Changes in river fish assemblages associated with vegetated and degraded banks, upstream of and within nutrient-enriched zones

Abstract

Fish assemblages in two reaches of the Hawkesbury-Nepean River were studied to determine the separate and combined effects of modifications to the riparian vegetation and nutrient enrichment on the composition of fish assemblages. Fish were sampled along vegetated and degraded banks where no vegetation was present, and upstream of and within zones of nutrient enrichment, associated with discharge of treated sewage effluent into the river and run-off from nearby urban areas. Although the species composition differed between river reaches, both the number of fish species and total fish abundance were significantly greater in habitats adjacent to vegetated banks. Five species [Anguilla reinhardtii Steindachner, Macquaria novemaculeata (Steindachner), Cyprinus carpio L., Hypseleotris compressa (Krefft) and Myxus petardi (Castelnau)] were significantly (P < 0.05) affected by riparian degradation, with mean abundances ranging from 3.3 to 13.0 times larger than adjacent to vegetated banks. Species richness, total abundance and abundance of M. novemaculeata and H. compressa were also higher in zones with low levels of nutrient enrichment. Multivariate analyses showed that while eutrophication has a major effect on fish assemblages in the Hawkesbury-Nepean River, even greater effects may result from clearing of riparian vegetation leading to bank degradation.