Ecology of urban streams
Freshwater fish distribution in an urban environment
Ongoing urbanisation in lower stream reaches can adversely affect the ecology of river ecosystems by altering native fish community structure. For diadromous fish species (i.e., species with a marine larval phase), unimpeded access to and from the sea is necessary for life-cycle completion. Therefore, providing fish with opportunities of upstream passage is particularly important to maintain and enhance fish populations in urban settings, particularly given the prospective growth of our cities. As part of her MSc research, Kati Doehring studied the effects of urbanisation on freshwater fish species by comparing fish species community composition between urban waterways and unimpacted waterways. Kati also investigated the effects of culverts as barriers on species distributions and experimentally tested the remediation of migration impediments by different fish pass designs (Fig. 1).
Figure 1. Road culverts can be major dispersal barriers to migratory freshwater fishes. We experimented with different ramps to try and find an effective fish pass that could be used in urban planning.
Analyses of fish community composition showed significant differences between urban impacted and un-impacted stream reaches with fish species sensitive to poor habitat quality generally being absent in urban waterways. Inland penetration of fish species was shorter in urban impacted than in un-impacted streams, potentially indicating upstream migration barriers by culverts. Upstream–downstream comparisons of fish abundance at 13 culverts showed significantly more individuals downstream, further indicating that culverts may have an effect on fish species distribution (Fig. 2).
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Figure 2. The residuals of the non-linear logistic regression of the mean distance inland
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Laboratory experiments showed that the climbing ability of poor-swimming fish species, such as inanga (Galaxias maculatus), depended on fish size and the length and slope of the ramp to climb, but not the availability of resting pools. Further, in situ experiments conducted at 13 culverts showed that fish could not pass (0% passage) undercut (perched) culverts unless aided by an experimental ramp (43% passage) (Fig. 3).
Figure 3. The number of inanga (Galaxias maculatus) able to traverse ramps, experimentally placed inside road culverts, were counted to try and determine effective fish passes for use in urban stream planning.
These results provide a rigorous framework for identifying and correcting barriers to diadromous fish passage. Kati found that in order to provide successful upstream passage at culverts, fish pass designs should include the knowledge of species with the weakest swimming ability and consider trade-offs in ramp geometry.
People involved in this project
University of Canterbury
Kati Doehring (Completed MSc student)
Angus McIntosh
Cawthron Institute
Roger Young
Nelson City Council
Paul Sheldon
Fish & Game New Zealand, Nelson / Marlborough Region
Neil Deans
Department of Conservation, Nelson / Marlborough
Martin Rutledge
Publications
Doehring & McIntosh 2008. The distribution of native freshwater fish in
the Nelson City Council Area. University of Canterbury report for Nelson City Council (pdf, 1.8mb).
Doehring, McIntosh & Young 2008. Have you seen any fish in Nelson City lately? Poster by the Freshwater Ecology Research Group, University of Canterbury (pdf, 2.6mb).