Not sure you have looked to add fish or want to add fish to the scenerio but in messing with the scenerio on my own I found a way to add fish in.
in local on_chunk_generated = function(event) just before the end of the function i added the following:
for i = 1, 10 do
local position = event.surface.find_non_colliding_position('fish', event.area.right_bottom, 10, 1)
if not position then return end
event.surface.create_entity {name = 'fish', position = position}
end
Basically it just puts 10 fish in the corner of the chunk that will spread out and swim around afterwards.
