Environment – Protecting gulf’s marine life

All of the sea around the island is now part of a Seafloor Protection Area (SPA), while the south-western part is a High Protection Area (HPA). Fishing, seafood collection and any disturbance of sea habitats is banned in HPAs. In SPAs trawling, dredging and Danish seining and disturbance of sea habitats is banned.

The aims of these and other Protection Areas around the gulf are to protect, restore and enhance marine life in the gulf by reducing human impacts. But how will we know if they are working?

Fortunately for those interested in Tiritiri Matangi, my colleague from the Supporters, John Sibley, has been monitoring the marine environment since 2020 and has collected some amazing details on the state of the seas and the plants and animals in the near shore.

Starting with the big picture and narrowing down to local impacts, the anticlockwise South Pacific Gyre brings warm nutrient poor water down the east coast of Australia and across the Tasman Sea to northern New Zealand. A branch called the East Auckland Current hugs the east coast of northland, flowing past the Hauraki Gulf and entering the Bay of Plenty. So, our east coast and gulf waters are typically warm and nutrient poor, and these currents tend to bring some tropical species to our shores. Climate change has increased the temperature and flow rate of the currents since the 1960s. Off Tasmania there has been a 50 percent drop in inshore plankton numbers, a tropical sea urchin invasion and rapid decline in cold-loving kelp beds.

Until recently our cool winter minimum sea temperatures of around 12ºC have prevented exotic northern species from becoming resident in the gulf. John’s measurements of sea surface temperatures have recorded winter minimums of 2ºC to 2.2ºC higher than the long-term average. Some tropical species may now survive and proliferate.

Normally, inner gulf waters are rather nutrient poor and badly mixed, but the right combination of winds and oceanic currents sometimes pushes nutrient-rich water far into the gulf as far as Tiri causing sudden plankton blooms. John’s plankton recording illustrates this, with occasional huge increases in plankton above typical levels of 10 to 100 organisms per cubic metre of seawater. These nutrient and plankton pulses will support baitfish shoals and they, in turn, will feed our kororā/little penguins and other seabirds. We don’t know how important these surges in productivity are for the wellbeing of our seabirds.

Another consequence of the oceanic currents is the arrival of two exotic Australian Caulerpa seaweeds (joining our nine native Caulerpa species). John speculates that fragments of these species have probably been arriving on our shores for millennia, but only now have been able to establish in our warmer winter sea temperatures.

I’m sure the HPAs and SPAs will benefit the local marine environment, but there will continue to be major impacts from beyond our shores.