Adjective: bearing some physical and/or behavioral resemblance of a fish.
My brother was making some goofy faces at the
dinner table, and I described him as pescatine, not knowing it's
not a word.
She stood there opening and
closing her mouth in a pescatine manner,
searching for a suitable
response.
"Oh, don't mind him, his mind just flits about in a pescatine way."