According to legend, the Menehune (roughly equivalent to Leprechauns) built the fishponds overnight.
The stones to build the ponds were passed hand to hand by each Menehune for several miles to accomplish the task.
No one was supposed to see them working, however a couple attempted to witness the feat and was caught doing so. They were changed to stone pillars.

The remnants of ancient fishponds are found throughout the Hawaiian islands.
http://www.pbase.com/image/25782829 , Big Island
http://www.pbase.com/image/25782830 , Big Island
http://www.pbase.com/image/28559364 , Oahu
The Hawaiians are the only Polynesian group to practice aquaculture on a sustained scale.