Read the water · The rulebook Satellite SST

HOW WE READ SST: THE RULES BEHIND THE SERIES

Every tip in the series carries a tag. These are the rules behind the tags, graded by how well they are established and listed with their sources.

No rule, no claim. Every interpretive sentence in the six posts maps to one of the 30 rules below, drawn from a 72-rule corpus built on 94 sources read on 2026-10-19: fisheries agencies, peer-reviewed tagging and oceanography papers, the trade's own reading guides, and the offshore forums.

How to read a tag

A tag such as sst_break_definitionA at the end of a sentence names the rule the sentence rests on and the rule's confidence grade. Tap it to land on the rule here. The grade sets the wording: an A-grade rule is stated plainly, a B-grade rule as what experienced crews find, a C-grade rule as anecdote, and an I-grade rule is a property of our own bake, never a claim about the ocean. Where a rule's caveat applies to the pass in hand, the post carries the caveat too.

GradeMeaningHow we write itIn this series
AAn agency (NOAA, CSIRO/IMOS, NIWA, the Marine Institute) or a peer-reviewed paper states it, or it is a property of the instrument or the physicsPlain statement6 rules
BTwo or more independent practitioners say the same thing, or one practitioner plus a consistent forum consensus"Experienced crews …", "the trade's rule is …"21 rules
COne practitioner, one forum thread, or anecdote; or the science is suggestive but not about fishingHedged, attributed, or left out of short form0 rules
IInternal: a property of Fishare's own bake or detectorPlain statement about our data, never about the ocean3 rules

Four things the rules never let a post say: that fish are present (the posts say where the water says to look); a number that did not come from the pass; a forecast frame described as an observation; and a competitor's name. Species temperature windows are context for which side of a line matters, by ocean, never a forecast of fish.

The break

Bsst_break_connected_warm_massregions: us-west, us-east, au-east

A warm patch only counts if it connects to the main warm water

When: A warm pocket or finger is visible; check whether it joins the current or eddy that supplies it.

What it means: Isolated warm patches are often river outflow or afternoon heating and hold no oceanic life; a finger connected to the Gulf Stream, EAC or Loop Current carries the bait with it.

Caveats:

Used in: Part 3.

Sources:

Asst_break_definitionregions: global

What counts as a break

When: Surface temperature changes sharply over a short distance. The trade's floor is about 0.5 °C (1 °F); commercial longliners look for 1–1.5 °C or more; 2–3 °C (3–5 °F) is a strong front. Fishare draws a line when the water changes at least 0.45 °C across a 22.3 km baseline after an 18 km smooth, and labels it with the warm and cool temperatures measured either side.

What it means: Two water masses meet. The seam concentrates plankton, debris and bait and is where pelagic predators patrol.

Caveats:

Used in: Part 1, Part 2.

Sources:

Bsst_break_persistence_three_daysregions: global

A break needs about three days to set up

When: The same break is visible on passes over 2–3 days (Fishare: 'holding N passes').

What it means: Plankton, then bait, then predators take days to assemble on a new boundary; a fresh break has little on it and a break that has held 3–5 days is at its best.

Caveats:

Used in: Part 1, Part 2, Part 3, Part 5.

Sources:

Bsst_break_strength_scales_with_deltaregions: global

Bigger step, more activity — but a small step can be enough in flat water

When: Rank breaks by their °C step; in warm, flat regions (tropics, SoCal summer) a 0.2–0.5 °C step can concentrate fish.

What it means: The delta is the first thing experienced readers look at; the more pronounced the boundary the more it concentrates life.

Caveats:

Where the trade and the science differ: Guides say sharp breaks fish best (T04, T05, T21); an NZ forum poster finds subtle changes the most productive (F08). Both are consistent with 'the strongest feature available in that water'.

Used in: Part 1, Part 2.

Sources:

Bsst_break_verify_on_boardregions: global

The chart finds the break; the boat confirms it

When: Always — before leaving the dock and while running toward the charted line, watch the transducer temperature and expect the seam a few miles from where the pass drew it.

What it means: Satellite SST is the top millimetre at the pass time; the transducer reads the top metres now. Offsets of 0.5–1 °F are normal and 3–4 °F happens under cloud edges or afternoon heating. Watch the gauge as you approach and fish the gradient you measure.

Caveats:

Used in: Part 2.

