Here's why and some alternatives:
* "Fish on" is a phrase indicating a fish is hooked. Hawaiian wouldn't use a single phrase like this. They'd likely describe the action more directly.
* Possible translations:
* "Iʻa ma ka laina" (ee-ah mah kah lie-nah): Literally "fish on the line."
* "Iʻa ua hopu ʻia" (ee-ah ooah ho-poo ee-ah): Literally "fish is caught."
* "Ua ʻike ʻia ka iʻa!" (ooah ee-keh ee-ah kah ee-ah): Literally "The fish has been seen!" This emphasizes the fish biting the bait.
The best way to express "fish on" in Hawaiian would depend on the specific context and what you want to emphasize.