For the complete engine maintenance story, start with the first article: Periodic Engine Maintenance and Spares.
This filter is located in the bottom of the pump, inside of the removable bottom cap. Twist the cap and the filter can be removed and cleaned. Problem is the mesh is so fine it is easily damaged, so be careful! Upon removing the cap (and catching most of the diesel in a plastic bag) the culprit was obvious. A collection of debris completely covered the filter. It had apparently never been checked. We cleaned it as carefully as possible (considering the circumstances) but still managed to damage the mesh a little. This was not part of our spares and there certainly wasn't one aboard, so we were forced to reinstall the damaged filter. We assumed the downstream filter cartridge would catch any debris before it reached the injector pump. We could only hope any remaining debris in the tank wouldn't damage the pump the mesh filter was designed to protect.
After reassembling the pump, we had to bleed the fuel injection system. Luckily, like most of the Universal engines used by Catalina, this engine had a self bleed return valve so it was relatively easy to bleed. Once that task was accomplished, the engine started and ran fine. We were able to make our way into the anchorage while we could still see the coral, drop anchor and have another round of rum punch.
The reason for the clogged pump filter can only be conjecture. First, the boat was a reserve boat, not the boat we had originally chartered. (The head gasket on our original boat's engine failed but that's another story.) The debris in the tank had probably accumulated over time and settled to the bottom. Second, it was a really great sailing day. It is likely the motion of the boat agitated the fuel and dispersed the debris throughout the tank until... Three, as we entered the calmer conditions of the harbor, the debris may have settled out again and been sucked up by the fuel pump.
In any case, this illustrates the need for periodic cleaning of the pump filter. On most occasions, when you check the filter it will be just fine. But occasionally you may find debris that will compromise the safety of your boat. The appropriate time to check the fuel pump filter is at the time you replace the fuel filter cartridge. You will have introduced air into the fuel system and need to bleed bleed it anyway. Why not clean the fuel pump filter at the same time?