Weed Management Plan for Hornwort in Lake Karāpiro 2016 to 2025 Prepared for LINZ on behalf of the Lake Karāpiro Aquatic Weed Management Group September 2016 Prepared by : D E Hofstra M de Winton For any information regarding this report please contact: Dr D E Hofstra Scientist Aquatic Plants +64-7-859 1812 [email protected] National Institute of Water & Atmospheric Research Ltd PO Box 11115 Hamilton 3251 Phone +64 7 856 7026 NIWA CLIENT REPORT No: HAM2016-071 Report date: September 2016 NIWA Project: BML7201 Quality Assurance Statement Fleur Matheson Reviewed by: Aarti Wadhwa Formatting checked by: Paul Champion Approved for release by: © All rights reserved. This publication may not be reproduced or copied in any form without the permission of the copyright owner(s). Such permission is only to be given in accordance with the terms of the client’s contract with NIWA. 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Contents Executive summary ............................................................................................................. 5 1 Introduction .............................................................................................................. 7 2 Background on the weed issue ................................................................................... 8 2.1 Biology and ecology of hornwort .............................................................................. 8 2.2 Status, impacts and values at risk ............................................................................. 8 3 Feasible control methods ......................................................................................... 10 Diquat ................................................................................................................................ 10 Harvesting ......................................................................................................................... 10 The future for weed control options................................................................................. 11 4 Key stakeholders ..................................................................................................... 13 5 Weed management strategy .................................................................................... 14 5.1 Goals ....................................................................................................................... 14 5.2 Objectives ............................................................................................................... 14 5.3 Key milestones ........................................................................................................ 16 6 Site Prioritisation Framework (SPF) .......................................................................... 19 7 Record keeping, Annual planning and Review ........................................................... 20 8 Risks to implementation .......................................................................................... 22 9 Acknowledgements ................................................................................................. 23 10 References ............................................................................................................... 23 Appendix A Designated activity zones and navigation features for Lake Karāpiro. .......... 25 Appendix B Reference Maps and Images ...................................................................... 29 Appendix C Summary of control methods ..................................................................... 35 Appendix D Framework for site evaluation and prioritisation ........................................ 36 Weed Management Plan for Hornwort in Lake Karāpiro Tables Table 1: Key stakeholder roles, responsibilities and interests. 13 Figures Figure 1: Goals and objectives for weed management in Lake Karāpiro. 6 Figure 2: Comparison of weed control tools - diquat and harvesting. 12 Weed Management Plan for Hornwort in Lake Karāpiro Executive summary Lake Karāpiro is a multi-use lake with intrinsic, cultural, environmental, recreational, sporting and utility values (e.g., hydro-power generation and water takes). These values are threatened by the submerged aquatic weed hornwort ( Ceratophyllum demersum ). Previously, a feasibility report commissioned by Waikato Regional Council in 2015 has described the issues, identified the options and outlined high level goals for weed management over the long term. Development of a weed management plan (WMP) for the lake was identified as the next step that would then be used to inform annual weed control operations. NIWA was engaged by LINZ, to develop a WMP for Lake Karāpiro on behalf of the Lake Karāpiro Aquatic Weed Management Group (Waikato Regional Council, Waipa District Council, Mana Whenua represented by Ngāti Koroki Kahukura, Land Information New Zealand and Mercury). The WMP includes a strategy to deliver on the high level goals and objectives (see Figure 1) for long term management of aquatic weed at Lake Karāpiro. A site prioritisation framework (SPF) is presented to guide annual weed control operations. Risks or barriers to implementation of the WMP are considered along with risk mitigation measures. The SPF is based on criteria for scoring and then ranking sites for control works. These criteria include: 1. External factors or pressures that influence what can be achieved (in terms of weed reduction) and when (e.g., public perception and events calendar). 2. Biosecurity, or the risk of weed transfer from Lake Karāpiro to other lakes. 3. The intrinsic, amenity and utility uses of the lake and the benefits derived from those uses. 4. Environmental dependencies, or the potential for habitat to influence the nature of the weed management outcome using current control methods. The primary risks to implementation of weed management in Lake Karāpiro relate to the multiple uses of the lake, public perceptions and expectations, certainty of funding and although effective, the weed control tool set is small and has its own use constraints. Planning, clear communication, pre and post-treatment monitoring, record keeping and timely review of plans and progress with a longer term review of the WMP (5 years) can be used to mitigate negative impacts from most of these potential risks and barriers to successful weed management in Lake Karāpiro. Weed Management Plan for Hornwort in Lake Karāpiro 5 Figure 1: Goals and objectives for weed management in Lake Karāpiro. 6 Weed Management Plan for Hornwort in Lake Karāpiro 1 Introduction Lake Karāpiro is a dammed riverine lake, the furthest downstream in a chain of lakes created along the Waikato River for hydro-generation purposes. The lake is large, of moderate depth and receives a very high nutrient load due to a rapid flow-through of relatively enriched water. Lake Karāpiro is a multi-use lake (Appendix A, designated activity zones), that is valued for its recreational, hydro- generation, cultural and environmental values. These values are threatened by the development of extensive and persistent hornwort ( Ceratophyllum demersum ) weed beds and the drifting weed that floats downstream. A collaborative group of key stakeholders representing Land Information New Zealand (LINZ), Waikato Regional Council (WRC), Waipa District Council (WDC), Mana Whenua (represented by Ngāti Koroki Kahukura) and Mercury (formerly Mighty River Power) identified the need to explore the options for managing the invasive aquatic plant hornwort in Lake Karāpiro over the long term. To inform the key stakeholder group WRC commissioned NIWA to undertake a feasibility study (Hofstra et al. 2015) to describe the issues for weed management, the available options for weed control, and to outline high level goals for weed management in the lake. The next step was to develop a weed management plan for hornwort in the lake, drawing on the feasibility study, which could be used to guide annual weed control operations. LINZ (with their biosecurity partner Boffa Miskell) have contracted NIWA to develop a weed management plan (WMP) for Lake Karāpiro. This WMP seeks to provide a shared view of hornwort management over the next 10 years (2016 to 2025). The WMP for Lake Karāpiro includes a: ° Summary of the weed issues and feasible control methods, ° Description of key stakeholder roles and responsibilities, ° Management strategy incorporating high level goals and objectives and key milestones to progress the goals, ° Site prioritisation framework (SPF) to inform annual control operations, ° Process for record keeping, and review of the WMP, and ° Risks to implementation are outlined. Preparation of the WMP was undertaken in two phases. A draft or ‘strawman’ Site Prioritisation Framework (SPF) for weed control works was initially developed (Phase One) for discussion with key stakeholder representatives during a workshop (Phase Two, 25 th August 2016). The workshop provided a mechanism to document and incorporate stakeholder perspectives into the SPF and hence into the WMP. Related to the WMP is the establishment