Ōkahu Bay

No, please don’t! Even if you happen to be uri o Ngāti Whātua Ōrākei. Chances are you’ll get really, really sick. Kūtai accumulated heavy metals, contaminants and bacteria in their flesh and Ōkahu Bay frequently is on the Safeswim high-risk list.  Our hope is that our grandchildren’s grandchildren’s grandchildren will one-day have tasty, edible, safe kaimoana from the Waitematā.

Ōkahu was once an important source of kai moana for Ngāti Whātua Ōrākei, with large shellfish beds of pipi and tuangi / cockles.  Historical records from mussel dredging show that there were dense beds of green-lipped mussels in the Rangitoto channel and other places in the Waitematā. It’s not clear how close to the Ōkahu shoreline these beds came, although we have found some large kūtai shells in and around the area we are working.  All up, over 500 sq.km of kūtai reefs were dredge fished from the inner Hauraki Gulf between 1910 – 1965.

Generations upon generations of kūtai went into forming the reefs. Fishers in the day recall the kūtai coming up like ‘rolls of carpet’. Marine scientists believe this fundamentally damaged the habitat, removing structure critical to the settlement of kūtai larvae / spat like filamentous algae, hydroids and hard shell surfaces. Sedimentation adds to the challenge, where poor land use practices have resulted in the loss of filtering wetlands and deforestation allowing large volumes of sediment to flow into the Gulf, making the water more turbid and the seabed conditions more muddy.

Despite shellfish beds being one of the most threatened marine habitats on Earth, there is still a lot to learn about shellfish bed ecology. Our subtidal, soft-sediment kūtai (Perna canaliculus), a species unique to Aotearoa / New Zealand, present some specific challenges, with different habitat needs across its lifecycle when compared to well studied shellfish such as oysters.

A single adult kūtai can filter between 150–350 litres of seawater per day.  Assuming each of our six plots has around 700,000 mussels that’s about 630+ million litres of filtration per day! It sounds big, but let’s not get too excited, it’s a drop in a bucket considering the volume of water in Ōkahu (and tidal currents etc). However, we will be keeping an eye out for any localised improvements in turbidity across the beds. Biodiversity improvement should be more apparent and we expect to see more species, like crabs and shrimps, juvenile fish and starfish to show up around the beds.

The presence of kelp (in our case Ecklonia radiata) is known to reduce the accumulation of sediments, reduce predation by starfish, and there is also evidence that kūtai growth is enhanced. By monitoring the health, growth rates and biological communities, it is hoped that the study will provide evidence that kelp can help to establish kūtai reefs.