Case study - Responding to a new marine pest

In 2007 large ascidians (Pyura doppelgangera) were found covering some rocks in the Far North. By May 2009 these ascidians were covering a much wider area than the original detection site. Using the Biosecurity Response Knowledge Base, MPI undertook an assessment of the potential impacts to New Zealand’s core values of the invasion. This indicated medium-level risks to the economy, the environment, social and cultural well-being and Māori values. The values at risk were assessed as outlined in the figure below.



  • Medium risk to the aquaculture industry, in particular, mussel farming (estimated to be worth $204m in 2008)
  • Approximately 80 per cent of the mussel spat that supplies the mussel industry is collected from the Ninety Mile Beach area, which is in the vicinity of the known location of the risk organism
  • The origin of the mussel spat supply is unknown, as is the contribution of intertidal mussel populations in the Far North to spat supply
  • The ascidian can form large, dense colonies, crowding out adult mussels in rocky intertidal areas
  • The local spat collecting economy could also be impacted
  • Pyura could inadvertently be spread to mussel farms via the seaweed bearing the mussel spat


  • Medium risk to native marine species and ecological processes
  • In Chile, a similar non-indigenous Pyura species is now the dominant species in the mid to lower intertidal area with the native mussel confined to the mid to upper intertidal area
  • Observations in the Far North showed that Pyura was the dominant organism in some rocky intertidal areas, suggesting that it had the potential to cause local-scale ecological disturbance to rocky intertidal communities, including competing with mussels

Social and cultural well-being

  • Medium risk
  • Recreational mussel gathering may be affected if Pyura displaces or significantly reduces mussel beds
  • Aesthetic values may be affected as Pyura forms dense mats, changing the intertidal landscape
  • On the other hand, the sea squirt may be used as fish bait, as is currently the case in Australia and South Africa for related species

Māori values

  • Medium risk
  • Iwi in the Far North rely on locally harvested shellfish
  • Cape Reinga is spiritually significant and the proximity of the detection of Pyura is of concern to iwi

A survey was undertaken to determine the extent of the infestation. The survey showed Pyura was present at many locations on the west and eastern coasts of the Far North, suggesting the species had been present for some time prior to the initial detection. MPI decided that eradication could not be achieved, given the widespread nature of the incursion. However, local iwi and stakeholders were very concerned about the marine pest and MPI decided to trial the feasibility of local elimination at three sites in the Far North beginning in 2010.

The results of the trial suggested that periodic clearances of the ascidian could suppress the populations at specific sites. The field team was comprised of personnel from iwi (who received training in marine biosecurity and field research methods, MPI and Northland Regional Council, and DOC contributed resources. The Te Hiku o te Ika Fisheries Forum, comprising representatives of mandated iwi organisations, participated throughout the trial. The Forum developed a marine biosecurity strategy for the Far North and is currently working with the Northland Regional Council to develop a long-term management plan for this marine pest.

Last updated at 1:26PM on February 6, 2018