Over the course of last week, extraordinary numbers of large sharks and dolphins feasted on bait balls along Tallow Beach at Byron Bay. Why?
Long-time locals and devoted surfers from the area have never seen anything like it.
Incredible drone footage showed sharks and dolphins feeding together, herding the bait balls into the northern corner of the beach.
The typically crowded surf spot was empty as tourists and locals watched the natural spectacle from the shore.
A unique set of conditions
Anyone who regularly goes in the water in Byron Bay, or anywhere along the NSW coastline, knows that water temperatures can change quite a bit, not just over the seasons, but from day to day.
The two main factors are the East Australian Current (the EAC) and coastal wind-driven upwelling.
Made famous in the film Finding Nemo, the EAC is an oceanic Western Boundary Current; a type of current which flows along the western boundaries of ocean basins and redistributes warm water from the tropics towards the poles, regulating global climate.
The winds over the tropics and subtropics are predominantly from the east, so warm tropical water is pushed up against the eastern sides of continents, and this water ‘escapes’ in a strong jet that flows from the equator towards the poles.
In the south Pacific Ocean, warm tropical water flows from north to south along the east coast of Australia.
Sometimes the water flows straight down the coast like a jet, sometimes it gets stronger and wiggles (‘meanders’), and sometimes it becomes unstable and turns away from the coast, pinching off large spinning bodies of water.
These oceanic eddies are the ‘weather systems’ of the ocean.
When the EAC moves away from the coast, cold water comes up from depth or from the south, and beachgoers notice cooler water.
Most NSW beachgoers know that the water gets cold when the northerly wind blows and, while upwelling is a common phenomenon, it is poorly understood by many.
People often ask, ‘Why is the water so cold in summer?’ and, ‘Why doesn’t the north wind blow warmer water down from the north?’
Well, if the world wasn’t spinning, wind blowing from north to south would push the surface waters from north to south.
But because the earth is spinning, northerly winds push the surface waters towards the east (to the left of the wind in the southern hemisphere, the opposite in the northern hemisphere).
So, along the NSW coastline, winds from the north, most common in spring and summer, push surface waters offshore. Colder water comes up from depth to fill the void.
Water from depth is not only colder, but also richer in nutrients, so upwelled water is sometimes greener and murkier.
What was different about early December 2025 in northern NSW was that we had a strong and anomalously warm East Australian Current that was close to shore (up on the continental shelf), and we had strong coastal upwelling.
This mixture of very warm clear water (the EAC) and nutrient-rich upwelled water is the perfect nursery for baitfish.
Varying ocean temperatures are the norm off NSW, as the winds are variable and the EAC is always swirling and changing.
We see this from maps of Sea Surface Temperature (SST), that has been measured from space since the 1960s, revolutionising weather and ocean forecasting.
With decades of satellite data, we have a good idea of the ‘average’ temperatures off northern NSW for this time of year.
The extreme bait ball and shark activity off Byron Bay happened when the EAC was 2–3°C warmer than average, exceeding the 90th percentile.
Yet close to shore, the temperature was cooler than average.
The wave buoy off Ballina and a fishing vessel (part of the emerging Fishing Vessels as Ships of Opportunity Program) measured 20–22°C water close to shore, with the EAC just a few kilometres offshore measuring 27–28°C.
This stark temperature front brings warmth, light and nutrients together enhancing productivity (phytoplankton and zooplankton growth) and concentrating prey and predators.
Local surfers, young and old, were itching to get back in the waves.
UNSW’s South-East Australia Forecast System (SEA-COFS) predicted the strong EAC to continue over the following week, although the bait balls cleared after four to five days. The SEA-COFS system brings the Global Mercator Ocean Model into the NSW region to capture processes on the continental shelf and close to shore.
Our research shows that over the last 30 years, the EAC is shifting poleward.
This means warmer water is penetrating further south, more often.
Waters off southern NSW and Tasmania are warming at two to three times the global average.
This warming has impacted kelp forests, causing mass devastation by transforming ecosystems from kelp-dominated to urchin-grazed, increased diseases in oysters, and boosted algal blooms, impacting fisheries and local biodiversity dramatically.
Extreme marine heatwave events provide insight into the potential future impacts of climate change.
Off northern NSW, this shift could possibly mean more warm temperature extremes in the EAC.
Byron Bay surfer, beachgoer, and expert oceanographer Dr Colette Kerry leads the ocean prediction team in the UNSW Coastal and Regional Oceanography group.
First published by UNSW Science.
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