Ecological Services of Grey Mullet Lost Through Fishing

The humble grey mullet may have been an important vector in transporting sediment through our estuaries.  They were in huge abundance 100 years or so ago, but now are present in just a shadow of their former numbers.

When grey mullet were in huge numbers they would have been an important vector in transporting sediment through our estuaries, by swallowing mud in the upper estuary and releasing it near the estuary mouth.  Unfortunately there are so few left that this service is no longer effective

When grey mullet were in huge numbers they would have been an important vector in transporting sediment through our estuaries, by swallowing mud in the upper estuary and releasing it near the estuary mouth. Unfortunately there are so few left that this service is no longer effective

There are historic photos of clinker dinghies in the Kaipara Harbour, loaded to overflowing with mullet, many huge by todays standards and upwards of 70 cm long.

In my youth I saw a few schools of grey mullet while snorkeling, but then they “disappeared from the face of the earth”. Only in the last eight or so years have they been starting to come back, and I now frequently see adult grey mullet in the mangroves of Whangateau, and juveniles are seen in the lower reaches of the Brick Bay stream. A couple of years ago I saw a school of around 200 grey mullet, swirling around like a school of kahawai, in the Whangateau 100 metres or so below the Ti Point wharf.

It would be a great pity if grey mullet fishing intensity was to increase and prevent a come-back of this fish as only recently it has been realised that it provides an important service which is only effective if the fish is present in great abundance.

Grey mullet feed largely by swallowing mud hoovered up from the estuary floor then digesting organic matter and organisms contained in the mud. Because of the relatively small amount of digestible material in the mud, considerable quantities are ingested to get enough food to sustain the fish. Once the food value is extracted, the “spent” mud is passed out through the digestive tract and back into the marine environment.

It so happens that the mullet feed in the upper estuary at high tide, then head down to the estuary entrance at low tide when they void their faeces. So the feeding activity of the mullet actually transports sediment from the upper estuary to the lower estuary and open sea.

Clearly the more mullet are in the estuary the more effective is this sediment transport system. Historically when mullet were really abundant this would have been a highly significant ecological service provided by the mullet, helping to maintain our estuaries in a clean and healthy state, in a similar way to abundant cockles filtering the water and maintaining it clear.

Now it has become clear that following removal of forest from the land and subsequent massive input of sediment to our estuaries, this is just when we desperately need the ecological services of the grey mullet. With abundant grey mullet in our estuaries they could help move large quantities of sediment down through the estuaries and help avoid sediment accumulation which has been linked to massive mangrove spread in some estuaries.

Perhaps we should recognise the valuable ecological services of the grey mullet by protecting them from fishing so they can build up their numbers to historic abundance and sizes? Only then could they help to repair the damage caused by large amounts of sediment accumulating in our estuaries.

Roger Grace
17 June 2015

Comments

  1. Ian Macdonald says:

    I can certainly claim to have seen the demise of mullet in our harbours over my lifetime. Living on the Kaipara Harbour as a young boy I remember seeing mullet in large numbers leap out of the water at the turn of the high tide in all the small inlets near us on the Arapaoa Peninsula. Dick Scott in his book “Seven Lives on Salt River” accounts that the Maori in the same part of the harbour would fish for mullet by paddling up an estuary inlet splashing their paddles. The mullet were so abundant that they took their catch with the frightened fish that landed into the waka. We would also fish for flounder by just walking along the edge of the incoming tide and standing on them. They were big fish. We also always had lots of small dogfish swimming around us at the same time. These are sights you would never see now. I blame set nets, fish finders and fishing competitions along with a good dose of greed.

  2. I recently spoke to a local who catches mullet to feed to his dog!!

  3. Wow, loved reading this Roger. Wouldn’t it be great if we could make Whangateau estuary a full marine reserve as a test lab to see with time how the estuary recovers and looks after itself, could be a pioneering case study for more full marine estuary reserves in the future. Well just some dreaming, nice if it could happen. Regards Darryl

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