The monitoring of breeding success in blue penguins at Oamaru has shown that the penguins there have a high breeding success compared
to sites further north. In 1998, Mihoko Numata compared the breeding success
and nest attendance patterns of blue penguins at Motuara Island in the
Marlborough Sounds with that observed at Oamaru. She found that foraging
trips at Motuara were longer than at Oamaru and this was reflected in
lower chick survival and growth rates. This disparity was thought to be
due to a difference in food availability.
A study carried out in Oamaru in 1994 by Maree Fraser found that the main
diet item at Oamaru was slender sprat - a species not found at Motuara,
but it was not known if there were also differences in foraging behaviour.
In 2000, Thomas Mattern from the Institute of Marine Sciences in Kiel,
Germany, came to Otago University to investigate the foraging behaviour
of blue penguins at Motuara and Oamaru. Thomas used radio tracking at
determine where the penguins were foraging and time-depth recorders (TDRs)
to determine how long and deep they were diving. He also looked at the
breeding success at both sites to see how this was affected by the foraging
Thomas fitted small radio transmitters to the backs of penguins that
were raising small chicks and used two tracking stations to follow the
penguins at sea and triangulate a position. This required long days at
the stations, as the penguins would be at sea for 15 hours. After much
difficulty with equipment and poor weather, sufficient tracks were gained
to get a picture of where penguins forage at the two sites. Simply put,
the penguins at both sites tended to forage in areas of relatively shallow
water. At Oamaru, the penguins had a much larger area available to exploit
when compared to Moutara Island. Birds with young chicks undertook single
day foraging trips up to 25km offshore and travelled up to 75km in a day.
Dive, dive, dive
TDRs fitted to birds with chicks in the guard stage revealed that dive
behaviour at the two sites was markedly different. Motuara birds regularly
dived to 20m, while Oamaru birds only occasionally exceeded 10m. Despite
making deeper dives, the Motuara birds also made 30% more dives, resulting
in them spending twice as long diving per foraging trip than their counterparts
To increase foraging success, penguins have two options: Travel further
or dive deeper. Oamaru birds have a wide area of shallow water in which
to forage, while their Moutara counterparts have a small area within Queen
Charlotte sound. Consequently, Oamaru birds extend their foraging horizontally
and Motuara birds extend their foraging vertically.
Prey availability plays an important part in the success of the penguins.
Periods of apparent starvation at Moutara regularly reduce chick survival,
while such events have not been recorded at Oamaru. The analysis of diet
samples taken at Moutara has not yet been completed, but the prey items
are expected to differ from those taken in the colder waters off Oamaru.
Learning what prey species are taken and how local conditions affect their
availability will be an important step towards undertanding why the penguin
breeding success varies markedly between locations and years.