Will they stay or will they shag off? Kāruhiruhi and the lower reservoir restoration

Written by Chris Gee (volunteer and boat skipper) and Ellen Irwin (Lead Ranger, Conservation)

In 2021, Zealandia Te Māra a Tāne carried out a project to restore the mouri and water quality of the lower reservoir, Roto Kawau. As part of this project, the water level of the reservoir was lowered by six metres, creating an expanse of bare, fractured mud. An important aspect of this whole project, and long before this happened, was implementing a wildlife hazard management plan to identify any specific hazards to Zealandia species presented by the lower reservoir operation, and to consider mitigation approaches where necessary.

There are several kinds of waterfowl present in Zealandia’s reservoirs, but some of the most obvious and charismatic species are the kawau/shags: Zealandia has four resident shag species (kāruhiruhi/pied shag, kawau pū/black shag, kawau paka/little shag, kawau tūī/little black shag). Birds typically roost and nest in the trees around the lower reservoir but spend parts of the day foraging for food in the harbour.

Two years before the operation, an intern at Zealandia researched shag feeding and diving behaviour over winter, and observed almost no foraging behaviour, indicating that the manu/birds did not likely rely on the exotic perch, which were being removed as part of this project, as a key food source. In addition, the operation was going to take place in late autumn/winter and so would have little effect on breeding.

However, the lowering of the water level had the potential to influence shag behaviour in the short term and was identified as a potential risk in our wildlife management assessment for the project. To keep an eye on this, the team called up volunteer Chris Gee, a Zealandia boat skipper who also leads monthly shag nest counts (in addition to many, many other tasks).

So how did the shags react to the water disappearing?

In Chris’s own words:

As a boat skipper I get plenty of opportunity to observe the behaviour of the various shag species that use the lake environs for roosting and nesting. In order to assess any impact of the reservoir project, observations of the roosting behaviour seemed to be the best way to go. Of the four species we see in the sanctuary the most straightforward to study are kāruhiruhi/pied shags.

(Sidebar: Kawau/black shags have nested the last two summers but we rarely see them outside the breeding season. Kawau paka/little shags are regular visitors but roost and nest in the trees and can only be observed from the boat. Kawau tūi/little black shags are occasional roosters, have yet to nest in the sanctuary and again roost in an area only seen from the boat.)

The method I decided to use was to focus on those kāruhiruhi that roost on the fallen trees at the southeast end of the reservoir between the shag lookout and the pontoon walkway as a representative sample. There are also many kāruhiruhi that roost in trees around the reservoir. To count roosting birds I assumed that any birds present at dawn had roosted overnight and I used the Met Office posted time of sunrise as my ‘observation time’. Rather than try to count the birds in situ, I chose to take a photograph using a telephoto lens from the Round Lawn observation fence which I could then blow up on a computer screen at home to count the number of birds. For a complete set of observations I planned to take photos at ten minute intervals from SR-30 to SR+30. In practice it was too dark at SR-30 to take a suitable photo and also on some days at SR-20 the exposure required was too long to get a useable photo.

The overall conclusion must be that lowering the reservoir level and then re-filling has had no impact at all on the number of kāruhiruhi roosting.

?There are, however, a few interesting observations that can be drawn:

1. From watching the birds while photographing, the first birds leave the roost to head out to sea between 15 and 20 minutes before sunrise.
2. Generally the number of roosting birds is weather-dependent, with lowest numbers in fine weather and highest when there are southerly winds on the coast.
3. In fine weather the birds appear slower to head out, the number only decreasing slowly between sunrise and 30 minutes after sunrise. The observation on 30/6 was the day after an extreme southerly storm battered the south coast and in the half hour of watching nearly two thirds of the roosting birds left.

The other interesting observation is where the birds roost.

(All the above photos by Chris Gee)