Abstract:
Banded kokopu (Galaxias fasciatus) are a native endemic fish that inhabit many first to third order streams and rivers throughout lowland New Zealand. Their presence is reliant on stream shading, cool water temperature, riparian vegetation cover, adequate habitat and availability of invertebrates for consumption, particularly invertebrates of terrestrial origin. This study tested whether the extent of riparian cover in forested streams affected terrestrial insect contributions to the stream. This study also tested whether varied insect availability in drift then influenced banded kokopu diet.
Eight forest sites were chosen throughout the Waitakere Ranges to represent a range of vegetation cover types, from 65-100% overhead vegetation cover. All sites were assessed during summer, and four of those eight sites during winter, to provide seasonal comparison while maintaining a cross section of the cover variable. Measurements of allochthonous invertebrate drift were obtained during both day and night. Fish gut contents were also obtained from each site.
This study found that significantly higher densities of insects in drift occurred in summer during both day and night time collections. Although many significant relationships were found between cover and insect abundance during day and night at summer and winter, there was no unequivocal evidence to confirm that increases in riparian cover above 65% caused a subsequent increase in the density of terrestrial insects in drift.
Banded kokopu diet consisted of 87.6% terrestrial components during summer and 58.7% during
winter, giving an average diet composition of 73.1% terrestrial insects. There was little evidence to suggest that banded kokopu diet tracked the provision of terrestrial prey abundances in drift. Banded kokopu were selective in their consumption habits, consuming a similar community of insects from the proportion of those available.
Comparison of these results with other studies suggest that abundant food was available to fish in streams utilised in this study, allowing banded kokopu to be selective in their diet choices from the proportion of food opportunities available. However, a lack of suitable feeding habitat and residential habitat restricted the abundance of banded kokopu at these locations. It is possible that where dense populations of banded kokopu exist, increased competition with conspecifics may cause prey consumption to closer track diet availability.