Gamefish often, but not always, are found in clear, blue oceanic water that borders on more productive, greener water with higher chlorophyll. I’ve designed the colour scale of the chlorophyll maps to highlight the detail in these low chlorophyll waters, so that the colour range covers zero to 2 mg/ litre of chlorophyll. Nearshore water often has much higher chlorophyll, but the satellite sensors cannot easily distinguish between chlorophyll and other pigments contained in particles originating from the land. Areas with river outflows will be coloured by combinations of sediment, particles from the land (called terriginous material), and chlorophyll from phytoplankton. This is why it’s hard to distinguish between “dirty water” and water with high chlorophyll content. Fishing Maps gets around this problem to some extent by just plotting the lower levels of chlorophyll, and colouring all of the higher chlorophyll and the “dirty water” pink. It’s not ideal, but it works pretty well.
On the 9th December 2025, a dramatic chlorophyll front was visible off the Mahia Peninsula, marked by the red arrow in Figure 1. The front bordered an area of higher chlorophyll shown within the white circle, which was located near the head of a dramatic canyon cutting into the shelf (black arrow). The 4-day chlorophyll maps (Figure 2) provide more information about the development of the chlorophyll front. The maps for 8th and 9th of December (Figure 2) suggest that the elevated chlorophyll may have been fed by nutrients coming onto the shelf from the canyon head. However, there is another potential source of nutrients that could have fueled the phytoplankton growth and higher chlorophyll. This is the cool water streamer (which looks like a river), coming from south of Mahia on the 6th and 7th December (red arrow in Figure 3). The boundary between this cool water streamer and the warm water on the shelf created an SST front, marked with the red arrow in Figure 4. The streamer of cool water faded on the 8th and 9th of December, and a warm patch of water developed in the same area as the chlorophyll patch. The fronts faded away because there were no longer strong temperature differences. The remaining warm surface water and more nutrients from the cool water would support phytoplankton growth which would produce a patch of chlorophyll. It is impossible to know exactly what was going on without sub-surface data, but we can still get a lot of information from the satellite maps.
The bottom line for game fishers is to know that the dramatic chlorophyll front was very likely due to phytoplankton accumulating chlorophyll in the area where there was an SST front formed by a cool water streamer. Phytoplankton grow quickly in warm water with enough nutrients, and will create a patch of chlorophyll in a couple of days. This is an important point, and I keep hammering it. Patches of feed take time to accumulate, and it is the persistence of features like fronts over several days that will determine whether plankton feed patches will be detectable. The fish are highly mobile, and less predictable based on the oceanography. But the plankton that enrich the areas where fish feed respond to the ocean physics over several days, and this is why the fishing maps are so useful.
