An oily film in mid-summer is most likely caused by insect cases that were concentrated along the shore by wind after a hatch.
(An oily film in mid-summer; is most likely caused; by insect cases) (insect cases; were concentrated; L:along the shore; by wind; T:after a hatch)
An oily film in mid-summer insect cases
is most likely caused were concentrated
by insect cases
L:along the shore by wind T:after a hatch