Tomahawk Lagoon Study

Our group of 20 strong water testers have been working on feeding back to the community about the birds they have observed over the past 18 months.

The data we have analysed covers the period from January 2020- July 2021.

Following the algae bloom in February 2020 we noticed that the amount of Canadian pond weed, seagrass, water fern and duckweed were growing at higher rates than we had previously seen during the period before.

By the 5th of August 2020 there was a significant drop in the amount of Canadian pond weed, with only debris of this apparent. Seagrass was still abundant. We attributed this to the abundance of Black Swans which had been documented since the 3rd of June 2020.

The abundance of the swans continued through to the end of our observations for 2020. When we resumed observations in 2021 we noticed that there were still a large number of young swans on the lagoon. Young Black Swans can’t fly for the first year and with the intense breeding in 2020 there were still a large number of young swans who hadn’t left.

In 2021 we noticed a decrease in the water level of the lagoon at site one. Our first recording was taken on February 10th with a depth of 36cm. The depth kept decreasing until its lowest point on the 7th of April 2021 where it was only 18cm.

Swan numbers at this time were also decreasing. On May 5th 2021 we noticed the first death of a young swan.

Over the next few weeks there were more young swans dying. By June there were 4 dead at site.

In June this was being noticed by the public and the ORC got involved, they checked the ducks and the swans to see if there was any pollution in the lagoonORC Investigating- ODT.

The July report showed that ORC Death Unsolved- ODT that the ORC was not sure what caused the of the death of the swans. From our observation our inference was that the young swans were starving to death due to the lack of vegetation growing in the lagoon. This could have been caused by the decrease in water depth in the early months of the year or the over abundance of young swans on the lagoon for the past year.

On the 24th of March 2021 we observed the continual drop in water levels of our site. This reduction brought new birds to our site. A Marsh Crake and a Coot.

Marsh crake

New Zealand status: Native

Conservation status: Declining

Found in: Dense wetland vegetation throughout New Zealand, but mainly in the South Island

Threats: Predation, habitat loss

Links between diet/habitat

Diet

Marsh crakes primarily feed on invertebrates like snails, worms, beetles, insect larvae, mosquitos, flies They also eat sand seeds of aquatic plants.

They also love finding birds and spiders to eat as a treat.

Habitat

Dense wetland vegetation throughout New Zealand, but mainly in the South Island

The Marsh Crakes habitat is almost entirely what the habitat is at the lagoon, and the lagoon also has a lot of the marsh crakes diet.

Why was our bird at the lagoon?

The reason our bird was at the lagoon was that it likes dryer locations. And because the water level for a few weeks was strangely low, it established itself there, hiding amongst the swampy water and dying rapou.