and ecological constraints., Key insights from our simulations are that (i) the spatial extent of the study reach critically affects detectability of barrier effects; (ii) the number of recaptured individuals that cross a barrier has greater effect on detectability than the total number of recaptures on both sides of the barrier; and (iii) detectability of asymmetry in barrier permeability and of altered movement behaviours increases with both linear fish density and effect size., Synthesis and applications. The proposed dispersal models, incorporating asymmetric barrier permeability and changes in movement behaviours of fish confronting a barrier, are of broad importance in the quantification of habitat connectivity in streams and rivers. Our simulation approach provides precise guidelines for improving the sampling design by adjusting the spatial extent of the study reach based on a priori knowledge of ecological constraints. This study highlights the importance of evaluating the detectability of the effects of barriers such as road crossings and carefully planning the sampling design of mark-recapture studies before conducting costly field trials and provides quantitative tools to help achieve these goals. [ABSTRACT FROM AUTHOR]

Summary 1. Habitat degradation and fragmentation are growing concerns in ecology, yet distinguishing between the effects of these processes can be a challenging task. In riverine landscapes, roads impact fish populations mainly through: (i) restriction of passage of individuals (fragmentation) and (ii) reduction in habitat quality downstream by increases in sediment load (habitat degradation). These two processes can be differentiated during road construction projects. 2. This study examines the impacts of a highway expansion and the presence of existing highway crossings on local population density of brook charr Salvelinus fontinalis in stream reaches traversed by the highway. Density was estimated on three consecutive summers in 212 sections distributed among 36 streams. This extensive sampling design focused on replicated comparison of sites upstream and downstream of highway crossings and included different stages of highway construction (before, during and after) and types of highway crossing (low, intermediate and high passability). Mixed models were used to examine the impacts of the highway on population density. 3. Highway construction activities had no detectable effect on density. However, existing highway crossings appeared to have a strong effect on density, which differed markedly between upstream and downstream sites near highway crossings of intermediate and low passabilities. 4. A Markovian movement model yielded estimates of passability that were consistent with the classification of crossing types and provided evidence for the restriction of upstream movement at crossings as a plausible explanation for observed differences in local population density. 5. Synthesis and applications. Habitat fragmentation resulting from restriction of passage at highway crossings had markedly greater effects on local population density than short-term impacts arising from construction activities. The modelling approaches used in this study can be useful management tools for the conservation of mobile fish species in fragmented riverine landscapes. [ABSTRACT FROM AUTHOR]