Will Fenn and other types of traps be illegal to use after 1 April 2020 for other species?

No, Fenns and several other trap designs can still be used to trap relevant pests provided they are listed on the relevant STAO and you act in accordance with the requirements.

If you are targeting stoats, you will need to change to a new stoat approved design/set-up as listed on the relevant STAO.