Conclusions
ˇ Sediment accumulation in clean gravels is approximately 30% higher at the middle reach of this study (Site B), than at the most upstream reach (Site A). The most downstream reach is even worse, with 350% more sediment accumulating in gravels at Site C than at Site A.
ˇ Modelling predictions suggest that spawning success for brown trout is only likely to be achieved at the upstream reach in this study (Site A). However, redd viability is probably close to the limits and may be adversely affected by any further increase in sedimentation rates. Further work is required to verify this.
ˇ Sedimentation rates at the middle reach have increased from levels that were present prior to 1990 and have caused trout to abandon their main spawning area.
ˇ Riparian habitat improvement in the Styx Mill Conservation Reserve may have helped improve trout redd viability in this reach (Site A).
ˇ If sediment inputs to the river are not reduced in the near future, the trout population will probably be unable to replace itself, and the fishery will become locally extinct.
ˇ Possible methods to reduce sedimentation effects on trout reproduction include stricter discharge enforcement, riparian corridor implementation, instream habitat restoration, and dredging deep sediments that have accumulated downstream.
Recommendations
Presently, spawning in the upper reaches of the Styx River remains viable, but is probably vulnerable to any increase in sediment inputs. It is not likely that trout will be able to successfully spawn again in the middle reach unless significant reductions in sediment inputs are achieved.
A combination of all of the suggested management options is probably necessary to restore the health of the Styx River, and ensure that it remains home to a diverse range of fish, invertebrates, and birds. However, abating sediment-laden discharges, and providing effective riparian protection from stock and non-point source pollution are immediate priorities. Towards this end, monitoring of stormwater discharges to the river should be increased, and fences to prevent livestock access to the river should be constructed. Options to undertake significant riparian planting along the reaches of the river above, and adjacent to, the remaining gravel-bottomed habitat should also be investigated.
Acknowledgments
I would like to express my thanks to several people who have assisted me during the course of this project. Mark Taylor at NIWA for his help, as well as for sharing data on trout redd distributions, copies of relevant technical reports, and for use of the GPS unit. Kelvin Nicolle and Jonet Ward at Lincoln University supervised the project and provided logistical assistance, monitoring equipment, and most of the icecream containers. Thanks to Trish Lockington (Environment Canterbury) also for kindly making data available from the flow recorder at Radcliffe Road. Particular thanks to Christine Heremaia at the Christchurch City Council (Water Services Unit) for her assistance and for arranging funding to cover some of the petrol costs associated with this project. I would also like to thank Lyn Torgeson (Lincoln University) who helped clarify some issues associated with hydrological processes. Finally, I would also like to thank my wonderful fiancé, Karen Rounds, for her willingness to review the manuscript.