EXECUTIVE SUMMARY

There is ongoing concern regarding the potential impact of the degraded Taharua River on water quality in the upper Mohaka River. Hawke’s Bay Regional Council (HBRC) has undertaken a longitudinal survey in the Mohaka River to assess the effects of the Taharua River on the ecology of the Mohaka River. The survey involved the collection of benthic invertebrate and environmental data at two sites upstream of the confluence and up to fourteen sites downstream over four sampling occasions. The food base for adult trout is one area of concern which we address in this report through analysis of the invertebrate (density and biomass) and environmental data.

Size-structured invertebrate density and biomass data were summarised and visually assessed. Multivariate analyses were undertaken to detect spatial patterns in invertebrate density and biomass data, and assess changes in invertebrate community assemblages in relation to the environmental data. The relationship between invertebrate density and biomass and potential changes in community composition, in light of the potential effect of invertebrate prey availability to adult trout, were also explored. This involved the application of adult trout prey quality indices recently developed by Cawthron Institute (Cawthron).

Finally, statistical analyses were undertaken to assess whether there was a change in standardised invertebrate density and/or biomass with increasing distance downstream. We found:

• A distinct and statistically significant change in invertebrate community assemblage based on abundance data in the Mohaka River below the Taharua confluence on all sampling occasions. On three occasions this change was driven by an increase in dipteran diversity and abundance associated with increased periphyton cover and biomass. On the fourth occasion periphyton cover and biomass was relatively similar at the downstream sites to upstream, but the invertebrate community structure downstream of the confluence still differed significantly from upstream.
• No indication from the size-structured invertebrate data that food would be limiting to trout up or down stream of the Taharua River confluence. However, the change in community assemblages downstream suggested that a benthic-browsing feeding strategy would be a more successful than drift-feeding.
• On the three sampling occasions, drift-feeding and benthic-browsing trout prey indices indicated the quality of food for adult trout was reduced below the Taharua confluence (for at least 0.5 to 1.0 km downstream).
• Longitudinal patterns in benthic density, biomass and quantitative trout prey index scores, beyond 1 km downstream, were  probably confounded by the inflow of other tributaries or segment-scale influences. The biomass of net-spinning caddis (Aoteapsyche) was significantly higher beyond 3.7 km downstream of the confluence than upstream—most likely due to their association with seston (mainly drifting algae). The pattern in the drift-feeding and benthic-browsing trout prey indices (TPIs) confirmed a significant reduction in the quality of the invertebrate food base for adult trout downstream of the confluence for a distance of typically 0.5 km, and up to 3.7 km depending on occasion.
• The densities along with TPI and QTPI scores in the Mohaka River longitudinal survey compared favourably to 88 rivers sampled throughout New Zealand suggesting a generally good quality of food in the river for trout.