Scores of dead little blue penguins that have washed up on northern beaches this year probably died of starvation in extreme sea conditions, researchers say.
And they warn that climate change and its big storms could mean many more mass deaths of cherished bird and other vulnerable marine species.
Extreme weather, including several ex-tropical cyclones and record-hot sea surface temperatures, led to high numbers of reports of dead penguins washing up on beaches across the upper North Island.
The Department of Conservation's Tauranga office has had an average of five calls a week since February, and Western Bay Wildlife Trust's Julia Graham said the trust had 58 calls about dead penguins in two weeks.
"All of the penguins were dead, or died within a short time of arriving on land," Graham said.
Researchers who did necropsies of 11 birds collected in March and April from Omaha Beach and Tawharanui Peninsula near Auckland indicated that they were starving when they died.
Forest and Bird seabird expert Dr Karen Baird and Massey University's Dr Daniel Thomas, along with students, found all of the birds had depleted fat reserves and had started using pectoral muscle for energy, resulting in wasting.
Ten of the birds had empty stomachs, one had eaten a small amount of grass, and all showed other signs of starvation.
"These birds starved after experiencing a series of severe weather events, which may have prevented them from feeding, and these results may also explain the deaths of many hundreds of other little penguins around northern New Zealand," Baird said.
"We also found that most of these birds had recently completed moult suggesting they also hadn't been able to put on enough condition prior to the moult."
Moulting could take three to four weeks, during which the birds could not feed.
"Once they head back to sea they need to forage successfully to quickly regain lost body weight."
Baird noted the record marine heatwave in the Tasman Sea, which also affected the East Auckland Current as it flowed into the Hauraki Gulf, raising sea surface temperatures around the north of New Zealand.
"High temperatures could affect productivity and hence food supply but high temperatures have likely exacerbated a series of summer storms causing turbidity and hence poor visibility at critical times for little penguins."
Penguins were visual predators, and needed to see to be able to feed.
"We are concerned that with climate disruption causing high sea temperatures, summer storms could become more common and what has been previously seen as a one in 20-year event could become more frequent," she said.
"Populations like little blue penguins and other species can recover from infrequent bad events by breeding, but if it happens much more frequently, the population doesn't get a chance to recover.
"I think it's a wake-up call that we can't expect always to have penguins, unless we start to think more about how we can actively manage them."
The now-dissipating marine heatwave was the most significant recorded in New Zealand, and would be considered unusual even under climate change projections for 2050. But new research has found such events were becoming more frequent as the planet warmed.
Hundreds of shearwaters, petrels, fairy prions and shags, along with dead poisonous pufferfish, were also reported to have washed up along coastlines in January.
Little penguins were widely distributed around New Zealand, and other populations such as those in Oamaru have not suffered the same fate.
"We have very little information on the population trend of little penguins and necropsies like those performed here provide crucial data," Baird said.
"Outside of reserves and protected populations their distribution appears to be shrinking and populations declining.
"We don't know what the population is in the Auckland region or how much these deaths could have affected it."
• Additional reporting by Zoe Hunter of Bay of Plenty Times