Shamrock Megalodon

Shamrock Megalodon is gone for good: the wiki tags it Unobtainable, so you cannot catch it today. It was a St. Patrick's themed Limited fish from Ancient Isle, tied to Lucky or Clover Cluster bait, Clear weather, daytime and Spring. Its description says it was rarer than a typical Megalodon.

What remains is the record. Average weight was 58,575 kg, from 50,050 kg to 144,910 kg, and its maximum ranks 45 of 663 fish. At C$477 per kg, the average fish was worth C$27,934, which ranks 69 of 742 documented fish by average value. It gave 31,426 base XP.

For context, Phantom Megalodon is the other Limited fish listed for Ancient Isle, and the rods sold in that zone are Krampus's Rod, Phoenix Rod and Stone Rod. Those rods are still the ones to look at if Ancient Isle is your destination.

Verdict: there is nothing to farm here. If you own one from the past, its value per kg is high enough that it is worth keeping as a collector piece. If you do not, use these numbers only to understand what the event fish were worth.