A fish-passable barrier to stop the invasion of non-indigenous crayfish

The invasion of non-native signal crayfish into European ecosystems has resulted in a drastic reduction of native European crayfish, with adverse effects on ecosystems and fisheries. This study aimed to determine whether native crayfish can be protected by physical barriers that do not hinder fish migration, but prohibit the upstream migration of non-native crayfish. Laboratory experiments were carried out to test a barrier design consisting of a gently-inclined, smooth, prismatic, cross-channel structure. Barrier efficiency appeared to depend on barrier roughness, barrier slope and flow velocity directly above the barrier crest. The maximum barrier slope that can be climbed by crayfish decreases with increasing flow velocity in a non-linear way. This observation is in agreement with the physics of crayfish locomotion as demonstrated by applying Newton’s laws of motion to crayfish. Contrary to general acceptance, signal crayfish do deliberately deploy their swimming capacities to pass barriers, proving the general belief that crayfish only swim as an escape response to be untrue. This suggests that crayfish are able to pass all barriers regardless of barrier slope or barrier roughness if the flow velocity is below the maximum velocity against which crayfish can swim. Nevertheless, physical crayfish barriers are an effective method to protect indigenous crayfish in streams with sufficiently high flow velocities. Promising barrier locations are pre-existing structures such as fish ladders alongside weirs, where flow velocities are controlled, sedimentation risks are low, maintenance is done regularly and the bed profile is suitable to connect barriers to.