Blue penguin foraging
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 strategies.

Radio-controlled penguins
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 at Oamaru.

So what?
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.

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