Rationale for changing management determination from Biomass to Total Permissible Dissolved Nitrogen Output

​​Finfish farming in Macquarie Harbour has expanded since 2005, peaking in 2014-15 with a standing biomass of over 20,000 tonnes. Since early 2017, the maximum permissible biomass to be held in Macquarie Harbour has been progressively lowered by the Director, EPA to allow for environmental recovery in the harbour.

In recent years biomass was capped at 9,500 tonnes, however, despite compliance with the standing biomass being maintained, annual feed inputs have gradually increased since 2018. This increase in feed inputs appears to be due to a change in harvesting strategies in Macquarie Harbour and has resulted in an increase in total annual production.

By setting a Total Permissible Dissolved Nitrogen Output (TPDNO), the EPA are confident that this regulatory tool will limit finfish production in Macquarie Harbour.  There are a number of reasons why TPDNO has been phased in as the preferred regulatory tool to limit finfish production in Macquarie Harbour, and these include:

  1. Standing biomass limits cap the total amount of fish being farmed at any point in time; they are not a measure of production over time. Actual production depends on the growing harvest strategies employed on the farms. For example, companies can increase production by extending the growing season without exceeding the standing biomass limit.​​

    Under the previous determination, the industry in Macquarie Harbour was able to increase production by around 10% from 2019 to 2021 while remaining compliant with the biomass limit. The move to TPDNO provides a more robust limit that reduces the opportunity for production creep.

  2. Biomass numbers are derived from daily calculations of estimated growth, mortality and harvesting of finfish. These estimates are based on measurements of small sample sizes, coupled with computer-modelling generated by a company's fish production tracking system. The resulting data outputs cannot be directly measured or independently audited.

  3. The setting of a TPDNO for a defined area over a specified period provides a better regulatory mechanism to cap production because it limits the amount of nitrogen which can be released to the environment via feed inputs.

  4. Dissolved nitrogen output is a calculated value, using standardised methods to reflect nitrogen content in feed and science-based knowledge on the fate of nitrogen in salmon. In a regulatory setting, TPDNO is a more effective tool as it provides a clearer auditing and compliance enforcement pathway compared to biomass limits.

To ensure that transition from biomass limits to TPDNO is based on sound principles, the following rationale was used:

  1. ​​​There is no constant mathematical relationship between a standing tonne biomass limit, which reflects a specific point in time, and TPDNO which sums nitrogen output values over a longer time span. As such, it was not possible to convert the 9,500 tonne biomass limit into an equivalent TPDNO.​​

  2. ​​The TPDNO specified in this determination was calculated from feed input and nitrogen output values available for 2019 and as such, provides for a 10% reduction from 2021 production levels. ​