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The discovery of almost 80 dead grass carp in Gulf Harbour’s canals last month dismayed residents, who are now offering to assist with improving water quality there.
The section of canal where the dead fish were found borders Gulf Harbour Reserve and flows alongside Laurie Southwick Parade and part of Bella Vista Drive, out to Hobbs Bay.
Lack of oxygen in the water was identified as the culprit in the fish’s demise. However, there are two schools of thought as to what caused those low levels.
Auckland Council compliance manager, Adrian Wilson, says Healthy Waters staff and contractors removed the dead fish, while pollution officers investigated, concluding that the deoxygenated, green and smelly water resulted from natural causes. Oxygen levels in water are affected by things such as weed growth and how fast the water flows.
“It could also have been due to a lack of food, or salt water in the canal, as these grass carp can’t tolerate salt water,” he says. “The fish escaped from the upstream lakes during the floods in January last year.”
Bella Vista Residents Association secretary, Kerry Hart, says his discussions with contractors who were removing the fish indicated that turning the pump back on at the now disused Gulf Harbour golf course would increase the flow from those waterways into Regency Lake, upstream, with benefits along the whole system.
Hart says residents are keen to step in and look after the edges of the waterway near their homes and would also like to put more carp in there to keep weeds down.
“Volunteers from our residents’ group are prepared to help keep the weed under control around our estate, if council can help with equipment. We want the wildlife to flourish,” he says.
Council is following up with its Healthy Waters team and will then discuss options with the residents.
D’Oyly wetland also low on air
Another local waterway, the wetland in D’Oyly Reserve, has also recently caused a stink due to low oxygen levels. Neighbours of this wetland called the paper last month to complain about the water being stagnant and this was confirmed when students from Whangaparāoa College did water quality readings there as part of their studies into stream health. Assisting them on March 26 was Restore Hibiscus & Bays restoration adviser (schools and community training), Julia Clavel. She says there is a scale for milligrams of dissolved oxygen per litre of water, with 6-9 milligrams being ideal. The water in D’Oyly Reserve was found to be below six milligrams. Clavel says there was also a weed called parrots feather present, which indicates low oxygen and raised phosperous levels. There were large numbers of introduced mosquito fish (Gambusia affinis), which also thrive in stagnant water. Very few other creatures were found. Water clarity was also tested and found to be average for an urban stream. Council’s head of planning, Healthy Waters, Nick Vigar, says the situation will improve with more rain, as it is due to seasonal fluctuations in dissolved oxygen and aquatic weed levels. “During the dry summer months, the water level goes down,” Vigar says. “That means the water sits still for longer. Oxygen is added when the water is disturbed, so during low flows, oxygen cannot mix with the deeper water. Additionally, lower water levels allow aquatic vegetation to grow faster due to increased sunlight. The vegetation consumes oxygen at the surface of the pond preventing it from mixing deeper in the water. Generally, we would expect to see these conditions change as we move into winter with increased rainfall. We regularly check all our stormwater ponds to ensure that they are still operating as designed. Our team has removed parrots feather weed from this watercourse before and staff regularly check it with that in mind.”
