Habitat fragmentation and nonindigenous species are 2 of the leading causes of species loss globally. Thus, scientific information is needed to assess their interactive effects on the biota of lotic ecosystems. We tested the hypothesis that culverts (i.e., pipes) at road–stream intersections elevate flow velocities and differentially slow upstream locomotion of native relative to nonindigenous crayfishes. We: 1) mapped culvert locations and measured their flow; 2) quantified movement of Orconectes propinquus (native) in culverts with velocities of 2, 31, and 42 cm/s; 3) compared the movement of Orconectes rusticus (nonindigenous), O. propinquus, and Orconectes virilis (native) in a culvert with velocity of 30 cm/s; and 4) used a recirculating flume to determine the water velocity that impeded upstream movement (impedance velocity) of each species. Culverts had higher velocities than upstream riffles during baseflow conditions. Orconectes propinquus moved upstream in culverts with flows >30 cm/s, but individuals slipped more frequently and traveled slower than those tested at 2 cm/s. Orconectes rusticus moved upstream faster in culverts and had an impedance velocity ∼6 cm/s faster than O. propinquus (34.0 ± 1.2 cm/s, mean ± 1 SE) and ∼8 cm/s faster than O. virilis (32.0 ± 1.1 cm/s). Culverts impeded crayfish upstream movement differently among species. Nonindigenous species tolerated higher flows, so culverts may create a filter that favors the spread of invasive species. Culverts should be designed to keep water velocity <30 cm/s to mitigate flow effects on crayfish passage, thereby minimizing the possible combined effects of fragmentation and nonindigenous species introductions.
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