Fish Sampling and Metrics
Biologists in the Biomonitoring and Aquatic Studies Section (BASS) assess the biological health of Vermont's aquatic environment through macroinvertebrate
and fish communities.
(Note: The Vermont Department of Health's Fish Consumption Advisory recommends
that people limit their consumption of some fish caught in Vermont waters.)
Fish Sample Collection
Stream fish community sampling is generally conducted during the late-summer, early fall index period on wadeble-sized streams and rivers. Fish assemblages
are sampled from a variety of stream types ranging from cold water streams commonly supporting one to eight species to warmwater streams with up to 20 species.
One or two backpack DC electrofishing units are used to stun fish for identification and enumeration. Each site is electrofished in a systematic fashion
beginning at the downstream margin of the section and continuing upstream to the head in the section. One to three passes are made over the entire section.
- Representative Sampling:
The general method of fish assemblage sampling used by the VTDEC has been to sample stream sections which are representative in habitat of the overall
stream reach in which they are located. For example, if a stream reach is of high gradient with a predominance of riffles, then the section selected for
sampling will reflect that character by including a predominance of riffles. An effort has also been made to include all habitat types within a section
so that the full assemblage present may be sampled. For most sites a subjective habitat analysis is conducted which includes visual estimates of quality
and quantity of the following physical factors: in-stream cover; substrate composition; pool-riffle ratio; bank stability; type of dominant riparian vegetation;
relative stream discharge; water clarity and percent canopy. Sampling efficiency is also noted in the field notes. For many sites, a more quantitative habitat
evaluation is undertaken using a series of measurements of velocity, depth and substrate at regularly placed points on perpendicular transects. Physico-chemical
parameters measured on-site are water temperature, specific conductance and alkalinity.
Two Indices of Biotic Integrity (IBI) are used to evaluate fish assemblage health at most sites. The Mixed water IBI (MWIBI) is a modification of Karr's
original mid-western U.S. streams index (Karr, Assessment of biotic integrity using fish communities. Fisheries 6(6):1981). The nine-metric MWIBI is specifically
formulated for Vermont streams that support five or more native species. The six-metric Coldwater IBI (CWIBI) is used on small coldwater streams supporting
two to four native species. Both indexes are multi-metric and score in a range 9-45 (corresponding to poor to excellent). Currently neither index can be
applied to slow- flowing, sand-bottomed streams or very large non-wadeable rivers.