you can introduce fishs as part of some medicines , just use imagination and real life.
Even makin quest u know " bring mad tribal shaman 20 teeth fishs so he can make some ritual to save his people" and make this type of fish a rare spawn other one chief from lets say San Francisco buys some types to makin his fame poison fish food - some HP , + AP?
(i know this is not a brillant ideas but hey on wasteland in fallout2 more crazy thingz happend and this is just example
)