The Southern Oscillation Index or SOI
ENSO, or the El Niño Southern Oscillation, is tracked globally using an index, the Southern Oscillation Index, which measures surface air pressure fluctuations across the Pacific. Extreme events of either El Niño or La Niña, are likely to impact penguin breeding success wherever such climatalogical conditions impact foraging activity.
West Coast Penguin Trust observations
Through its penguin monitoring, the West Coast Penguin Trust has observations that relate closely to these climate phenomena.
Firstly, in 2015, with a strong El Niño event, we saw tawaki chicks starve with breeding success close to zero at the Jackson Head study colony. At the same time, our scientist, Dr Thomas Mattern, was tracking tawaki from the same colony, and found them foraging much further off shore and deeper than seen the previous year and since. That high foraging effort is reported in his Tawaki Project report here.
In 2022, breeding success of little penguins or kororā took a sharp nose dive with the peak of a ‘triple dip’ phase of La Niña events. We halted our foraging study that year to avoid any extra pressure on penguins that were clearly stressed. Around the same time, over 90% of the bull kelp at the Punakaiki Marine Reserve died off due to the prolonged marine heatwaves.
Last year, the 2025 breeding season for kororā appeared to be adversely affected by a weak La Niña event, as we saw penguins expending very high foraging effort during the egg incubation stage and presumably the consequent poor breeding success from our study colony.
So, what is going on and why can both sides of the Southern Oscillation Index impact penguin foraging and thus breeding success?
How may penguin foraging efforts be affected?
Curiously, both El Niño or La Niña events can result in warmer sea surface temperatures, and the natural assumption is that warm water drives penguin prey fish to cooler water, perhaps out of reach by distance or depth.
However, the reality is that ENSO reorganises wind patterns and ocean circulation differently, affecting each penguin’s specific prey, at their specific foraging depth, in their specific location.
So we need go look at the differences between the two penguin species that call the West Coast home.
Tawaki can dive to around 100m (with most dives to 20 or 30m) and, as Dr Mattern has found, over a few weeks can swim halfway to Antarctica and back, covering distances of 7,000km or more in one go. During the breeding season affected by El Niño at Jackson Head in 2015, they were ranging 100km offshore to find food, but that meant they were not only gone too long to feed chicks but probably returned without sufficient food for chicks.
Kororā forage within around 20m of the surface most of the time, with rare maximum dives of around 30-40m. They usually stay close to the breeding colony when raising chicks, 10–20km offshore and up to around 36km from home. During breeding, they pursuit-dive for prey generally in waters less than 100m deep. Last year, we saw them going beyond 100km from home during egg incubation, into waters several hundred metres deep, putting stress on both penguin parents and sadly small chicks were found dead.
We’re essentially comparing a bird that can reach 100m depth against one constrained to roughly 40m depth. That’s not just a size difference — it defines entirely different ecological niches, and entirely different vulnerability profiles.
Why La Niña hits kororā on the West Coast so hard
La Niña drives increased rainfall and storm activity on the West Coast. The effects compound in a way that specifically targets kororā’s constraints.
Thermal stratification. Prolonged warm La Niña sea surface temperatures create a strong, stable thermocline — a warm, buoyant surface layer sitting on colder water beneath. Nutrients and prey are pushed below this barrier. We believe tawaki can simply dive through it. Kororā, hunting mostly in the top 20m, are foraging in the warm, nutrient-depleted surface layer above where the fish have retreated.
Sediment and turbidity. Turbidity caused by sedimentation can affect seabirds that hunt visually, including penguins. Sedimentation events are both cumulative — where sediment accumulates slowly over time — or catastrophic, where sediment is rapidly deposited, often following severe rainfall and storm events. La Niña’s intense West Coast rainfall generates exactly this: rivers run brown, coastal waters turn turbid, and kororā — visual hunters — lose the ability to detect prey.
Kororā may head for river mouths to take advantage of the high amount of nutrients running off the land, which fuel productive ecosystems, but this behaviour makes them vulnerable to increases in sediment after heavy rainfall. There’s a cruel irony here: the very habit that draws them to productive river-mouth zones becomes a liability when storms hit, turning those areas into murky, sediment-laden water that defeats visual hunting. During our foraging study in 2025, we saw two of the three birds monitored head along the coast to the Hokitika river mouth, perhaps supporting this theory.
Kororā cannot escape such poor local conditions.
What about El Niño conditions?
In a strong El Niño event, the food web under the Tasman Sea can collapse — there is little food regardless of how far or deep a penguin can swim, which is why even the capable tawaki and their chicks may starve. In La Niña, the food isn’t necessarily gone from the water column — it’s below the thermocline, or hidden in turbid water. Tawaki, 60cm tall and weighing 4kg, can punch through both barriers. Kororā can’t.
It’s not just strength, it’s the physical architecture of the dive. A heavier, more streamlined bird with larger oxygen stores can reach greater depth. A 1kg bird, however perfectly adapted to its niche, simply cannot overcome physics to follow prey into water it cannot reach or see through.
What’s particularly concerning for the forecase major El Niño event is whether kororā — already stressed by the recent run of La Niña and heatwave seasons — will enter it in good enough condition. Body condition going into a bad season matters enormously for whether birds abandon eggs, successfully raise chicks, or simply survive.
We will continue to explore what ENSO means for penguins on the coast.
In the meantime, research shows that while ENSO remains a naturally occurring cycle, human-caused climate change is supercharging both El Niño and La Niña events, making them more extreme. Increased greenhouse gases and background warming are driving stronger temperature swings, more volatile rainfall patterns, and longer-lasting extreme events worldwide.
So we must continue to do our bit, however small – it all adds up, to reduce our carbon footprint through our travel, energy use and purchasing power.
