Fishers often tell me that satellite maps need to be really recent and very high resolution for them to be any use for detecting fronts. As a result, they rely on single pass imagery for the most recent SST and dismiss the cloud-free SST as old news. I’m going to tell you that this is a mistake, and show you why in this blog.
Satellite mapping apps will often provide a long list of single images, often presented as thumbnails that fishers can sort through to find an image for their target fishing area. They may tell you that the SST is available very frequently, but it is really important to check both the dates and the times on these single images. I have seen a stack of thumbnails where most of them are obscured by clouds, and the remaining few useful images are as much as 24 hours old. In other words, what looks like recent imagery is not that recent when the cloudy images are removed. This is not always the case, but it often is, especially in regions like New Zealand which have a lot of cloud cover. Some fishing app companies also tell clients that they process the SST imagery in-house to provide the highest resolution possible, but if they are using the free satellite data from NASA, NOAA and the European Space Agency, the highest resolution (at present) is 0.01 degrees or about 1 kilometer, regardless of whether they process it or not. This is the same resolution that is available as cloud-free SST.
Should you use the most recent SST data, or look for frontal zones, or both?
I download 1 kilometer resolution data from a NOAA satellite every 3 hours, at 00:00h, 03:00h, 06:00h, 09:00h etc. In many cases the patchy coverage caused by clouds prevents determining where the SST front or temperature breaks are. So the most recent satellite maps may not get you very far because of cloud cover, and the resolution will still only be about 1 kilometer. Here is an exceptionally good series of images for the Bay of Islands, New Zealand where the changes in SST are visible. I stack 6 images together to form this series map, so each image is separated by one hour. The most recent image was 7 hours old at 09:00h, which was the time that this series was updated. There are three fronts clearly visible in these images. I’ve marked them by the black lines and blue arrows. By comparing the hourly images you can see how the fronts are changing.
Compare the frequent SST series maps (above) with the cloud-free SST maps over the most recent 4 days (below). It is easy to see the same frontal zones in the cloud-free SST images in each of the four days. I have marked the fronts with black lines for easy reference.
The same pattern can be seen in the fronts map (below), where the black lines have been placed along the fronts.
The SST frontal zones are clearly defined in the cloud-fee SST (see below) by the closely spaced white-coloured temperature contour lines. To make this even clearer, I calculate the temperature gradients and plot them as the green and yellow lines in the fronts maps (see below). The areas with the strongest fronts are either yellow or red.
The 4 cloud-free SST images have several advantages compared to the frequent SST series maps.
(1) The cloud-free SST shows that the frontal zones were persistent over 4 days, which is long enough for the plankton and baitfish to respond to small scale flows and nutrient dynamics.
(2) The cloud-free SST shows the frontal zones in much more detail than the single pass image because the cloud-free image is constructed from 5 different sensors on separate satellites, and is combined with modelling and in situ (meaning in ocean) measurements when these are available.
(3) The variability over 4 days defines frontal zones that can be fished, which is more valuable information than the location of a single temperature break that may form and dissipate, and reform with the tides.
This example illustrates the answer to the question “is it better to use the most recent SST data, or to look for frontal zones, or both?” The answer is that both types of information can be combined to determine the best target zone to fish.
Key Points
- Temperature breaks are also called fronts.
- Targeting the exact position of fronts detected in the most recent satellite images is likely to be less useful than targeting broader frontal zones that persist over several days. This is true because because individual fronts move, within broader frontal zones, and plankton accumulation in frontal zones takes several days to develop.
- Combining the most recent single pass SST images with four days of cloud-free SST imagery provides a good method for locating promising frontal zones to target for game fishing.
- The answer to the question “Is it better to use the most recent SST data, or to look for frontal zones, or both?” is both.