[Biradallo]

You're not fishing the Noosa river?

Seriously though, sometimes this sort of thing just happens.

It could be a spontaneous mutation. That's one of the ways evolution occurs.

Fin deformities and absences common with highly inbred fish such as farmed salmon. With the way the bass fishery got flogged and the habitat was reduced, I'd expect that there is a degree of inbreeding in the population.

The fish looks healthy so it doesn't seem to have affected it's foraging.

I don't recognise your name, so if you're new, welcome to Scribbly
There's and introductions sticky if you feel inclined to tell us something about yourself.