Coastal aquaculture provides one of New Zealand’s biggest opportunities to generate new wealth from the primary production sector. Uncertainty about potential environmental effects of aquaculture expansion is a major impediment to realising this potential.
We have developed numerous techniques to determine the local environmental effects of marine farms and the effect of environmental conditions on crop yields.
Sustainable development of marine farming mean providing opportunities for investors while maintaining coastal ecosystem health and integrity. Rapid advances in technology to bring high-value species such as kingfish and rock lobster into commercial culture provide fantastic opportunities to grow export earnings, but uncertainty about the effects that further aquaculture expansion might have on the environment is a barrier to realising aquaculture’s growth potential.
For example, concerns have often been raised in council and environment court hearings that there are too many farms in the Pelorus Sound growing area and that the carrying capacity had been exceeded. However, the complex processes at work have not been analysed and systematic environmental surveying has not been undertaken to back decisions up by scientific data. There is an urgent need for quantitative data and guidelines to show the way towards sustainable development and management of aquaculture.
There were four linked objectives:
- determine the effects of environmental variability on aquaculture areas
- measure the responses of the cultured organisms to that variability
- identify and measure the influences that aquaculture operations have on the environment
- determine the ecological and social significance of those effects and develop approaches to mitigation.
This project has taken significant steps towards ensuring all sectors of New Zealand’s growing aquaculture industry are clean and green. We have shown how climatic drivers correlate with nutrient and phytoplankton production, and how closely linked this was with mussel production in Pelorus Sound over the past decade. This has allowed the mussel industry to plan for variable production through different management practices, which is already bearing fruit for them in the current year in which La Niña conditions have prevailed and lower mussel production was predicted.
Increased ability for industry to predict crop yield and respond accordingly
The analyses confirmed that El Niño-Southern Oscillation (ENSO) affects oceanic conditions in central New Zealand and Pelorus River flow, both of which supply nutrients to Pelorus Sound. Together, these climatic effects strongly influence the abundance of phytoplankton and organic matter underpinning mussel food supply, which in turn affects mussel yield.
This finding was very significant because it confirmed that a major decline in mussel yield from 1999 to 2001 was not caused by too many farms, but by climatic drivers affecting food supply in the growing waters. The analysis identified the key environmental factors on which to base predictions of crop yield, which is of particular relevance to the mussel industry. The research team subsequently predicted good mussel growing conditions during El Niño weather conditions in 2006/07 and this was confirmed by the mussel industry.
The robustness of our predictions was confirmed again in 2007/08 when a dry winter was followed by La Niña weather in spring/summer. Key mussel farmers are now using the results of this project, together with NIWA’s monthly Climate Outlooks, to more accurately project crop harvests for their factories. They are also adjusting farm stocking densities to compensate for predicted changes in growing conditions.
We have developed reliable techniques to measure the feeding processes (e.g., filtration, ingestion, assimilation, egestion) of bivalves such as mussels and oysters in response to variable food supply. These have been used to develop validated ecophysiological models for each species that have then been used to predict shellfish growth and ecosystem responses under a range of farming scenarios.
An unexpected result of our research was the discovery that chlorophyll-a is not a reliable indicator of available mussel food, even though it has been used for that purpose for many years. Instead, the particulate nitrogen content of the water (includes live phytoplankton and detritus) and the carbon content of the phytoplankton cells are much better indicators of mussel food. Our research subsequently uncovered some explanations.
- First, that non-living particles are a significant part of the diet of mussels.
- Second, large portions of the total chlorophyll in the water sometimes comprise very small picoplankton, which greenshell mussels are unable to ingest.
- Third, the nutritional value of the plankton depends on the mix of taxa (diatoms, dinoflagellates, etc.); this mix changes with environmental conditions, and chlorophyll a measurements do not accurately detect such variations.
- Finally, contrary to the popular belief that mussels ingest whatever food is available, we found evidence that greenshell mussels can ingest phytoplankton species selectively, based on food quality.
These findings will allow us to predict crop growth and carrying capacity of aquaculture areas with much greater accuracy than has been possible using models based on total chlorophyll measures. Knowledge of food assimilation processes of cultured organisms is essential to understanding the conditions that produce healthy, fast-growing crops, and knowing what the cultured species put back into the environment is essential to understanding and mitigating environmental effects. Both are needed to balance increased production with maintenance of ecosystem integrity (i.e., sustainable aquaculture).
The cultured animals are key indicators of that balance. We have shown that no two species respond in the same way, but we now have a suite of models that can predict crop yields, pelagic and benthic effects and indicate carrying capacity for a range of cultured species under multiple farming and environmental scenarios. These have been successfully used in more than 100 assessments of proposed marine farm effects and a comparable number of Fishery Resource Impact Assessments.
Limits of Acceptable Change (LAC) framework
The Limits of Acceptable Change (LAC) framework focuses on the level of environmental impact that an activity has under various management settings, to prevent significant adverse environmental impacts during resource use.
NIWA, Environment Waikato and the Wilson Bay Marine Farm Consortium collaborated to implement LAC at the Wilson Bay Aquaculture Management Area (AMA), in the eastern Firth of Thames. This 3,000 ha AMA harbours the largest block of marine farms in New Zealand. To implement LAC, the stakeholders identified tractable indicators of environmental change, agreed upon levels of acceptable change in the indicators, and instituted management responses to apply if monitoring shows they are exceeded.
LAC requires robust monitoring to measure environmental performance relative to the indicators. In a New Zealand first, we used hydrodynamic modelling to design a marine monitoring programme at the Wilson Bay AMA, to ensure the monitoring could separate phytoplankton depletion arising from mussel farms, from that due to natural environmental variation. Using this monitoring, we have shown insignificant phytoplankton depletion at the farm zone since its inception in 2001. This result has been verified by three independent approaches (dynamic biophysical modelling, intensive plankton sampling, and nutrient mass-balance analysis).
LAC was also applied to manage potential effects of the Wilson Bay farm on bottom-living animals, and the associated monitoring has demonstrated insignificant effects in that environment. Given this favourable environmental performance, the flexibility of the LAC framework was demonstrated by a reduction in monitoring requirements that was agreed to by all parties. The LAC management framework has allowed development at Wilson Bay to proceed to new stages in its development under its consent conditions, while providing assurance to the mussel industry and Regional Councils that the activity is environmentally sustainable. We are now working with Councils and stakeholders toward adapting the LAC approach to managing finfish aquaculture.
The interaction between industry managers, resource managers and scientists provides a consistent and transparent management framework for moving forward with sustainable aquaculture. The implementation of the LAC management framework has delivered industry confidence and demonstrable ecosystem integrity. This is its first use in aquaculture internationally, and in New Zealand it will have widespread influence in the $200M mussel industry and in other forms of aquaculture, including the nascent kingfish farming industry.