Marine Mammals

The Cawthron Institute has studied all known marine mammal species in Whangarei Harbour and Bream Bay to understand where the risk areas are and what we need to do to minimise or eliminate the impact of our proposal.

Their study has reviewed:

  • Relevant marine mammal species and populations
  • Location of these populations and seasonal behaviours
  • Important habitats, such as nursing or feeding areas

Special consideration has been given to nationally critical and endangered species – including Bryde’s whales; Orca; and Bottlenose dolphins.


The Cawthron experts identified possible direct and indirect effects on marine mammals, including four regular or seasonal visitors to Whangarei Harbour and Bream Bay: bottlenose dolphins; Orca; Bryde’s whales; and common dolphins.

The possible effects include the risk of vessel strike, entanglement in vessel equipment, behavioural or physical impacts due to underwater sounds, and change of habitat affecting the ecosystem or feeding.

Overall, the likelihood of the effects having an impact on marine mammal populations was considered to be low. Provided we follow our expert’s advice in terms of mitigation, avoidance and monitoring possible effects, the overall risk is deemed to be acceptable.

We will be following their advice by making sure:

  • Simple and common sense boating behaviour is followed
  • Taking the ‘best practicable option’ method in choosing a dredge and disposal plan (i.e. one that minimises noise and duration)
  • Implementing a ‘safety zone’ when operating in daylight hours, where if a marine mammal is sighted within a set perimeter of the vessel, dredging activity is suspended
  • Creating a marine wildlife management plan, in partnership with the Department of Conservation, that allows for monitoring by:
    • Conducting visual sightings and periodic passive acoustic monitoring in the area before, during, and after dredging and disposal activities
    • Using monitoring as a learning tool for the future, and to inform any changes needed for future maintenance dredging

If you’d like to know more of the nuts and bolts of the effects these are outlined in the summary document here.

You can read further detail in the related downloads section below.

We’ve spent a lot of time looking at the best options for dredging and disposal. Take a Deeper Dive into how this looks here.

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