BMI Collection

BMIs are animals without a backbone that can be seen with the naked eye, and have to the ability to cling to bottom surfaces such as rocks, leaves or roots. They include crustaceans, mollusks and annelids but in many aquatic environments most of the macroinvertebrate community are the larvae of aquatic insects. These communities are important links in the food web between producers (leaves, algae) and higher consumers such as fish, and are key indicators of biological integrity in streams, rivers and wetlands.

Depending upon the stream environment a variety of methods and equipment are used to collect benthic macroinvertebrates from wadeable streams. In rocky-bottom streams, WV Save Our Streams recommends using a two-pole screen-barrier net (commonly known as a two-pole kick-net) or a single pole rectangular style kick-net (sometimes called surber on a stick). Both types should be equipped with 500-micron mesh netting. Two-pole kick-nets require at least two people working together to collect the sample. Groups may purchase other nets as needed but only the two-types discussed here are recommended for rocky-bottom collections. D-nets are used for macroinvertebrate collections from low gradient and wetland habitats. Low-gradient streams and wetlands require a much more intensive collection procedure. Ultimately, the types of nets used depend upon the goals and objectives of the volunteer monitoring program and the kinds of streams they plan to monitor.

Benthic Macroinvertebrates

BMIs live in a wide variety of aquatic environments. In lakes, wetlands and large river systems they are common in shallow edge microhabitats along shorelines in tangles of vegetation, roots, and leafs, in gravel shoals or along rocky and undercut banks; some kinds burrow in mud and sand of in shallow flowing water. In streams and swift-flowing rivers they are more common and diverse in rocky areas (riffles), but are also found in runs, which are sampled when riffles are not present. The collection procedure described here is designed for rock-bottom streams from riffle habitats.

Choose the Best Habitats

Your goal is to collect macroinvertebrates from three different riffle areas. (If the riffles are as wide as the stream then multiple samples could be collected within the same riffle.) The riffles should have different characteristics (i.e. different composition but mostly cobble and gravel and different velocities). Often different types of riffles hold different varieties of macroinvertebrates, so to properly assess the biological conditions you need to collect a representative sample. Of course your choices of habitats will ultimately depend upon what your reach provides. Once you’ve chosen your location always approach from the downstream end, sampling the site farthest downstream first. This approach insures that the sample is representative of its location and reduces the chances of biasing your second and third sample.

Get Into Position (Place the Net)

Select an area approximately the same width (or slightly less) than the width of your kick-net. Most two-pole screen-barrier kick-nets have about 1-meter or 3-foot width. The width of the single-pole rectangular kick-net is about ¼ to ½ meter. So if you are using a smaller diameter-net you will need to collect more samples (2-3 samples using the rectangular kick-net is equivalent to 1 sample with the screen-barrier kick-net). The net holder will place the net snugly against the bottom of the streambed; rocks can be removed if necessary to make sure you have a close-fit. Once you are satisfied with the position, line the front of the net with rocks heavy enough to hold the net in-place. However, be careful to choose rocks that are not too heavy or too wide or high. Large rocks will damage the net and will influence how the macroinvertebrates flow into the net thus making capture less successful. The net holder now tips the kick-net backward at about a 45-degree angle from the water’s surface. This provides greater surface area and more even flow into the net. If the net is held to high some of the macro-invertebrates will wash around the sides and not be captured in the net. While holding the kick-net backwards the net-holder must make sure that water does not wash over the top of the net. Have one 3 ½ to 5-gallon bucket ready before you begin collecting the sample. The bucket will be needed during sample collection.

Begin Disturbing the Streambed

The second person approaches the sample area from upstream and determines the approximate sample-size. Once the area is delineated, the sampler begins disturbing the streambed directly in front of the net. The process starts with rock rubbing. First, pick-up all large rocks (cobble size and larger) and inspects them. You are looking for snails, clams and caddisfly cases. These animals often cling very tightly to the rocks and are not removed by just a simple rub with the hands or a small brush. If the rocks have any of these animals, remove them from the rocks and place them inside the bucket. Move the rocks you picked-up towards front and slightly into the net; brush all sides of the rocks with your hands or a small vegetable-brush to dislodge other clinging macroinvertebrates. Continue the rock rubbing process until the larger sized stones have been thoroughly cleaned. If the rocks cannot be lifted from the streambed, simply rub them where they lay. As much as possible the rock rubbing should proceed from the upstream portion of the sample area towards the front of the net. After the rocks are rubbed they should be placed aside (outside of the sample area) so they are not rubbed a second time.

