Learning the ropes: mussel spat ropes improve fish and shrimp passage through culverts

Abstract

Summary Culvert pipes are regularly used around the world for conveying stream flows underground, through embankments or under road crossings. Installation of these features can have significant negative effects on the passage of freshwater biota and potentially exclude many species from large areas of river networks. We investigated the installation of mussel spat ropes as a potentially rapid and cost‐effective tool for improving passage of freshwater biota through culvert pipes where internal barrel conditions impede passage. We assessed passage success for two fish species, juvenile rainbow trout Oncorhynchus mykiss (Walbaum 1972) and adult inanga Galaxias maculatus (Jenyns 1842), and one migratory shrimp, Paratya curvirostris (Heller 1862), through culverts of differing length (3 and 6 m), slope (1·5 and 3°) and flow (0·24 and 0·75 L s−1). We hypothesized that ropes would enhance the passage success of these three species, but success rates would differ between species and trial combinations. Ropes resulted in a reduced water velocity within culvert barrels and significantly improved passage success for all three species. Shrimp benefited most by the presence of ropes, being unable to negotiate any of the pipe combinations in their absence, but exhibiting varying rates of success across all combinations with their presence. Both G. maculatus and O. mykiss were able to negotiate some of the non‐roped pipe combinations, but as the level of difficulty increased, successful passage was only achieved with the ropes present. Synthesis and applications. We conclude that this relatively inexpensive and easy‐to‐install tool has the potential to substantially improve passage for a range of aquatic biota through various culvert scenarios. We consider that ropes would be particularly useful in situations where internal culvert access is difficult and where various culvert parameters (slope, flow, length) result in internal barrel hydraulics that would normally limit or exclude passage of aquatic biota.