Subsurface Temperature for Fishing — What Fish Actually Sit In
Surface SST tells you what the sun sees. Subsurface temperature tells you what your fish thermoregulate against. Read it at 20 different depths (0.5–800 m) for AU offshore + game fishing.
BiteCast keeps Surface SST and Subsurface temperature as separate layers on purpose. Surface tells you what the sun sees; subsurface tells you what your fish thermoregulate against at depth. They're both useful — but for different decisions.
What it actually is
Subsurface temperature is the modelled bulk water temperature at a chosen depth in the water column. BiteCast offers 20 discrete depths from 0.5 m (surface representative) down to 800 m (deep-drop territory). At any depth, the layer shows a coloured raster of temperature across the AU domain.
Use the depth slider on the left edge of the map to cycle through depths. The colour bar adapts so eddy boundaries stay visible at each depth (deeper water is colder, so the slider reuses the SST palette but with depth-aware ranges).
Use SST to find structure (fronts, eddy edges, convergence lines). Set the Subsurface depth to your typical trolling level (50–80 m for yellowfin, 30 m for mahi, 100+ m for bigeye). Confirm fish are actually in the right water at that depth.
How it's modelled
- Source: Copernicus Marine Service GLO-PHY 1/12° analysis. The same dataset that drives Altimetry (SSHA) + MLD + Th-Depth + Eddies layers.
- Method: physics + data-assimilation model (HYCOM-equivalent). Calibrated against Argo float profiles, satellite altimetry, surface SST, and sea-level anomaly.
- Latency: 24–48 hours behind real-time. Daily refresh.
- Resolution: ~8 km horizontal × 20 depth levels vertical.
Why use it
1. Confirm fish are in the right water at depth
SST might show 22 °C at the surface, but if Subsurface at 80 m is showing 18 °C, your fish (which target 21–23 °C) are above that depth. Set your spread accordingly.
2. Find eddy structure that SST can't see
Some eddies have weak surface signatures but strong subsurface temperature contrasts — the BiteCast Eddies layer (gradient computed from Subsurface) is sharper than Subsurface alone for finding these.
3. Pick a deep-drop target depth
For broadbill swordfish, bigeye tuna, hapuku — set Subsurface to 200–400 m and look for temperature anomalies that suggest upwelling or shelf-break convergence.
Standard depth selections (AU offshore)
- 0.5 m (surface): Compare against SST. Diurnal skin warming visible as a 1–3 °C difference on calm sunny days. Treat as the “bulk surface” reference rather than SST itself.
- 50 m: Upper-thermocline trolling depth for most pelagics. Best general-purpose depth for offshore game.
- 75–100 m: Yellowfin tuna patrol depth. Eddy edges show sharply here.
- 150 m: Deeper yellowfin + bigeye habitat. Below most thermoclines in summer.
- 200 m: Albacore + bigeye. Often the “permanent thermocline” in AU offshore water.
- 400 m+: Deep-drop reef species (hapuku, blue-eye trevalla, bar cod, gemfish).
Reading Subsurface in context
Subsurface alone is a depth-frozen view of water temperature. Pair with:
- SST — surface comparison. Big surface-to-subsurface differences often mark fronts.
- Altimetry + Eddies — the Eddies layer is computed from Subsurface horizontal gradients. Switching between them at the same depth lets you find boundaries.
- MLD + Th-Depth + Th-Wall — water-column structure context.
Common mistakes
- Treating Subsurface as a measurement. It's a model. Calibrated, accurate, but with ±0.5–1 °C variability. Use as a planning input, not gospel.
- Reading too coarse a depth. Setting the slider to “surface” defeats the purpose — use SST instead. Go 50–150 m for offshore game.
- Ignoring forecast latency. Daily update means the field is up to 24–48 hours old. Fast-moving features can drift.
- Reading Subsurface in isolation. Always pair with thermocline structure. A point at 22 °C at 80 m means little without knowing whether 80 m is above or below Th-Depth at that location.
Related
Frequently asked
Why have a separate Subsurface layer when SST exists?
SST is satellite skin temperature — the top 10 micrometres of the ocean. Subsurface is modelled bulk water temperature at the depth you select. They can differ by 1–3 °C on calm sunny days because of diurnal skin warming. Use SST to find structure (fronts, eddies on the surface). Use Subsurface to know what your fish are actually in at trolling depth.
What depth should I set for offshore game fishing?
Match it to your target species and the thermocline. For yellowfin tuna in the EAC, set Subsurface to ~50–80 m to see what the fish are in at trolling depth. For deeper-running species (bigeye, broadbill swordfish), 100–200 m. The depth slider in BiteCast cycles through 20 standard HYCOM-equivalent depths.
How is subsurface temperature modelled?
Source: Copernicus Marine Service 1/12° physics analysis. Latency 24–48 hours behind real-time. Resolution ~8 km horizontally. It's a model, not a measurement — but the Copernicus model is calibrated against Argo float profiles + satellite altimetry + SST + sea-level anomaly, so it's the best-available representation of subsurface water without a CTD cast.
Will it match my boat's transducer temp?
Often yes within ±0.5 °C, depending on depth. The model represents bulk water; your transducer also reads bulk water at transducer depth. Differences are usually small. Differences of more than 2 °C suggest you're sitting on a localised front the model resolution can't resolve — useful info.
Can I read fronts on Subsurface like I do on SST?
Yes — and they're often sharper at depth than on the surface. Eddy boundaries in particular show as steep colour breaks on Subsurface at 50–100 m even when the surface signature is diffuse. The Eddies layer (horizontal temperature gradient) is built directly from Subsurface for this reason.