My retainer won't go after any fish other than Crayfish?

[Orophin]

I would have to double check the list, but it seems like your retainer will only go after fish that are classified as "Crafting Materials".

On a side note, keep in mind as well your retainer won't be able to go past level 5 on fishing until you raise your own level. Your retainer will always have a cap of 5 levels under your class unless your class is level 50.