skip navigation
New Hampshire Department of Environmental Services

Fish Community Assessments
Sampling Techniques

Sampling Techniques for Fish Community AssessmentsMembers of the fish community represent the apex of the aquatic food web, and a survey of the resident community can tell a great deal about an ecosystem's overall health. The Biomonitoring Program's fish assessment methodology is intensive and is adapted from EPA's Rapid Bioassessment Protocol. The protocols are currently designed for wadable streams and rivers and use a backpack-mounted shocking unit for the capture of fish.

Surveys are conducted from the beginning of June through September. Sampling during these months is representative of a stable fish population. Fish tend to remain in a particular localized area, as spawning and migratory activities are not usually taking place, and the collected fish assemblage is most likely to include the full range of resident species. Rivers and streams during these months are probably at their lowest and least fluctuating flows, making collection efforts less difficult.

Since many species of fish inhabit very specific sub-habitats within a stream, all dominant habitat types within the reach are sampled. This includes pools, riffles, and runs as well as other types of habitat that may be present (beaver ponds, for example). The sampling distance is 150 meters, and at least two representatives of all habitat types present will be sought in this reach. The 150 meter sampling length helps to ensure that all the species found in the stream segment are likely to be captured.

Once a stream segment is selected, electrofishing efforts use a single pass over the reach. This deviates from other protocols which call for multiple passes over the selected reach. For our program, we are not interested in gathering every single organism in the reach. Most important to our data collection is the diversity of species present and relative abundance.

Four people are involved in the in-stream shocking process; one individual operating the backpack shocker, two people netting the stunned fish, and the fourth person carrying an aerated holding tank to retain the stunned fish. Sampling takes place in an upstream direction, eliminating problems of turbidity introduced from the survey crew while maximizing the capture of immobilized fish that are drifting downstream. All possible precautions are taken to avoid fish mortality, which is typically extremely low. Fish are removed from the electric field as soon as possible so as not to expose them to additional shocking. The level of effort at each site is measured by actual shock time, and sampling is standardized over the 150 meter length of stream. In other words, we try to have electric current flowing for roughly the same amount of time at each site.

When electrofishing is completed for a reach, the number of individuals of each species is recorded. Young-of-the-year fish are recorded for non-forage species, but not included in data enumeration. This consists of fish less than 25 mm in length. Adult fish (and/or those >25 mm) are inspected for external anomalies (lesions, tumors, deformities, parasites, fungus, etc.) and noted, and then they are returned to the waterbody. External anomalies that are caused by pollutants are extremely rare in New Hampshire, as are anomalies that occur naturally in fish.

Mercury contamination in fish is an issue that has received much attention in the state. Fish collections are made routinely by the NHDES's programs in cooperation with New Hampshire Fish and Game Department to monitor concentrations in fish tissue. Samples collected by these programs are analyzed for mercury by the Department of Health and Human Services. The results are used to assure the safety of consuming fish caught in New Hampshire waters.

NH Department of Environmental Services | 29 Hazen Drive | PO Box 95 | Concord, NH 03302-0095
(603) 271-3503 | TDD Access: Relay NH 1-800-735-2964 | Hours: M-F, 8am-4pm

copyright 2014. State of New Hampshire