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Protected fish nursery wiped out by mud

Bright colors, juvenile fish and clear water: These are good quality bryozoan colonies near D'Urville Island.  At Separation Point, however, the seabed now features dead debris and soft featureless mud.

Niwa

Bright colors, juvenile fish and clear water: These are good quality bryozoan colonies near D’Urville Island. At Separation Point, however, the seabed now features dead debris and soft featureless mud.

A fish nursery established in 1980 off Abel Tasman National Park has been destroyed by sediments coming off the land, a preliminary review of data has been found.

The 146-square-kilometre protected area was off Separation Point, between Tasman and Golden Bays, at the top of the South Island.

It harbored colonies of “bryozoans”​ – small, colorful marine animals that sometimes form structures that are similar to coral.

It was thought juvenile snapper, tarakihi, cod, leatherjackets (creamfish) and other fish hid and fed in the colonies.

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The area was known as “the coral” to local fisherman in the 1950s and 1960s. They caught lots of small and young fish there. Locals suggested dredging and trawling should be banned to protect the fish.

The area is now “barren”, states an internal Niwa​ document obtained through the Official Information Act.

Niwa staff led by Dr Mark Morrison undertook surveys off Separation Point-Te Matau in August 2020 and February 2021.

“A very troublesome picture emerged. Very few bryozoan colonies or other [prized] habitats were observed, and the few species seen … were dominated by [sea squirts].”

A bryozoan, Telopora lobata.  Young fish like this habitat.

Abby Smith

A bryozoan, Telopora lobata. Young fish like this habitat.

They saw “many large patches” of dead shells, bryozoan fragments and other debris, surrounded by soft featureless mud.

Few fish were seen on the videos, with no juvenile blue cod present. Similarly, baited traps had very low catch rates, mainly of larger, older blue cod. No juvenile snapper were seen on video or caught.

“In summary, the area is now a barren desert in terms of… habitats and juvenile fish,” the document concludes.

Underwater visibility was also poor, even though the researchers experienced good weather.

Another look at high quality bryozoan habitat and associated sponges, ascidians, encrusting coralline algae and two leatherjacket fish.

Niwa

Another look at high quality bryozoan habitat and associated sponges, ascidians, encrusting coralline algae and two leatherjacket fish.

A “likely smoking gun” was Cyclone Gita, which swept directly across the area in 2018 and caused widespread destruction on land.

But that event came on top of decades of environmental decline on land. While there is no longer forestry in the national park, there are forestry blocks on the park borders, said Bruce Vander Lee, project director at Project Janszoon, a philanthropic trust restoring and preserving the Abel Tasman.

The wider Nelson-Tasman-Golden Bay region brims with forestry blocks and farms. A 2006 study showed that during floods, sediments can come down the Motueka River and drift around to Separation Point and into Golden Bay.

“This suggests that adverse land-based impacts from sedimentation may be significant, despite full protection from [bottom-contact] fishing methods,” Morrison said.

Morrison cautioned that the recent conclusions were not final and would need to be peer reviewed.

There were earlier indications that mud was affecting the bryozoans. A 2003 study found the colonies alive but growing only at their tips, which suggested “sedimentation stress”.

Apart from the lost fish nursery, marine biologists mourned the loss of the bryozoans and other marine life that has disappeared from Separation Point.

A blue cod caught and released during a 2010 DOC fish survey.

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A blue cod caught and released during a 2010 DOC fish survey.

Bryozoans date back 65 million years but are little known. They are invertebrates that prefer water 40 meters to 80m deep – too deep for recreational scuba divers. They rarely wash up on beaches for people to find and aren’t edible, said Professor Abby Smith​​ of the University of Otago’s Department of Marine Science.

Also known as “moss animals”, “sea mats” or “lace coral”, they need clear and flowing water and rocky seafloors. Sediments are fatal, she said.

Illegal fishing sometimes occurred at Separation Point.

It’s not known if bryozoan colonies can be reestablished but it would be difficult at Separation Point while the mud remains and sediments continue to come off the land.

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