Finfish Farming Effects: Nutrient Balance and Carrying Capacity

To assist Environment Waikato with planning changes that may provide for finfish farming, this study compares new N additions from finfish farming with aquatic ecosystem processes of the Firth, riverine and oceanic additions, and losses through hydrographic export, denitrification (the microbially-mediated loss of N to the atmosphere) and mussel harvest. It combines information from a nutrient mass-balance budget for the Firth and estimates of Firth primary production with estimates of N discharged to the marine environment during fish feeding. Key findings are: 1. On average, riverine supply of inorganic and organic N to the Firth is greater than the supply arising from mixing across the boundary between the Firth and the Hauraki Gulf. 2. The Firth is a strong net sink for inorganic N, indicating that it denitrifies large amounts of nitrogen gas to the atmosphere on a net basis (about 10,800 t N y-1). 3. Nitrogen discharged to the marine environment from fish farming is estimated at 60 kg N per tonne of fish production, using a feed conversion ratio of 1.3 . About 85% of this will be in dissolved forms (ammonium, urea, nitrate, the sum of which is called dissolved inorganic nitrogen DIN here), and the rest is in particulate form. 4. To place the potential N discharged by fish farming into context, scenarios ranging from 1,000 to 10,000 tonnes of fish production1 per year were evaluated at the two FCRs. At a production of 2,000 tonnes and FCR = 1.3, N discharged was estimated to be small relative to other Firth-wide N processes, sources and sinks: These percentages increase by about 25% for FCR = 1.5. 5. The analyses of this report consider the sizes of fish farm discharges relative to Firth-wide ecological processes, sources and sinks involving N. It is certain that N discharged from fish farms, as proportions of areal primary production, denitrification, and loading from other sources (rivers and oceanic) will be much higher local to the WBMFZ than over the Firth-wide scale. 6. If, at the local AMA scale, such discharged N causes significantly increased organic supply sub-oxic conditions could form, threatening fish farming. 7. Mussel harvesting removes N from the ecosystem, estimated at about 6 kg N per tonne of green weight mussel harvested. 8. Because of the focussing of N discharge at the local scale (described above), only mussels growing within the perimeter of effects caused by that focussing will be relevant for remediation. 9. The uncertainties introduced by the focussing of effects mean that the local-scale effects of discharged N need to be examined more closely, and are a strong reason to support better-resolved dynamic bio-physical modelling of the local area, including coupling with sedimentary and oxygen dynamics and effects of mussel harvest. Remediation by other forms of co-culture (e.g., algal, deposit feeders) should be also be considered. 10. It is recommended that defensible, locally applicable ‘limits of acceptable change’ are designated for adaptive management of WBMFZ fish farm development. This should be informed by the modelling and by meta-analyses of known fish farm effects from other studies. Purpose: The purpose of the report is to provide perspectives on the relative magnitudes of ecosystem and farm processes under various intensities of finfish farm development, to inform Council decision-making about sustainability of finfish culture in the region. The primary focus of the study is at the Firth-wide scale, but makes inferences about impacts at the local AMA scale.

Contents Page: Executive Summary 1. Introduction 2. Methods 2.1. Finfish species 2.2. Budgetary approach 2.2.1. Water budget 2.2.2. Salt budget 2.2.3. Budgets of non-conservative nutrients 2.3. Fish feed N discharged to the marine environment 2.4. Nitrogen content of mussels 2.5. Scenario designation 3. Results 3.1. The Firth N cycle 3.2. Quantifying the nutrient budget and primary production 3.3. Assessing influence of finfish aquaculture 3.4. Nitrogen removal through mussel harvest 4. Discussion 4.1. Significance of fish farm N discharge in the context of the Firth ecosystem 4.2. Protection of ecosystem services: Firth vs local scales 4.3. Implications for co-culture 4.4. Conclusion 5. Acknowledgements 6. References 7. Appendices 7.1. Appendix 1: Stoichiometry of N fluxes and calculation of primary production 7.2. Appendix 2: Estimating nitrogen derived from yellowtail kingfish aquaculture 7.3. Appendix 3: Estimating the N content of mussels 7.4. Appendix 4: Accuracy and precision

Firth of Thames and Hauraki Gulf

See below

This study combines data from three sources: - a water, salt and nutrient mass-balance budget for the Hauraki Gulf and adjacent Firth of Thames, based on ship samples obtained in 2001—2002, and protocols developed within the ‘Land-Ocean Interactions in the Coastal Zone’ (LOICZ) programme of the International Geosphere-Biosphere Programme. - Firth primary production, i.e. organic matter fixation, determined from ship samples (Gall et al. 2002; author’s unpubl.information). - N additions associated with kingfish aquaculture from information provided by finfish aquaculture specialists in New Zealand and Australia.

Gaps in collection: n/a Data quality: Varied, see report.

Report available from Environment Waikato website.

Part of general assessment of environmental effects for finfish farming in the Wilson Bay marine farming zone, Firth of Thames.

Complete

Related information: Assessment of Visual and Natural Character impacts of finfish farming. Baysian network model - impacts of finfish farming. National literature review on effects of finfish farming.