Assessing the detectability of road crossing effects in streams: mark-recapture sampling designs under complex fish movement behaviours

1. 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. 2. 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 markrecapture sampling designs under different combinations of design and ecological constraints. 3. 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. 4. 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.