To sort the sample, place the composited sample into the mesh bottomed sorting tray, similar to
equipment described in EPA's RBP Maunal and by Caton (1991). The tray is a 250-micron mesh bottom
that is evenly divided into numbered sections.
Place the mesh bottomed tray into the plastic outer tray and add approximately 3 cm of water to
facilitate the even distribution of debris. In the field, place the tray on a level tripod platform.
Evenly distribute the material in the tray and lift the mesh bottom tray out of the water.
Use the random number table to select a minimum of one-quarter of the sections. Use a cookie cutter
or other similar device to segregate, and remove the macroinvertebrates from the selected squares
with a small spatula.
Distribute the contents of the four squares into a separate white plastic tray with a small quantity
of clean water. All the macroinvertebrates are removed with forceps and placed in a labeled vial of
alcohol. An inside paper and pencil label is recommended as well as an exterior label.
A minimum of 300 specimens or ¼ of the tray is sorted. If necessary, randomly select additional
squares to attain the 300 organism minimum sample size. All organisms are completely removed from
all sub-sampled squares to avoid biasing the macroinvertebrate sample toward the larger, more
visible species. A tally counter is recommended. Keep track of the number of squares sub-sampled in
order to estimate the original macroinvertebrate density in the stream. Identify the
macroinvertebrates to the taxonomic level desired.