For the mountain stream fish investigated as a marker, several fish are captured at the same date and time at the same point, and the internal organs are removed to remove the digestive organs.

Immerse the removed internal organs in 1 mol of sodium hydroxide aqueous solution to dissolve the protein.

The microplastic remaining in the digestive organs is removed using a microscope.

(The period for dipping the sodium hydroxide aqueous solution is 14 days)