Sources:

Cloud

Icloud_hole_is_missing_not_edgeregions: global

Cloud is a hole, never an edge; the rim beside a cloud reads cold

When: A sharp boundary on a single pass coincides with the edge of missing data.

What it means: The last pixel or two beside a masked cloud are partly cloud-contaminated and read cold; a 'break' along a cloud edge is in the sensor, not the water. Fishare never draws a line through or along a hole.

Caveats:

Used in: Part 1, Part 2, Part 3.

Sources:

Itoo_cloudy_to_callregions: global

When cloud covers the water, say so

When: Too few clear pixels over the grounds to scan.

What it means: An unscanned pass is 'we could not look', not 'no break'. The honest miss builds more trust than a guess.

Used in: Part 1, Part 4, Part 6.

Sources:

Pass age

Bchl_image_is_olderregions: global

Chlorophyll is a day or two older than the SST pass

When: Comparing a chlorophyll frame with an SST pass.

What it means: Ocean colour needs daylight and no cloud; one frame a day at best versus 6–12 SST passes. Project the colour edge forward by the break's drift before trusting its position.

Caveats:

Used in: Part 5.

Sources:

Bpass_age_and_freshnessregions: global

Say when the pass was taken; an edge older than a day has moved

When: Reading any pass.

What it means: The screen the night before is not what you find; 24–48 h old data can send you to a feature that has moved or faded. Fishare stamps every pass with satellite and UTC time.

Caveats:

Used in: Part 1, Part 3, Part 4, Part 6.

Sources:

Bsst_break_moves_track_speedregions: global

Track the break across passes; project where it will be when you arrive

When: The break's position differs between passes.

What it means: Water moves; a 24–48 h old image can send you to structure that has moved or dissipated. Direction and speed of the break are as useful as its position.

Caveats:

Used in: Part 3, Part 4, Part 6.

Sources:

Composite and pass

Bcloud_free_products_are_modelsregions: global

'Cloud-free' SST is modelled where it is cloudy

When: A gap-filled (L4 / cloud-free / multi-res) field.

What it means: Gap-filled fields combine sensors and a model to paint under cloud; they show general zones but not ultra-sharp edges and can hold yesterday's water.

Caveats:

Used in: Part 3, Part 6.

Sources:

Bcomposite_vs_passregions: global

Use the single pass for the edge and the composite for the picture

When: Choosing between a 24 h / 72 h composite and the latest clear pass.

What it means: Composites fill cloud holes but mix times — a 72 h composite can place a 2-day-old edge next to a fresh one — and blur or smear sharp edges. Cloud-free layers are model-filled and can be days stale. The sharp edge belongs to the newest pass.

Caveats:

Used in: Part 2, Part 3, Part 4, Part 5.

Sources:

Skin, sun and the morning edge

Ahourly_morning_edgeregions: global

Trust the pre-dawn frame; afternoon warm patches may be sunburn

When: Calm, sunny conditions; comparing a morning and an afternoon pass or hourly geostationary frames.

What it means: Under calm sun the top millimetre warms by up to 5 K above the foundation temperature; the warming is patchy and can mimic a break. A real water-mass boundary is still there at dawn; diurnal warming is not. Crews use 1–6 am single-granule images.

Caveats:

Used in: Part 1, Part 2, Part 3, Part 4, Part 6.

Sources:

Askin_vs_boat_thermometerregions: global

Satellite subskin is not the boat's bulk temperature

When: Always, when comparing a chart number to the transducer.

What it means: Infrared and microwave sensors read the top millimetre; the transducer reads the top metres. They agree at night or in wind and diverge by up to several degrees under calm sun; typical chart accuracy is quoted at 0.5–1 °F. Compare the step across the break, not the absolute value.

Caveats:

Used in: Part 1, Part 4.

Sources:

Chlorophyll

Bchl_coastal_runoff_false_positiveregions: au-east, us-west, us-east

Flood run-off and river plumes look green but are cold and lifeless

When: High chlorophyll/turbidity hugging the coast after heavy rain, or a river plume.

What it means: Sediment and freshwater read as 'green' but the water is cold, dirty and not the plankton-rich oceanic green that holds bait. The Columbia outflow likewise looks warm without being connected to oceanic warm water.

Caveats:

Used in: Part 5.

