Predicting road culvert passability for migratory fishes

Abstract

Aim Our goal was to predict road culvert passability, as defined by culvert outlet drop and outlet water velocity, for three fish swimming groups using remotely collected environmental variables that have been shown to influence the passability of road culverts. Locatio Laurentian Great Lakes Basin, north‐eastern North America, on the Canada–USA border. Methods We generated four boosted regression tree models, one for road culvert outlet drop and one each for the three culvert outlet water velocities, and predicted the probability of impassable road culverts on low‐order streams (Strahler 1‐4) based on the models. Independent variables in the models included the upstream area draining to the culvert, slope at the culvert, stream segment gradient and stream reach gradient. Results Gradient of the stream segment was the most important predictor in the outlet drop model, while upstream drainage area was the most important predictor in the three water velocity models. A majority of road culverts on low‐order streams are estimated to be passable even for weaker swimming fishes. Moderate to highly impassable road culverts are distributed across many low‐order streams throughout the basin, but particular regions are predicted to have higher densities than others due to topography. Main conclusions Predicted passability of road culverts by migratory fish is related to natural gradients in topography and stream size. While the probability of any particular culvert being impassable is low, the vast number of culverts in the basin means that, together, they could pose a greater challenge to migratory fish than dams. Our modelling framework could be used in any region where culverts are prevalent in the riverscape. The resulting estimates of passability to fishes can guide surveys towards the most problematic hydrological regions and structures and contribute to broad‐scale prioritization of barrier removals to restore ecological connectivity.

Enhancing ecosystem restoration efficiency through spatial and temporal coordination

“…For road crossings, we assumed that all intersections with streams of Strahler order >4 were likely to be bridges (22) and therefore fully passable to migratory fishes (∼7.4% of the road crossings in our analysis). For structures over streams with a Strahler order ≤4, we used structure-specific passability estimates from ref.…”

“…For structures over streams with a Strahler order ≤4, we used structure-specific passability estimates from ref. 22. In brief, field-surveyed data from 2,235 culverts across nine watersheds in the GLB were used to create a statistical model linking culvert passability to geographic information system (GIS)-derived landscape geomorphic covariates.…”

“…For each of these structures, we estimated the direct economic cost of restoring full passability (removal of dams or retrofitting road culverts) and the net upstream habitat that would become available, and we used estimates of the current passability of each culvert (22). Barrier passability is defined as the proportion of fish able to pass through or over a barrier to migrate upstream.…”

Effects of Road Crossings on Habitat Connectivity for Stream‐Resident Fish

“…It is difficult to determine how the frequency of fish passage problems at road crossings in the Pine‐Popple watershed compares with other regions because other inventories used different assessment protocols and only assessed a subset of the crossings in their respective study regions (e.g., Coffman et al ., ; Poplar‐Jeffers et al , 2007; Park et al , 2008; Anderson et al ., ). In theory, the occurrence of high water velocities and outlet drops should be more common in steep terrain, and there is some evidence for this pattern (Park et al , 2008; Januchowski‐Hartley et al ., in press). However, if fish swimming performance varies in parallel with hydraulic characteristics, the result could be a relatively uniform distribution of fish passage problems.…”

Conserving migratory species under human impacts and climate change

“…Indeed, barriers might benefit species which can climb by hindering predators without climbing ability (e.g., estuary-associated species) while meanwhile increasing the predation pressure on other species that live downstream of barriers (Cooney & Kwak, 2013). However, the passability of culverts may also vary with local landscape and stream characteristics such as the gradient of the stream segment and the size of upstream drainage area (Januchowski-Hartley et al 2014). Because many diadromous (e.g., gudgeon, flagtail, goby and eel) and estuary-associated fish (e.g., snappers, jack, trevally and barracuda) are socioeconomically important species for inland and coastal fisheries in Fiji (Jansen, Parkinson & Robertson, 1990; cichlids were recorded .…”