A Multi‐Method Approach to Assessing Barrier Effectiveness in Preventing the Spread of Invasive Signal Crayfish ( Pacifastacus leniusculus )

Relieving barriers and increasing free flowing rivers is a global imperative to restore habitat connectivity for migratory fish stocks. While reducing river fragmentation will certainly improve biodiversity, the spread of non‐native species throughout a river system may be facilitated as an inadvertent outcome. Environmental assessment of river systems tabled for barrier alleviation is thus essential to appraise the risk posed by non‐native species on a site‐by‐site basis. Non‐native freshwater crayfish are species of concern globally and are implicated as a stressor to native salmonid populations and multiple invertebrates. Here, we demonstrate and compare the use of trapping, remote underwater video (RUV), baited RUV (BRUV) and eDNA to risk assess the implications of removing in‐stream barriers and invasive signal crayfish ( Pacifastacus leniusculus ) throughout the River Calder catchment, United Kingdom. There was no difference between the detection probability of trapping, remote underwater video (RUV) and baited RUV (BRUV) for signal crayfish. No crayfish were detected by eDNA in any of our sites, suggesting a lack of utility in eDNA for rapid and accurate crayfish species occurrence at our sampling level. Distance from barriers played a key role in determining crayfish presence, with populations of crayfish more likely to be found directly downstream from the nearest upstream barrier. Crayfish abundance was higher at locations with lower Oxidation–Reduction Potential (ORP), although it is unclear whether low ORP is caused by the crayfish. There were 53 barriers encapsulated by our survey; over half (50.9%) of these barriers were found to have invasive crayfish both above and below them, and a further 17% had crayfish directly below them. Both tributaries that had reported presence of native white‐clawed crayfish ( Austropotamobius pallipes ) had several barriers between them and the nearest record of signal crayfish; it is recommended that these barriers remain in place and are potentially reinforced, while those that have been identified as breached be the focus of barrier alleviation. In‐stream barriers may be creating micro‐habitats and relief from strong flow which support crayfish persistence and therefore removal may facilitate flow regimes which are less conducive for non‐native crayfish.