When Luke Hinkle, of Purakaunui, was fishing in Glenorchy’s Diamond Creek recently, he hooked a trout.
But as he was reeling it in, he noticed something a bit odd about it — the fish boasted an extra fin on the back of its head.
"It didn’t seem to notice it was unusual," Mr Hinkle said.
He took a quick video on his phone of the trout, which weighed about 500g, before he released it back into the creek.
University of Otago developmental biologist Associate Prof Caroline Beck said the abnormality of the "mohawk trout" looked like an ectopic median fin, and was most likely an error in the fish’s cell development.
In chickens, adding the Fgf into the tissue between the two pairs of forming limbs at three days of development induced formation of a third limb.
"Because only one mistake is needed it could happen spontaneously as in your fish.
"Once the process of fin formation is triggered, a fin will form."
Prof Beck said there were also frogs in Colorado, in the United States, which had extra hind limbs caused by a parasitic worm disrupting tadpole development.
It was not the first time a mohawk trout had been caught in the South — in 2014 a Southland fisherman also caught a brown trout with an extra dorsal fin.
Southland Fish & Game had the 2.5kg fish mounted and displayed at its Invercargill office.