Some volunteer programs choose a timed approach to rock rubbing since this is a rather intensive and somewhat time consuming step. The recommended time frame for adequate rock rubbing is four - six minutes (or less) depending upon the abundance of cobbles and boulders within your sample area. If you choose a timed approach you should make a note on the survey data sheet and record the time frame you use. The recommended kick-time is two minutes, so your entire sample should take about six - eight minutes to collect.

Remove the Kick-Net from the Streambed and Capture the Collection

This is a very important step; since the sample collection is laborious you do not want to lose any of the macroinvertebrates collected by sloppy procedures here. Very slowly remove the rocks that have acted as anchors to hold the kick-net in place, rub them off while you remove the rocks, or you may choose to rub them before using them to anchor the kick-net in place. While the net-holder grabs and holds the top of the net in-place the kicker grabs the bottom edge of the net near the handles. The net is removed with a scooping motion, the kicker moved slightly forward and upward while the holder keeps the net steady so that no macroinvertebrates are lost from washing over the top of the kick-net. Both persons then pick-up the net and roll it into a loose cylinder, securing the ends and taking it to the shoreline. The collection bucket should be at the ready to accept the contents of the kick-net.

Kick-Net Removal

Place the Net into the Bucket

Slightly unwind the net so that it fits inside the bucket. With a smaller bucket or a spray bottle, wash the contents of the kick-net into the bucket. It will take several minutes and several washes to knock loose most of the macroinvertebrates. Between each attempt, remove the net and check for macro-invertebrates that have not been dislodged. Often these hardy clingers are found near the edges of the kick-net along the bottom-side and in the seams of the net. Be sure to check the opposite side for macroinvertebrates that may have crawled in an attempt to escape. You must be very careful not to overfill the bucket. If the bucket begins to fill with stream water more than about two-thirds its heights, remove some of the water by seining it through the kick-net (hold the net tightly on the bucket and pour off the water) so that the water is poured off and the macroinvertebrates remain in the bucket. The process is complete when you are satisfied that the kick-net has been thoroughly washed and most of the macroinvertebrates are now in the bucket.

Remove the Captured Invertebrates from the Bucket and Begin Sorting

The goal of this step is to remove all captured macroinvertebrates so that they can be observed, identified and counted. WV Save Our Streams recommends that you use several shallow white trays. The best way to start is by trapping the macroinvertebrates as they are poured from the bucket. Before starting the steps below, remove all larger materials that may have been collected with your sample from the bucket. Make sure to check these for macroinvertebrates before they are discarded. At certain times of the year leaves and other debris are very plentiful in the stream and this material must be sorted. (It is common to find many kinds of macroinvertebrates in leaf-packs; this material is one of their favorite places to live.) The best way to deal with the leaves is to remove as many as possible, place them in smaller bucket or container and wash them to remove the macroinvertebrates. Pay close attention to the leaves that appear chewed and have begun to decay. Newly fallen leaves are less likely to have many macroinvertebrates. You can use a second bucket with your kick-net on top, and then pour the captured organisms over the net so that they are trapped against the net. The pouring is stopped periodically so that the macroinvertebrates can be removed from the net and placed into the collection trays. Small forceps are the best tool for this job; however the macroinvertebrates can also be removed by hand. The easiest method is to use a wash bucket or EZ-strainer. The EZ-strainer is available in a variety of mesh sizes and fits nicely inside a 3 ½ or 5-gallon bucket. Your collections are poured directly into the EZ-strainer, or into a second bucket and then into the strainer if additional washing is necessary. See additional information for some homemade equipment ideas.

If your organization decides to complete independent surveys that include aquatic collections, you must apply for and receive a Scientific Collection Permit from the WVDNR. See additional information to view the SOS Program's permit.

BMI Sorting

Additional Information

  • Benthics

    This resource is designed to provide a better understanding of the wide vareity of aquatic invertebrates found in our rivers, streams and wetlands. In addition to images, general information is included about the distinguishing features of the aquatic stage that aid in identification, and a scale for the organisms feeding group, tolerance, size range and habitat. Larval and adult images are also provided for many families.

  • Kick-Net

    Many professional and volunteer monitoring programs throughout the United States use a rectangular style kick-net. The reasons are simple; it is easy to use and versatile.

  • Low-Gradient Stream Procedures

    The procedures described here are mostly for advanced volunteer monitors; however level-one streamside procedures can be modified if collections from low-gradient (muddy bottom) streams are necessary.

  • Volunteer Manual

    USEPA's Volunteer Monitoring: A Methods Manual was written to provide a better understanding of the concepts and procedures needed to evaluate the conditions of streams and rivers.