I don’t know about mutants but they could have superior genes that allow them to get to huge sizes. I’m guessing that the fairly moderate climate allows them to grow year around and they have a plentiful food source, and that is why they get so big. Lakes can also have genetically distinct sub-populations of species of fish. The sub-populations May mix during parts of the year but during the spring they go back to their different spawning grounds. Hope this answers your questions a little.
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