EXECUTIVE SUMMARY

Waikato Regional Council (WRC) is undergoing development of a Marine Management Model (MMM) for the eastern Waikato coastal marine area (covering the Firth of Thames and wider Hauraki Gulf) that will assist in addressing a range of resource management issues, such as aquaculture development, biosecurity risks, and oil spill response. The first stage of the MMM has included the construction and calibration of an underlying 3-D hydrodynamic model and production of 2-year hindcast datasets on hydrodynamic conditions. This report represents the completion of stage one and focuses on the application of models that utilise the hindcast datasets to in turn forecast potential seabed and water-column effects of finfish aquaculture in the Wilson Bay and Coromandel Marine Farming Zones. The models and outputs described in this report have been made accessible to WRC staff and are intended to assist in the guidance in the consenting and monitoring of aquaculture developments as well as wider state of the environment monitoring in the Waikato coastal marine area.

Finfish aquaculture (e.g. kingfish, hapuka) is likely to be introduced in the near future in the Wilson Bay Marine Farming Zone within the Firth of Thames, and more extensively in the designated Coromandel Marine Farming Zone (CMFZ) in the Hauraki Gulf. The potential extent of effects that may arise from finfish aquaculture and the addition of artificial feed (nutrients) to the environment are relatively unknown for the region and hence is the focus of the initial modelling efforts. The following objectives are addressed:

• Provide a brief background on finfish aquaculture effects and approaches to forecasting effects using modelling tools;
• Describe how the modelling tools can be used in the assessment process for proposed developments;
• Describe the seabed and water-column effect modelling tools, their configuration, strengths and limitations;
• Configure and run the models based on several scenarios and present example results.

There are many different types of potential ecological effects associated with aquaculture, ranging from direct effects on the seabed to biosecurity risks to interactions with marine mammals and wild fish. The types of effects that can be quantified are conducive to modelling applications aimed at forecasting potential effects under a range of development scenarios. In the case of finfish aquaculture, these include effects of organic deposition on the seabed beneath and around the farms and the transport of nutrients in the water column and wider environment.

Seabed effects immediately beneath and in close proximity to finfish farm structures can be reliably quantified using a variety of established indicators (sediment chemistry and biotic) which can then be used to determine an overall enrichment stage (ES). The size and intensity of organic enrichment gradients around farms (commonly referred to as a depositional footprint) is strongly influenced by water depth and current speeds, which together constitute the dispersive properties of a site.

Simple methods and tools are provided to extract bathymetric and hydrodynamic data from the hindcast datasets, and together with information on cage configuration and feed inputs, use a 2-D depositional model (DEPOMOD) to predict depositional flux to the seabed. DEPOMOD outputs are then used to compare differences in the level of solids deposition or flux among modelled locations. These outputs can be used to estimate ES and determine whether initial feed levels fall within pre-determined acceptable limits.

In addition to enrichment of the seabed, feed-added aquaculture results in the addition of dissolved nutrients into the water column, which can be transported long distances from farms and influence important biological processes such as phytoplankton production. As a first step in modelling water-column effects, the transport of nutrients was modelled by releasing passive tracers during the 2-year hydrodynamic model runs. An important aspect of the tracer modelling is that each passive tracer group is modelled individually. This means that when the passive tracer modelling runs are complete, different feed loading scenarios can be quickly combined to investigate cumulative effects from several aquaculture sites and varying river (land-use) inputs. Tracer concentration results from the model can be scaled into real-world units; in the example scenarios, the tracers were scaled to total nitrogen concentration units that are based on inputs from both finfish aquaculture (feed) and rivers.

A range of feed and weather scenarios were carried out to provide example outputs for both the seabed and water-column modelling. The model results showed that the deep, high-flow waters of the CMFZ will assist in mitigating both seabed and water-column effects. In the case of seabed effects, results indicate that the maximum feed levels used in the scenarios for the CMFZ would likely result in effects that fall below the maximum enrichment stage limit used in the Marlborough Sounds.

Model outputs utilising passive tracers assisted in visualising differences in potential nitrogen loading between finfish farms, major rivers, and the ocean. Following runs of two weeks, the surface concentrations of total nitrogen downstream of simulated fish farms were an order of magnitude lower than the surface concentrations associated with the region's major rivers. The initial results for the model also show that the Wilson's Bay Marine Farming Zone inputs have the potential to interact with riverine-sourced nitrogen inputs in the Firth of Thames. WRC's decision to limit nitrogen inputs to the region appear to be sensible given the potential for cumulative effects.

Modelling is by nature an iterative process, whereby models are continually improved and developed. The models described in this report are considered 'first stage' tools for gauging aquaculture effects associated with a range of development scenarios. It is envisioned that the hydrodynamic and aquaculture effects models will continue to be improved, and that over time, additional models contributing to the overall MMM will be further developed and expanded to encompass more complex processes ( e.g. chemical and biotic processes,cumulative effects) and issues (biosecurity risk, coastal hazards, oil spills).