Interpreting Temperature and Chlorophyll Breaks on SST

Sea surface temperature charts can help you drive directly to the hot bite — but only if you understand what they’re telling you. 

Sea surface temperature charts and chlorophyll charts a la Fishdope are one of the best fish-finding, fuel-saving tools available to a modern offshore angler. Of course, Fishdope incorporates numerous other sources of intel to help find the fish (ranging from spotter planes to the Hot bite Report Map), but the wiggles, squiggles, and colors on those charts deliver reams of information. The question is, when you look at them do you understand exactly what they’re telling you? 

Reading SST Charts 

We all know that temperature breaks equal structure to offshore anglers, and we watch our temp gauge for changes in temperature as we look for these breaks. The more dramatic and sudden the change is, the better. What we’re seeing here is “convergence zones” where different water masses are meeting each other. Anything in the water that’s being pushed by the currents gets squeezed together between those masses of water — think of two conveyor belts running in different directions abutting each other, smooshing together everything from phytoplankton to sargassum.  

Read Next: Finding Fish: Wide Beam vs. Narrow Beam Transducers

SST charts give you a satellite’s view of all those different convergence zones. The basic idea of looking at the SST charts and determining where those conveyor belts are concentrating the food and flotsam is made easier by color coding and/or adding contour lines. Simple enough, right? 

But there are some subtleties to consider. First off, think about the temp break as if it was a physical item, then ask yourself just how attractive this structure would look. Training yourself to think of the temperature differences in physical terms will go a long way in helping determine where fish are likely to be. Are you looking at a more or less straight edge, for example, or does it have a finger or bulge popping out? Now translate the answer into physical terms: most anglers understand that a shelf may hold fish, but a shelf with an underwater point or bulge sticking off of it is likely to hold more fish right around the anomaly. Same goes here, when it comes to temperate structure. Dittos for an eddy, a pocket of water that’s significantly warmer or cooler surrounded by a water mass of different temperature. The physical structure with the most similar appearance would be an underwater hump or hole, which again, we all know could be a big fish-attractor.  

sea surface temperature charts can help us locate where the fish should be

Fishdope goes beyond SST and incorporates many data sources, but SST is always critical.

Also consider exactly what the temperatures are on either side of the break. Some species favor cooler water and others favor warmer water, and in some cases, one side or the other of a break may fall outside of a species’ comfort zone. In this case the fish will likely travel along the break, as opposed to crossing it. Now combine that with an anomaly along a break, and the side of the anomaly falling into the species’ favored temperature zone could act to concentrate fish in a huge way. 

Reading Chlorophyll Charts 

Chlorophyll charts display the amount of plankton — aka food — and where it’s concentrated. But plankton can be a curse as well as a blessing, because it reduces water clarity. As a result, the best areas to look for fish tend to be where the water with high chlorophyll content meets water with low chlorophyll content. Again, color coding and/or contour lines can be used to illustrate the differences in the water and again, the more dramatic and sudden the change is, the better. 

chlorophyll charts help to find out where the fish are most likely to be

Chlorophyll charts tell you where the food chain should be most active.

Beyond this, when it comes to using chlorophyll charts to determine where fish are likely to be you’re essentially looking for the same things (big breaks, anomalies in breaks, and isolated patches) as you are when looking at temperature. In fact, quite often the temp charts and the chlorophyll charts will overlap to a significant degree. 

Assembling the Fish-Finding Data 

You thought we were done? Nope, not quite yet. Now, we need to put this all together with bathymetry data. SST and chlorophyll on its own may lead you to fish, but when you find good-looking indicators and they just so happen to interact with physical structure and depth changes, the pot gets sweetened yet again. So, when you’re looking at some attractive water boundaries, they happen to hit an edge, pinnacle, or canyon, and the three factors of temperature, chlorophyll, and physical structure converge, it’s probably time to lay in your waypoint. 

using the latest SST shots on your MFD gives you the best chance to find fish

The timeliness of your SST view is critical; being able to pull up the latest shots right on your MFD gives you a leg up.

Okay, we have a lot to think about already, but now Mother Nature throws us a curve ball. Currents, water masses, breaks, and chlorophyll concentrations are constantly moving. How current the charts are, when you look at them, and when you arrive at the offshore grounds can vary quite a bit. And if 24 or 48 hours have passed between when the snapshot was taken and when you arrive at the spot, there’s a good chance you’ll troll around for hours looking for temperate structure that’s long gone. It’s important to scroll back through time to look at three or four days of data prior to a trip, and pay attention to the direction and speed of travel a break is taking. Then, you can predict where it will be when you’ll arrive on site. Also look for strengthening or weakening of the boundaries through the past few days, as water does mix, and breaks do strengthen and/or deteriorate over time. 

Learn how to read SST and chlorophyll charts like a pro, and you’ll be bloodying the decks in no time… right? Maybe. Finding the fish doesn’t mean you can get them to bite. Plus, there will be days when you locate an utterly magnificent looking area that’s completely barren. There will be other days when you plug the boxes even though it looked like the spot you caught ‘em at should have been a desert. But most of the time the data these charts deliver will prove to be an excellent tool that can help you do less looking and more catching. If, that is, you understand what it’s trying to tell you. 

Lenny Rudow …has been a writer and editor in the marine field for over two decades, and has authored seven books. He is currently the Angler in Chief at Rudow's FishTalk Magazine, is Electronics and Fishing Editor for BoatUS Magazine, and is a contributing editor to several other publications. His...