What fish / insects will become unavailable after the end of March?

Solution 1:

I've found this list of Fish and bugs leaving before the end of March, courtesy of Polygon.

Two locations which may not be immediately obvious are River (Clifftop) and River (Mouth). Based on my experience with past games in the series, along with some reading along, Clifftop appears to be any river not on the lowest level, and River (Mouth) is the area where the river connects with the sea.

Northen Hemisphere

There are 7 fish / insects that won't be available starting in April in the Northern Hemisphere.

Southern Hemisphere

There are 39 fish / insects that won't be available starting in April in the Southern Hemisphere.