I think there are four possibilities here - 1.) while she was stabbing the gnome and stealing the fishing pole, the fish managed to get away, and may be calling in its friends for a revenge feast. 2.) She ate the fish as a post-murder victory meal. Backstabbing gnomes is hard work, after all. 3.) She threw it back, because... ew, fish. 4.) The fish killed them both, and is now using the orc as a meat-puppet.