What does chlorophyll tell me about where to fish?

What the chart is showing you

Every color on the chlorophyll layer is an amount of chlorophyll, and the legend says how much. Bright green is plenty, deep blue is little. You are looking for the place where one turns into the other.

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1. 1 The day. Drawn from that morning’s report and the layers stored for that day.
2. 2 Legend and layer time. Chlorophyll in mg/m3, from 0.063 to 13. The steps are uneven, about four times each. The stamp under the bar says the satellite saw this water on Monday, October 5, 8 PM.
3. 3 Green water. Along the reef tract and the coast: productive water, where the bait chain starts.
4. 4 Blue water. Dark blue is the low end of this legend: low in plankton, where pelagics travel. On this map it lies farther offshore, east and southeast of the reef-side green-to-teal stretch the report described.
5. 5 Spot 1. The report’s top pick, 9.3 NM out, on the front where green starts to thin.
6. 6 The color line. Green fades to teal and then blue over a few miles. A gradual change, not a hard line.
7. 7 Layer picker. Switch the water underneath. Sea Surface Temperature and Surface Currents are in this list.

How to read it on October 7

Four reads, in the order a captain would make them.

1. READ 1

   Read the legend first

   The steps on this legend are 0.063, 0.24, 0.89, 3.3 and 13 mg/m3. Each is roughly four times the one before, so a patch that looks only a little greener can hold several times the chlorophyll. Read the colors against the legend, not against another day’s chart.

2. READ 2

   Find where the green thins

   Green water hugged the reef tract and the bay side. The imagery read had the water turning cleaner and lighter several miles seaward of Molasses, about 5 to 10 NM, with nothing within that stretch reading truly blue in the satellite imagery it analyzed. The dark blue on this map lies farther offshore, east and southeast of that stretch. The report called the change gradual to moderate rather than a hard line.

3. READ 3

   Ignore the extremes

   The zone averaged 1.53 mg/m3. The heaviest cell, 88.6, sat far outside the zone up the Gulf side, well above anything this legend draws. The report’s words: do not chase those big numbers.

4. READ 4

   Fish the edge, not the middle of the green

   Green water holds the bait, so the fishable edge is where the reef-tract green thins into cleaner water. The report’s plan was the first clean water outside the 30 ft contour, which is where its chlorophyll front sat, 9.3 NM out.

What to do with it

A color line on its own is a maybe. The report’s ranked spots count it as one edge among several: on October 7, spot 1 had a temperature break and a chlorophyll front overlapping. The imagery read put the warm band’s inner edge and the color line together 5 to 8 NM southeast of Molasses Reef, and the report’s advice was to work that seam. The convergence map page shows how that stacking works.

To check a spot against this chart, about a minute:

1. Open the report’s map and switch it to Chlorophyll-a (Ocean Color).
2. Find the spot’s pin. Is it on the line where green turns to blue, or in the middle of one color?
3. Read the layer’s time under the legend. Chlorophyll can be days old by the time the report is written.
4. Switch to the temperature layer. A color line that sits on a temperature step is a much better bet than either alone.

What the chart cannot tell you

• Two products, two times. This map’s stamp names Copernicus GLOBCOLOUR L4 chlorophyll daily, as of Monday, October 5, 8 PM. The report used a different chlorophyll product: its source note names VIIRS DINEOF gap-filled chl-a daily, observed October 5 at 12:00 UTC. Its data note counted that read at 45 hours old and said it came from a backup because the preferred source was unavailable.
• Cloud hides it. A satellite cannot see water color through cloud. The report’s VIIRS DINEOF backup was a gap-filled product, so some of that chlorophyll read was estimated where cloud hid the water.
• Thin reads happen. On October 2 the Alligator Reef report’s chlorophyll read covered only 3 data points. The report said so and leaned on the satellite imagery for the color line instead. A thin read is a reason to trust the line less, not more.
• Near shore, color is not only plankton. Close to the coast, sediment and shallow bottom also change the color, so the biggest inshore numbers are not a guide to bait.
• It is one clue. Chlorophyll tells you about the plant life in the water. It does not tell you where fish are standing or whether the day is fishable.

What this looked like on the water

That morning’s report told anglers to fish the first clean water outside the 30 ft contour, where its chlorophyll front sat 9.3 NM out, and to work the seam 5 to 8 NM southeast of Molasses Reef where the warm band and the color line overlapped. Its imagery read found nothing truly blue within the 5 to 10 NM stretch seaward of the reef that it described; the dark blue on this map lies farther offshore, east and southeast of that stretch. So the target was a shade change near the reef, not a blue-water edge.

FishIntel is for information only. It is not for navigation or safety-of-life decisions.