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Worldwide, human-mediated disruption of river networks by artificial structures negatively affects migratory fish species. To creatively solve part of this problem, we assessed the effectiveness of mussel spat ropes for improving fish passage past perched culvert structures. We used a before–after–control–treatment design to test our hypothesis that relative abundances of ‘young-of-the-year’ migratory fish species would increase following rope installation in a New Zealand stream. Results following remediation indicated a significant three- to four-fold increase in fish abundance relative to a control stream. Although four species (banded kokopu (Galaxias fasciatus), redfin bullies (Gobiomorphus huttoni), longfin eels (Anguilla dieffenbachii) and shortfin eels (Anguilla australis)) were present at both the treatment and control stream sites, the increased fish relative abundance at the treatment site was driven primarily by an increase in ‘young-of-the-year’ banded kokopu. The present study has shown that mussel spat ropes can significantly improve fish relative abundances above severely perched culverts, but may not be effective for all species present. Future use of ropes for evaluating passage efficacy for other species, and for a wider range of barrier situations globally, may greatly extend the application of this cost-effective tool.

The use of mussel spat ropes to enable the passage of banded kokopu (Galaxias fasciatus) past perched culverts was tested under laboratory conditions. A 0.5 m high simulated perched culvert was fitted with one of two UV stabilised polypropylene rope types: "Russet Loop" and "Super Xmas Tree". Four randomised replicate trials using each rope type were conducted to test their respective abilities to provide passage for "fresh-run" banded kokopu whitebait (migrating post-larval juveniles). There was no statistical difference in performance between the two rope types, with both performing equally well over each 3 h trial. A high proportion of fish used the ropes to successfully negotiate the structure (mean >85% individuals per trial irrespective of rope type). The high rate of successful passage over a short time period suggests that these ropes may be an effective, affordable and potentially easy retrofit to perched culverts, particularly for native species with similar climbing capabilities to banded kokopu.

The ecological responses of large rivers to human pressure can be assessed at multiple scales using a variety of indicators, but little is known about how the responses of ecological indicators vary over small spatial scales. We sampled phytoplankton, zooplankton and macroinvertebrates and measured river metabolism and cotton strip breakdown rates (loss in tensile strength) in contrasting habitats along a 21‐km urban‐industrial reach on a constrained section of the Waikato River, New Zealand's longest river. Rates of gross primary production (2.8–7.8 g O2/m2/d) and ecosystem respiration (3.5–12.7 g O2/m2/d) did not differ consistently between near‐shore (2–3 m from river side) and far‐shore (ca. 10 m from side) locations, urban and industrial reaches or between autumn and spring sampling occasions. Rates of cotton decay (−k) ranged from 0.014 to 0.112 per day and were typically faster at far‐shore locations and in the section of river receiving industrial inputs, but slower in spring compared with autumn. Nonmetric multidimensional scaling analysis of phytoplankton and zooplankton data did not reveal spatial patterns relating to pressure or location (embayment, edge, mid‐river). However, the macroinvertebrate ordination suggested distinct communities for the mid‐river benthos compared with near‐shore communities and a distinction between sites in the urban reach and the industrial reach. Our results suggest that large‐river macroinvertebrate communities and cotton decay rates can be influenced to varying degrees by reach‐scale pressures and local habitat conditions. Monitoring designs in spatially complex rivers should account for habitat heterogeneity that can lead to differences in structural and functional indicator responses. Copyright © 2012 John Wiley & Sons, Ltd.

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