Sources:

Bchl_colour_line_blue_meets_greenregions: global

Where blue meets green is the money zone

When: A chlorophyll edge: clear blue (low chl) against green (plankton-rich) water.

What it means: The colour line is where the food chain meets the clear water predators hunt in. Bait (including squid) gathers on the edge if the current does not disperse it.

Caveats:

Used in: Part 5.

Sources:

Bchl_species_preference_cleanregions: us-west, nz, au-east

Offshore tunas hold on the clean side; bait lives on the green side

When: A colour edge with clean blue water on one side.

What it means: Bluefin, albacore and yellowfin patrol the clean side and make brief feeding forays into the green; inshore species tolerate dirtier water.

Caveats:

Used in: Part 5.

Sources:

Achl_values_bandregions: us-west, hawaii, nz, global

The useful chlorophyll band is roughly 0.2–1 mg m⁻³; too blue is a desert

When: Chlorophyll values on the chart.

What it means: The basin-scale 0.2 mg m⁻³ contour is where albacore and loggerheads migrate; crews look for the first colour break with values near 0.5–1 beside clear water. Very blue water past the first break has little life.

Caveats:

Used in: Part 5.

Sources:

Bchl_with_temp_break_stacksregions: global

A colour edge on the same line as the temperature break is the best signal

When: The chlorophyll edge coincides with the labelled SST break.

What it means: 'Not temp vs chloro — temp AND chloro.' The Pass Read's check 'chlorophyll edge on the same line' is this rule.

Caveats:

Used in: Part 2, Part 5.

Sources:

Bsst_break_in_sterile_blue_waterregions: global

A break with no chlorophyll holds passing fish only

When: The temperature break sits in uniformly blue, low-chlorophyll water on both sides.

What it means: Without the base of the food chain the boundary cannot hold bait; fish cross it but do not stay.

Caveats:

Used in: Part 5.

Sources:

Sea-surface height

Bssh_confirms_sstregions: global

A break with an SSH gradient under it is a real water-mass boundary

When: The labelled SST break lies on or near a strong SSH gradient or an eddy rim.

What it means: Altimetry sees the dynamics beneath the skin and persists for weeks; it separates a current edge from surface heating or a cloud artefact.

Caveats:

Used in: Part 2, Part 5.

Sources:

Current

Bssh_gradient_is_currentregions: nz, au-east, us-east

Tight SSH contours mean strong current; fish where they cross a temperature break

When: SSH contours are close together and cross the SST break.

What it means: Surface geostrophic current runs along SSH contours; where it crosses a thermal boundary the water converges and the bait is pinned.

Caveats:

Used in: Part 2.

Sources:

Warm eddies

Awarm_eddy_edge_fish_perimeterregions: us-east, au-east, nz, hawaii, japan

Fish a warm-core eddy on its perimeter, not its centre

When: A warm pocket (anticyclonic, SSH high) is drawn or named near the grounds.

What it means: The rim is where the eddy's warm water shears against cooler surrounding water; nutrients and bait concentrate on the margin while the centre is downwelling. Longline data put Gulf yellowfin on anticyclonic eddy edges; Kuroshio swordfish concentrate in and near warm-core eddies.

Caveats:

Where the trade and the science differ: one US chart service and Great Days Outdoors call SSH highs 'downwelling, basically devoid of life' (T09, T10). Peer-reviewed fisheries data (S14 yellowfin, S33 swordfish) and East-coast captains (T06, T15) fish their edges. Resolution: the CENTRE is poor, the EDGE is good.

Used in: Part 2.

Sources:

Under the surface

Bsubsurface_confirms_break_depthregions: global

If the step is still there at 50 m, the break is a water-mass boundary

When: Compare the temperature at 50 m (and 100 m) across the surface break.

What it means: Surface heating and cloud artefacts are skin-deep; a current edge or eddy rim extends tens to hundreds of metres down. The Pass Read's 'still 21 °C at 50 m' is this check. Subsurface charts also reveal breaks hidden at the surface.

Caveats:

Used in: Part 2, Part 4.

Sources:

Species temperature windows

Bsst_break_fish_run_along_not_acrossregions: global

When one side is out of the species' window, fish run along the break

When: warm_c is inside the species' window and cool_c is below it (or vice versa).

What it means: The break is a wall to that species; they travel along it on their side, and an anomaly (finger, point) on that side concentrates them.

