Eddies + Altimetry for Fishing — How to Find Them

What eddies are

Eddies are rotating water masses that detach from main ocean currents and drift across open ocean. In AU waters they're typically 100–200 km in diameter, moving at roughly 10 km/day, persisting for weeks to months. The East Australian Current throws them off constantly — every offshore angler on the AU east coast fishes EAC eddies whether they think of it that way or not (see EAC fishing guide).

Two types:

• Warm-core eddies — detached lobes of warm tropical water sitting south of where they should be. They rotate anticlockwise in the Southern Hemisphere and sit slightly higher than surrounding ocean because the warmer water column is less dense. Productive for tuna, marlin, mahi.
• Cold-core eddies — opposite. Rotate clockwise (Southern Hemisphere), sit slightly lower (negative sea-surface height anomaly), often formed by upwelling that brings nutrient-rich deep water up. Less productive on their own but the boundary between a cold-core and warm-core (when they meet) is a premium feeding zone.

How BiteCast surfaces eddies

Two layers, used together:

Altimetry (SSHA)

The Altimetry layer shows sea-surface height anomaly (SSHA) in centimetres — the bump or depression in the ocean surface relative to the long-term mean. Source: Copernicus DUACS L4. Contours every 5 cm. Warm-core eddies show as positive SSHA (red, +5 to +25 cm); cold-core as negative SSHA (blue, −5 to −25 cm). The bold teal zero line is the convergence front — where warm meets cold.

This layer answers where the eddies are.

Eddies (horizontal temperature gradient)

The Eddies layer shows the rate of horizontal temperature change in °C per 110 km, computed at the active subsurface depth using a Sobel edge filter applied to the Copernicus HYCOM-equivalent analysis. Bright halos in this layer are literally the eddy edges — the places where temperature changes fastest in space.

This layer answers where the edge is. It's sharper than the Subsurface temperature raster because the gradient-magnitude calculation isolates the boundary regardless of whether it's a warm-core or cold-core feature.

How it's measured

• Altimetry source: Copernicus Marine Service DUACS L4 product. Satellite-altimeter measurements (Jason-3, Sentinel-6, SARAL, etc.) merged into a daily global gridded SSHA field at 1/4° resolution. Latency 24–48 hours. The same product RipCharts and Fishtrack use for their altimetry layers.
• Eddies (gradient) source: in-house computation from the Copernicus 1/12° physics analysis. We take the temperature field at the active subsurface depth, apply a Sobel filter for horizontal gradient, and serve as a separate COG. Refreshes daily.

Why eddies matter for fishing

Three things compound on eddy edges:

1. Bait gets concentrated along the boundary. Small baitfish + zooplankton don't cross sharp temperature gradients easily. The rotating water of the eddy pushes them outward against the colder/warmer water surrounding the eddy. They stack along the edge.
2. Predators stack inside the boundary. Tuna, marlin, mahi sit just inside the eddy edge and ambush bait drifting along the line. The eddy core is mostly empty; the edge is where the action is.
3. The convergence between two eddies is a premium feeding lane. Where a warm-core and a cold-core sit next to each other, the boundary between them is sharper than either eddy's outer edge — and persistent for the duration the two eddies remain neighbours.

How to read eddies in BiteCast

A typical sequence:

1. Open the Altimetry layer. Look for closed contours forming circular or elliptical shapes — those are eddies. Note whether they're positive (red, warm-core) or negative (blue, cold-core).
2. Identify the bold teal zero line. Tightly-packed contours flanked by +10/+20 on one side and −10/−20 on the other = top trolling lane.
3. Switch to the Eddies layer. Set the depth slider to 50 m (or your typical trolling depth). The bright orange/yellow halos are the eddy edges — usually slightly inside the SSHA contour line where they were on Altimetry.
4. Add SST. Look for a sharp temperature gradient running along the same edge — a 1–2 °C SST break stacked on top of an eddy edge is the gold zone.
5. Drop a pin and check the conditions card — Subsurface temp at depth, wind, current, solunar.

In context — eddies with other layers

• SST — surface temperature confirms the eddy is real and shows the sharpness of the boundary on the surface
• Subsurface temp — confirms what fish are actually in at trolling depth inside vs outside the eddy
• Chlorophyll — productive water often accumulates on the cold-water side of a warm-core eddy edge
• Currents — flow direction shows whether bait is being pushed onto or off the edge
• MLD + Th-Depth + Th-Wall — water-column structure inside the eddy is usually shallower MLD with sharper thermocline than surrounding ocean = compressed bait (see thermoclines deep dive)

Common mistakes

• Targeting the eddy core. The core is uniform water — no compression, no front. Fish there if you want to cross the eddy; don't fish there if you want to find feeding pelagics.
• Trusting SST alone to confirm an eddy. A warm patch on SST could be transient convergence or a cloud artefact. Confirm with Altimetry contours.
• Ignoring eddy drift. 10 km/day adds up. The eddy you planned around three days ago is 30 km away. Re-check the day-of.
• Ignoring the depth setting on the Eddies layer. The horizontal gradient is computed at the depth you've selected for the Subsurface temp layer. If you're fishing the upper thermocline, set depth ~50–80 m before reading.
• Assuming all eddies are productive. An eddy that's sat in cold water for weeks may have lost its temperature contrast — the edge is no longer sharp. Pair Altimetry with SST + Eddies to confirm the boundary is still hot.

AU-specific eddy patterns

• EAC eddies (NSW + southern QLD): the most productive eddy field in AU waters. Warm-core eddies peeling off the EAC drift south + east, providing offshore game fishing zones from Sydney down to Eden in summer.
• Leeuwin eddies (WA): smaller, less frequent, but the warm-core eddies that do form around Rottnest + Perth offshore are productive for billfish + mahi.
• Coral Sea eddies: different dynamics — typically larger, slower-moving, often associated with reef-edge upwellings. Productive for tuna + billfish year-round.
• Tasman Sea winter: EAC retreats north but eddies that broke off in summer can persist deep into winter, drifting around in the central Tasman. Sometimes they're findable 200+ km offshore from Sydney/Wollongong/Eden.

Related

• The East Australian Current — a fishing guide (eddies in context)
• SST layer explainer
• Reading thermoclines
• How to read sea-surface temperature
• The AI companion — “find me the closest warm-core eddy”