Category Archives: Great Mercury Island

Loss of seagrass

The Waitematā harbour was once host to enormous seagrass  (Zostera) beds which played a crucial role in providing habitat for juvenile fish. However the extensive harbour works, including port development and reclamations, had considerably reduced the seagrass beds by the 1930s. Then the remaining areas were virtually wiped out during the 1940s by an outbreak of fungal slime.

Waitemata harbour bridge
Port and waterfront development in the Waitematā harbour has contributed to the loss of extensive seagrass beds in the area.

The Tāmaki Strait was also an area with hosted seagrass beds. The steam trawler Doto, chartered by the government to do exploratory trawling in the Hauraki Gulf during 1901, found a net they hauled up after trawling along the Tāmaki Strait full of ‘grass and weeds’, indicating the extensive seagrass beds in the area.

Tamaki Strait looking from Waitawa Regeional Park
There is evidence that in 1901 there were extensive seagrass beds in the Tāmaki Strait, shown here from Waitawa Regional Park.

A major cause of seagrass loss is sediment from land development washing into the sea. When the health of a harbour or estuary deteriorates, seagrass is one of the first things to be lost. And with the seagrass beds goes part of the ability of the marine system to produce fish.

“If the sub tidal seagrass is lost, then juvenile fish production goes with it. It has a cascade effect.”

One of the few places with remaining seagrass beds in the Gulf is Huruhi Harbour at Great Mercury Island.

Huruhi Harbour, Great Mercury Island
One of the few places sub tidal seagrass beds can now be seen in the Hauraki Gulf is at Great Mercury Island.

 

 

 

 

 

 

 

Disappearing Crayfish

It was not until crayfish became scare around the shores of the Hauraki Gulf that they obtained any real commercial value. Before World War Two crayfish were everywhere and could be easily harvested amongst the rocky reefs close to Auckland.

“Initially there were sacks coming off Waiheke and North Head. Fishermen didn’t need to travel to get crayfish … the problem wasn’t lack of crayfish. They were everywhere … After the war, crayfish were worth very little, something like thruppence a pound.”

Crayfish in pots
Crayfish being harvested in pots around the Red Mercury Islands. Note the generally small size of the animals, many which are under the limit and therefore have to be returned to the sea.

Then things changed. New equipment, such as echo sounders, enabled crayfishermen to  locate new reefs to target. Improved gear such as winches enabled fishermen to lift many more pots a day and larger boats enabled them to travel further afield. Once the export market opened up for live crayfish in Japan very high prices could be obtained, reaching over $80 a kilo. This incentivised continued crayfishing even when catch rates plummeted. Recreational divers also target crayfish increasing pressure on the stocks.

“Now catch rates are so poor, at only around a third of a kilo per pot, that crayfishermen are reliant on high prices to survive.”

Crayfishing out from Whitianga
Hauling a crayfish pot in fishing grounds out from Whitianga with a poor catch.

Scientific models suggest that, prior to human arrival, crayfish were the third most ecologically important benthic invertebrate group in the Hauraki Gulf. However, the impact of fishing activity since then has been so great that they are now considered to be ecologically extinct. Stocks are probably less than a quarter of their original levels.

There has been poor recruitment of crayfish into the Hauraki Gulf over the past 6 or so years, and this has exacerbated stock depletion, as a high level of fishing activity has continued during this period.

This has had flow on effects throughout the food web, as crayfish prey on sea urchins which in turn graze on kelp. When the number of crayfish in a system is reduced, the sea urchin numbers rapidly increase due to a lack of predation and they end up stripping the kelp off the rocks. This results in ‘kina barrens’ which are depauperate of reef life. This is why it is important that crayfish stocks are rebuilt to ensure healthy reef systems throughout the Hauraki Gulf.

Kina barren
A kina barren showing what happens when top predators such as crayfish and snapper are overfished, the kina explode in numbers and strip the rocks of kelp and other seaweeds.

Hauraki Gulf Marine Park

Hauraki Gulf Marine Park map
Although many people think that the Hauraki Gulf only includes the area between Bream Head and Cape Colville, it is much bigger than this.

The Story of the Hauraki Gulf takes in the entire Hauraki Gulf Marine Park which covers around 1.4 million hectares of marine space and over fifty islands. Some of the larger islands include Great Barrier Island, Great Mercury Island, Little Barrier Island and Waiheke Island.

Great Mercury Island
Great Mercury Island is part of the Hauraki Gulf Marine Park