Caveats:

Used in: Part 2.

Sources:

Btemp_window_species_contextregions: global

Species windows are context, by ocean — use them to say which side, never to forecast fish

When: A break's warm and cool temperatures compared with the species window for that region.

What it means: Tagging and catch data give each species a comfort band that differs by ocean (NZ striped marlin 18–20 °C; Baja striped marlin 22–27 °C). The band tells you which side of a break a species will patrol and whether a break matters for it at all.

Caveats:

Used in: Part 2.

Sources:

Structure

Bsst_break_over_structureregions: global

A break over the shelf edge, a canyon or a bank ranks first

When: The labelled line crosses or runs along the 100-fathom line, a canyon wall or head, a seamount or bank.

What it means: Bottom structure deflects current and lifts bait; a water-mass boundary over it concentrates the ecosystem twice over.

Caveats:

Used in: Part 2.

Sources:

Forecast and observed

Bforecast_vs_observedregions: global

The 5-day forecast says where the break drifts; the pass says where it is

When: Using the modelled SST forecast alongside observed passes.

What it means: one US chart service and the trade predict where features will move because the imagery lags; a model forecast does that explicitly, but it is a model: the observed pass overrides it, and content must label forecast frames as forecast.

Caveats:

Used in: Part 3, Part 6.

Sources:

History

Ahistory_30_day_persistenceregions: global

A front that recurs over weeks is a frontal zone — the real hotspot

When: The 30-day history shows a break at the same place repeatedly (shelf-slope front, canyon tips, eddy corridor).

What it means: Predictable mesoscale frontal zones are where marine vertebrates reliably forage and where fisheries concentrate; a one-day break may be ephemeral.

Used in: Part 6.

Sources:

How we write

Ihonest_hedge_languageregions: global

Say where the water says to look; stamp every number; call the cloud

When: Writing any read.

What it means: Every number carries its pass time and satellite; the chlorophyll frame carries its own date; 'where the water says to look' never 'the fish are here'; 'too cloudy to call' when coverage fails; no competitor named; only public grounds; no 'real-time', no 'every eddy', no bare 'automatic break detection'.

Used in: Part 1, Part 2, Part 6.

Sources:

The series

  1. Part 1 — How to read a satellite SST pass
  2. Part 2 — SST breaks: why fish sit on them (and when not)
  3. Part 3 — Cloud-free SST composite vs a single pass
  4. Part 4 — Hourly SST frames and the morning edge
  5. Part 5 — Chlorophyll colour lines on an SST chart
  6. Part 6 — SST forecast vs observed: what to trust

The overview of all the layers is How to read SST and habitat maps; every region's newest pass is on the data status page. Open the map to read a live pass.

Data credits. Satellite SST passes and composites: NOAA ACSPO via NASA PO.DAAC. Chlorophyll: NASA OB.DAAC (IMOS ocean colour over Australia and New Zealand). Subsurface temperature, salinity and sea-surface height: Copernicus Marine Service. Every pass on the map carries its satellite and time; every chlorophyll frame carries its own date.

Olli, founder of Fishare · 19 October 2026

Offshore · satellite SST charts

EVERY PASS. EVERY REGION. BREAKS DRAWN.

Every satellite sea-surface temperature pass of the last 72 hours, checked hourly, in every region we cover, with the break already found and labelled. Free to open, no card needed to start.

Open the map

Keep reading

Part 1 of 6
How to read a satellite SST pass
A pass is one satellite looking down once. It is the least processed picture of the sea you can get, and the one most anglers never see, because most charts hand you a daily average instead.
Part 2 of 6
SST breaks: why fish sit on them (and when not)
A temperature break is a place where the sea changes its mind. Bait and the things that eat bait collect where it does.
Part 3 of 6
Cloud-free SST composite vs a single pass
The composite tells you where the water is. The passes tell you when.
Overview
How to read SST and habitat maps
All the ocean layers on one page.
Written by
OMV
Olli-Mikael Vaittinen

Olli-Mikael Vaittinen has fished his whole life. Fifteen years of fly fishing, guiding seasons on Norway's Lakselva — his favourite Atlantic salmon river — and a blue marlin landed in Vava'u, Tonga. Founder of Fishare — the app that puts the data behind the decisions every angler makes on the water.

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