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Responses of the fish community and biomass in Lake Ohinewai to fish removal and the koi carp exclusion barrier

TR 2017/10

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Report: TR 2017/10

Author: Grant W Tempero and Brendan J Hicks

About this report

The objectives of this research were to determine the biomass of koi carp (Cyprinus carpio) and other fish species in Lake Ohinewai in 2016 by two-sample mark-recapture methods, and to compare these estimates with similar studies conducted in 2011, 2012, and 2014.

The large-scale removal of invasive fish and installation of the one-way barrier resulted in significant changes to the invasive fish community composition.

Water quality indicators such as nutrient concentrations, Secchi depth and total suspended solids showed no improvement in the 5 years following the reduction in invasive fish biomass, except for chlorophyll a concentration, which declined with decreasing biomass of invasive fish from 2011 to 2014, and returned to previous concentrations as invasive fish biomass increased by 2016.

Reductions in koi carp biomass were not long lasting, and during the period of low carp abundance there was no convincing evidence for a corresponding improvement in water quality and zooplankton and phytoplankton community composition. Koi carp removal did appear to increase the abundance and size of shortfin eels.

Before any restoration programme is initiated it is recommended that a complete assessment of the system is undertaken to determine which factors are driving the decline in water quality. If reductions in invasive fish biomass are deemed necessary for ecosystem restoration, repeated removal programmes appear to be the only viable way of ensuring low biomasses of target invasive species are maintained.

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Responses of the fish community and biomass in Lake Ohinewai to fish removal and the koi carp exclusion barrier [PDF, 2 MB]


  Executive summary
1 Introduction
1.1 Objectives
2 Methods
2.1 Study site
2.2 Mark-recapture
2.3 Water quality sampling
2.4 Data analysis
3 Results
3.1 Fish abundance and biomass
3.2 Population size structure
3.3 Water quality
3.4 Plankton communities
4 Discussion
4.1 Fish population dynamics
4.2 Water quality
4.3 Comparison to previous reports
4.4 Recommendations
  Appendix 1: R statistical code (Ogle 2016) for Lincoln-Petersen fish population estimates for Lake Ohinewai

Appendix 2: Summary of fishing effort for the four mark recapture studies conducted from 2011-2016

  Appendix 3: Population estimates of fish in Lake Ohinewai from 2011 to 2016

Appendix 4: Summary of mean weights calculated from recaptured fish for four two-sample mark-recapture population estimates


Appendix 5: WRC peer review comments and responses by the authors for Technical Report 2017/10