Urbanisation influences on freshwater fish distribution and remediation of migratory barriers

Abstract

Ongoing urbanisation in lower stream reaches can adversely affect the ecology of river ecosystems by altering freshwater fish distributions. Diadromous fish species (those that migrate between the sea and freshwater) are particularly affected as unimpeded access to and from the sea is necessary for their life-cycle completion. To investigate the extent of the effects that urbanisation have on migratory freshwater fish species, I compared fish community composition between urbanimpacted and un-impacted waterways in the upper South Island, New Zealand. I also investigated whether culverts were barriers to inanga (Galaxias maculatus) juvenile migration, and experimentally tested different aspects of fish ramp designs for the remediation of juvenile inanga migration barriers. Analyses of fish community composition indicated significant differences between urban-impacted and un-impacted stream reaches. Fish species sensitive to poor habitat quality were generally absent from urban waterways. Inland penetration of fish species was reduced in urban-impacted than un-impacted streams, likely indicating culverts were upstream migration barriers. Laboratory experiments showed that climbing ability of inanga depended on fish size. Climbing also depended on the length and slope of the climb, but not the availability of resting pools. Experiments conducted at 13 natural culverts showed fish could not pass (0% passage) undercut (perched) culverts unless aided by an experimental ramp (44 % passage). To offer successful upstream passage for weak swimming species, culverts should ideally be box-shaped with widths and angles similar to the natural stream bed. Fish ramps should be designed to accommodate species with weak swimming abilities, considering trade-offs in ramp geometry. My results provide a framework for identifying and correcting barriers to diadromous fish passage, allowing managers and ecologists to maintain and enhance fish populations in urban environments.