Skip to main content
Critical alert


Together, we can stop the spread! For the latest updates and resources, visit the Government’s dedicated website:

Close alert

Evaluation of the Impacts of Finfish Farming on Marine Mammals in the Firth of Thames

TR 2008/27

Report: TR 2008/27

Author: Dr Samuel Du Fresne, (Du Fresne Ecology Ltd)


Aquaculture in New Zealand is dominated by the GreenshellTM mussel, however it is expected that finfish aquaculture will expand in coming years. Environmental Waikato is scoping a plan change that would allow finfish aquaculture to be developed within existing aquaculture management areas (AMA) currently used for mussel farming. This report seeks to identify those marine mammal species most likely to be at risk from such activities. Impacts, associated risks, and ecological consequences are identified and discussed.

The marine mammal species most likely to be encountered in the Firth of Thames include: short-beaked common dolphins (Delphinus delphis); bottlenose dolphins (Tursiops truncates); killer whales (orca; Orcinus orca); Bryde’s whales (Balaenoptera edeni/brydei); and various species of beaked whales. Additionally, the neighbouring Hauraki Gulf contains a high diversity of marine mammals, including those already listed, as well as: humpback whales (Megaptera novaeangliae); southern right whale (Eubalena australis); pilot whales (Globicephala sp.); and minke whales (Balaenoptera acutorostrata/bonaerensis).

Three possible effects of finfish aquaculture on marine mammals were identified: entanglement; habitat exclusion; and vessel disturbance.

Entanglement will be a greater risk for small cetaceans such as short-beaked common dolphins and bottlenose dolphins. Entanglement risk is currently well-managed by the aquaculture industry in areas of New Zealand where salmon farms exist, and there have been only three known cases of dolphin fatalities after becoming entangled in predator nets in over 25 years. Operational practices and net designs have improved such that entanglement should be a minor risk, however this will need to be monitored.

Habitat exclusion and vessel disturbance are potential risks for many marine mammals that utilise the Firth. A paucity of data makes assessment difficult; however clear mitigation strategies exist should future surveys and monitoring determine these risks to be significant.

Evaluation of the Impacts of Finfish Farming on Marine Mammals in the Firth of Thames [PDF, 371 KB]

  Acknowledgement i
  Executive summary  v
1 Introduction 1
2 Data sources  1
Marine mammal species encountered in the Firth of Thames  3
3.1  Short-beaked common dolphins – Delphinus delphis  3
3.2  Bottlenose dolphins – Tursiops truncatus  4
3.3 Killer whales, or orca – Orcinus orca  6
3.4  Bryde’s whale – Balaenoptera edeni/brydei  8
3.5 Beaked whales (various species)  9
3.6 Known stranding events in the Firth of Thames  10
3.7 Species known from Hauraki Gulf  11
4 Potential effects of finfish farming on marine mammals of the Firth of Thames  12
4.1 Entanglement  13
4.1.1 Description of effect  13
4.1.2  Species likely to be affected - entanglement  15
4.2  Habitat exclusion  15
4.2.1  Description  15
4.2.2  Species likely to be affected – habitat exclusion  16
4.3  Vessel disturbance – ship strike & underwater noise  16
4.3.1 Description  16
4.3.2 Species likely to be affected – ship strike  17
5 Risk and ecological consequences of identified effects  17
5.1  Entanglement 17
5.2  Habitat exclusion  17
5.3 Vessel disturbance 18
Research recommendations  19
Conclusions  19
  References  20
Appendix 1:  Definitions of conservation status used by IUCN 26
Appendix 2: Definitions of conservation status as used by Department of Conservation.  27