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Density-dependence and population variability of inanga (Galaxias maculatus (Jenyns)) in a small New Zealand pastoral stream.
I. G. Jowett and J. Richardson
National Institute of Water & Atmospheric Research Ltd, P. O. Box 11 115, Hamilton, New Zealand
Email: i.jowett@niwa.co.nz
The inanga population of a small stream was surveyed over four years to determine changes in density between spring immigration from the sea and their autumn spawning emigration. Over the first three years, inanga numbers declined by 50% to 75% between November and March, with densities of 3.0-10.0 fish per m-2 in November reducing to 1.3-2.6 fish m-2 in March. In the fourth year, the whitebait run was very poor and inanga densities were initially low but gradually increased so that by January the density of inanga was similar to previous years. Although we were not able to identify eel predation or water temperature as significant sources of inanga mortality, we found evidence of density-dependent population regulation, with a significant relationship between inanga density and the amount of usable habitat. The hypothesis that inanga densities were limited by the amount of habitat available was tested by experimental manipulation of in-stream habitat. Removal of in-stream debris, overhanging bank cover and the consequential in-stream habitat changes reduced population densities by a factor of four, showing the importance of in-stream cover and habitat to adult inanga.
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