Sustainable Aquaculture - Key Outcomes to June 2009

Here we provide a detailed summary of key outcomes from the Sustainable Aquaculture programme to 30 June 2009.

Predicting crop yields from environmental conditions

Extensive research on relationships between climatic and oceanic conditions and mussel yields in Pelorus Sound have resulted in increased ability for the aquaculture industry to predict crop yields and respond accordingly.

Our analyses confirmed that the 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.

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.

Predicting shellfish growth and ecosystem responses

Sustainable aquaculture is about balancing increased production with maintaining ecosystem integrity. The cultured organisms are the key to this balance. 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.

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.

We have shown that no two species respond to their environment 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 models have been successfully used in more than 100 assessments of proposed marine farm effects and a comparable number of Fishery Resource Impact Assessments.

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 (which 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:

  • Non-living particles are a significant part of the diet of mussels.
  • Large portions of the total chlorophyll in the water sometimes comprise very small picoplankton, which greenshell mussels are unable to ingest.
  • 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.
  • 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.

Determining the environmental impact of aquaculture operations

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.